The Milky Way[c] is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.
The Milky Way is a barred spiral galaxy with a D25 isophotal diameter estimated at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years),[10] but only about 1,000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that a dark matter area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years (613 kpc).[26][27] The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, forming part of the Virgo Supercluster which is itself a component of the Laniakea Supercluster.[28][29]
It is estimated to contain 100–400 billion stars[30][31] and at least that number of planets.[32][33] The Solar System is located at a radius of about 27,000 light-years (8.3 kpc) from the Galactic Center,[34] on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The Galactic Center is an intense radio source known as Sagittarius A*, a supermassive black hole of 4.100 (± 0.034) million solar masses.[35][36] The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.[37]
Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe.[38] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis,[39] observations by Edwin Hubble in 1923 showed that the Milky Way is just one of many galaxies.
Etymology and mythology
Main article: Milky Way (mythology)
In the Babylonian epic poem Enūma Eliš, the Milky Way is created from the severed tail of the primeval salt water dragoness Tiamat, set in the sky by Marduk, the Babylonian national god, after slaying her.[40][41] This story was once thought to have been based on an older Sumerian version in which Tiamat is instead slain by Enlil of Nippur,[42][43] but is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities.[43]
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so the baby will drink her divine milk and become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In another Greek story, the abandoned Heracles is given by Athena to Hera for feeding, but Heracles' forcefulness causes Hera to rip him from her breast in pain.[44][45][46]
Llys Dôn (literally "The Court of Dôn") is the traditional Welsh name for the constellation Cassiopeia. At least two of Dôn's children also have astronomical associations: Caer Gwydion ("The fortress of Gwydion") is the traditional Welsh name for the Milky Way,[47][48] and Caer Arianrhod ("The Fortress of Arianrhod") being the constellation of Corona Borealis.[49][50]
In Western culture, the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek γαλαξίας, short for γαλαξίας κύκλος (galaxías kýklos), meaning "milky circle". The Ancient Greek γαλαξίας (galaxias) – from root γαλακτ-, γάλα ("milk") + -ίας (forming adjectives) – is also the root of "galaxy", the name for our, and later all such, collections of stars.[51][52][53]
The Milky Way, or "milk circle", was just one of 11 "circles" the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, the Arctic Circle and the Antarctic Circle, and two colure circles passing through both poles.[54]
Common names
"Birds' Path" is used in several Uralic and Turkic languages and in the Baltic languages. Northern peoples observed that migratory birds follow the course of the galaxy[55] while migrating at the Northern Hemisphere. The name "Birds' Path" (in Finnish, Estonian, Latvian, Lithuanian, Bashkir and Kazakh) has some variations in other languages, e.g. "Way of the grey (wild) goose" in Chuvash, Mari and Tatar and "Way of the Crane" in Erzya and Moksha.
House river: The Kaurna people of the Adelaide Plains of South Australia called the Milky Way wodliparri in the Kaurna language, meaning "house river".[56]
Emu in the Sky: The Gomeroi people between New South Wales and Queensland called the Milky Way Dhinawan, the giant Emu in the Sky that it stretches across the night sky.[57]
Milky Way: Many European languages have borrowed, directly or indirectly, the Greek name for the Milky Way, including English and Latin.
Road to Santiago: the Milky Way was traditionally used as a guide by pilgrims traveling to the holy site at Compostela, hence the use of "The Road to Santiago" as a name for the Milky Way.[58] Curiously, La Voje Ladee "The Milky Way" was also used to refer to the pilgrimage road.[59]
River Ganga of the Sky: this Sanskrit name (आकाशगंगा Ākāśagaṃgā) is used in many Indian languages following a Hindu belief .
Silver River: this Chinese name "Silver River" (銀河) is used throughout East Asia, including Korea and Vietnam. In Japan and Korea, "Silver River" (Japanese: 銀河, romanized: ginga; Korean: 은하; RR: eunha) means galaxies in general.
River of Heaven: The Japanese name for the Milky Way is the "River of Heaven" (天の川, Amanokawa), as well as an alternative name in Chinese (Chinese: 天河; pinyin: Tiānhé).
Straw Way: In West Asia, Central Asia and parts of the Balkans the name for the Milky Way is related to the word for straw. Today, Persians, Pakistanis, and Turks use it in addition to Arabs. It has been suggested that the term was spread by medieval Arabs who in turn borrowed it from Armenians.[60]
Walsingham Way: In England the Milky Way was called the Walsingham Way in reference to the shrine of Our Lady of Walsingham which is in Norfolk, England. It was understood to be either a guide to the pilgrims who flocked there, or a representation of the pilgrims themselves.[61]
Winter Street: Scandinavian peoples, such as Swedes, have called the galaxy Winter Street (Vintergatan) as the galaxy is most clearly visible during the winter at the northern hemisphere, especially at high latitudes where the glow of the Sun late at night can obscure it during the summer.
Appearance
The Milky Way as seen from a dark site with little light pollution
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The Milky Way is visible as a hazy band of white light, some 30° wide, arching the night sky.[62] Although all the individual naked-eye stars in the entire sky are part of the Milky Way Galaxy, the term "Milky Way" is limited to this band of light.[63][64] The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Brighter regions around the band appear as soft visual patches known as star clouds. The most conspicuous of these is the Large Sagittarius Star Cloud, a portion of the central bulge of the galaxy.[65] Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. Peoples of the southern hemisphere, including the Inca and Australian aborigines, identified these regions as dark cloud constellations.[66] The area of sky that the Milky Way obscures is called the Zone of Avoidance.[67]
The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be visible.[68] It should be visible if the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1.[69] This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from rural areas when the Moon is below the horizon.[d] Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.[70]
The Milky Way as seen from Sajama National Park in Bolivia, an area with little light pollution.
As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations.[e] The Galactic Center lies in the direction of Sagittarius, where the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal hemispheres.[71]
The galactic plane is inclined by about 60° to the ecliptic (the plane of Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic, relative to the galactic plane. The north galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near β Comae Berenices, and the south galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the Milky Way arch may appear relatively low or relatively high in the sky. For observers from latitudes approximately 65° north to 65° south, the Milky Way passes directly overhead twice a day.[citation needed]
Astronomical history
See also: Galaxy § Observation history
Ancient, naked eye observations
In Meteorologica, Aristotle (384–322 BC) states that the Greek philosophers Anaxagoras (c. 500–428 BC) and Democritus (460–370 BC) proposed that the Milky Way is the glow of stars not directly visible due to Earth's shadow, while other stars receive their light from the Sun, but have their glow obscured by solar rays.[72] Aristotle himself believed that the Milky Way was part of the Earth's upper atmosphere, along with the stars, and that it was a byproduct of stars burning that did not dissipate because of its outermost location in the atmosphere, composing its great circle. He said that the milky appearance of the Milky Way Galaxy is due to the refraction of the Earth's atmosphere.[73][74][75] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 AD) criticized this view, arguing that if the Milky Way were sublunary, it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the Muslim world.[76]
The Persian astronomer Al-Biruni (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars".[77] The Andalusian astronomer Avempace (d 1138) proposed that the Milky Way was made up of many stars but appeared to be a continuous image in the Earth's atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence.[74] The Persian astronomer Nasir al-Din al-Tusi (1201–1274) in his Tadhkira wrote: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."[78] Ibn Qayyim al-Jawziyya (1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars".[79]
Telescopic observations
The shape of the Milky Way as deduced from star counts by William Herschel in 1785. The Solar System was assumed to be near the center
Proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. Galileo also concluded that the appearance of the Milky Way was due to refraction of the Earth's atmosphere.[80][81][73] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,[82] speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales.[83] The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Wright and Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.[84][85][86]
The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center.[87]
In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.[88][89]
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy
In 1904, studying the proper motions of stars, Jacobus Kapteyn reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions.[90] It was later realized that Kapteyn's data had been the first evidence of the rotation of our galaxy,[91] which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.[citation needed]
In 1917, Heber Doust Curtis had observed the nova S Andromedae within the Great Andromeda Nebula (Messier object 31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were independent galaxies.[92][93] In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[94]
The controversy was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 2.5 m (100 in) Hooker telescope. With the light-gathering power of this new telescope, he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way.[95][96]
Satellite observations
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Map of stars cataloged by the Gaia release in 2021, displayed as density mesh in the diagram
The ESA spacecraft Gaia provides distance estimates by determining the parallax of a billion stars and is mapping the Milky Way with four planned releases of maps in 2016, 2018, 2021 and 2024.[97][98]
Data from Gaia has been described as "transformational". It has been estimated that Gaia has expanded the number of observations of stars from about 2 million stars as of the 1990s to 2 billion. It has expanded the measurable volume of space by a factor of 100 in radius and a factor of 1,000 in precision.[99]
A study in 2020 concluded that Gaia detected a wobbling motion of the galaxy, which might be caused by "torques from a misalignment of the disc's rotation axis with respect to the principal axis of a non-spherical halo, or from accreted matter in the halo acquired during late infall, or from nearby, interacting satellite galaxies and their consequent tides".[100] In April 2024, initial studies (and related maps) involving the magnetic fields of the Milky Way were reported.[101]
Astrography
Sun's location and neighborhood
See also: Location of Earth
Map of stars cataloged by the Gaia release in 2021, overlay on top of artist's conception of the Milky Way overall shape
The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, between the Radcliffe wave and Split linear structures (formerly Gould Belt).[102] Based upon studies of stellar orbits around Sgr A* by Gillessen et al. (2016), the Sun lies at an estimated distance of 27.14 ± 0.46 kly (8.32 ± 0.14 kpc)[34] from the Galactic Center. Boehle et al. (2016) found a smaller value of 25.64 ± 0.46 kly (7.86 ± 0.14 kpc), also using a star orbit analysis.[103] The Sun is currently 5–30 parsecs (16–98 ly) above, or north of, the central plane of the Galactic disk.[104] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500 ly).[105] The Sun, and thus the Solar System, is located in the Milky Way's galactic habitable zone.[106][107]
There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsecs, or one star per 2,360 cubic light-years (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsecs, or one per 284 cubic light-years (from List of nearest stars). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than apparent magnitude 4 but 15.5 million stars brighter than apparent magnitude 14.[108]
The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun passes through the Galactic plane approximately 2.7 times per orbit.[109] [unreliable source?] This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth.[110] A reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation.[111]
It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a galactic year),[112] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Milky Way is approximately 220 km/s (490,000 mph) or 0.073% of the speed of light. The Sun moves through the heliosphere at 84,000 km/h (52,000 mph). At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).[113] The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic.[114]
Galactic quadrants
Main article: Galactic quadrant
A diagram of the Sun's location in the Milky Way, the angles represent longitudes in the galactic coordinate system
A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the origin of the mapping system.[115]
Quadrants are described using ordinals – for example, "1st galactic quadrant",[116] "second galactic quadrant",[117] or "third quadrant of the Milky Way".[118] Viewing from the north galactic pole with 0° (zero degrees) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are:
Galactic
quadrant
Galactic
longitude
(ℓ)
Reference
1st 0° ≤ ℓ ≤ 90° [119]
2nd 90° ≤ ℓ ≤ 180° [117]
3rd 180° ≤ ℓ ≤ 270° [118]
4th
270° ≤ ℓ ≤ 360°
(360° ≅ 0°) [116]
with the galactic longitude (ℓ) increasing in the counter-clockwise direction (positive rotation) as viewed from north of the Galactic Center (a view-point several hundred thousand light-years distant from Earth in the direction of the constellation Coma Berenices); if viewed from south of the Galactic Center (a view-point similarly distant in the constellation Sculptor), ℓ would increase in the clockwise direction (negative rotation).
Size and mass
Size
A size comparison of the six largest galaxies of the Local Group, including the Milky Way
The Milky Way is one of the two largest galaxies in the Local Group (the other being the Andromeda Galaxy), although the size for its galactic disc and how much it defines the isophotal diameter is not well understood.[11] It is estimated that the significant bulk of stars in the galaxy lies within the 26 kiloparsecs (80,000 light-years) diameter, and that the number of stars beyond the outermost disc dramatically reduces to a very low number, with respect to an extrapolation of the exponential disk with the scale length of the inner disc.[120][11]
There are several methods being used in astronomy in defining the size of a galaxy, and each of them can yield different results with respect to one another. The most commonly employed method is the D25 standard – the isophote where the photometric brightness of a galaxy in the B-band (445 nm wavelength of light, in the blue part of the visible spectrum) reaches 25 mag/arcsec2.[121] An estimate from 1997 by Goodwin and others compared the distribution of Cepheid variable stars in 17 other spiral galaxies to the ones in the Milky Way, and modelling the relationship to their surface brightnesses. This gave an isophotal diameter for the Milky Way at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years), by assuming that the galactic disc is well represented by an exponential disc and adopting a central surface brightness of the galaxy (μ0) of 22.1±0.3 B-mag/arcsec−2 and a disk scale length (h) of 5.0 ± 0.5 kpc (16,300 ± 1,600 ly).[122][10][123]
This is significantly smaller than the Andromeda Galaxy's isophotal diameter, and slightly below the mean isophotal sizes of the galaxies being at 28.3 kpc (92,000 ly).[10] The paper concludes that the Milky Way and Andromeda Galaxy were not overly large spiral galaxies, nor were among the largest known (if the former not being the largest) as previously widely believed, but rather average ordinary spiral galaxies.[124] To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter (24.3 mm (0.955 in)), the Milky Way would be approximately at least the greatest north–south line of the contiguous United States.[125] An even older study from 1978 gave a lower diameter for Milky Way about 23 kpc (75,000 ly).[10]
A 2015 paper discovered that there is a ring-like filament of stars called Triangulum–Andromeda Ring (TriAnd Ring) rippling above and below the relatively flat galactic plane, which alongside Monoceros Ring were both suggested to be primarily the result of disk oscillations and wrapping around the Milky Way, at a diameter of at least 50 kpc (160,000 ly),[126] which may be part of the Milky Way's outer disk itself, hence making the stellar disk larger by increasing to this size.[127] A more recent 2018 paper later somewhat ruled out this hypothesis, and supported a conclusion that the Monoceros Ring, A13 and TriAnd Ring were stellar overdensities rather kicked out from the main stellar disk, with the velocity dispersion of the RR Lyrae stars found to be higher and consistent with halo membership.[128]
Another 2018 study revealed the very probable presence of disk stars at 26–31.5 kpc (84,800–103,000 ly) from the Galactic Center or perhaps even farther, significantly beyond approximately 13–20 kpc (40,000–70,000 ly), in which it was once believed to be the abrupt drop-off of the stellar density of the disk, meaning that few or no stars were expected to be above this limit, save for stars that belong to the old population of the galactic halo.[11][129][130]
A 2020 study predicted the edge of the Milky Way's dark matter halo being around 292 ± 61 kpc (952,000 ± 199,000 ly), which translates to a diameter of 584 ± 122 kpc (1.905 ± 0.3979 Mly).[26][27] The Milky Way's stellar disk is also estimated to be approximately up to 1.35 kpc (4,000 ly) thick.[131][132]
Mass
A schematic profile of the Milky Way.
Abbreviations: GNP/GSP: Galactic North and South Poles
The Milky Way is approximately 890 billion to 1.54 trillion times the mass of the Sun in total (8.9×1011 to 1.54×1012 solar masses),[7][8][9] although stars and planets make up only a small part of this. Estimates of the mass of the Milky Way vary, depending upon the method and data used. The low end of the estimate range is 5.8×1011 solar masses (M☉), somewhat less than that of the Andromeda Galaxy.[133][134][135] Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km/s (570,000 mph) for stars at the outer edge of the Milky Way.[136]
Because the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M☉ within 160,000 ly (49 kpc) of its center.[137] In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kiloparsecs is 7×1011 M☉.[138] In a 2014 study, the mass of the entire Milky Way is estimated to be 8.5×1011 M☉,[139] but this is only half the mass of the Andromeda Galaxy.[139] A recent 2019 mass estimate for the Milky Way is 1.29×1012 M☉.[140]
Much of the mass of the Milky Way seems to be dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A dark matter halo is conjectured to spread out relatively uniformly to a distance beyond one hundred kiloparsecs (kpc) from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5×1012 M☉.[141][142][143] 2013 and 2014 studies indicate a range in mass, as large as 4.5×1012 M☉[144] and as small as 8×1011 M☉.[145] By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6×1010 M☉[146] and 6.43×1010 M☉.[141]
In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass,[147] [unreliable source?] with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen.[148] The mass of the Milky Way's interstellar gas is equal to between 10%[148] and 15%[147] of the total mass of its stars. Interstellar dust accounts for an additional 1% of the total mass of the gas.[147]
In March 2019, astronomers reported that the virial mass of the Milky Way Galaxy is 1.54 trillion solar masses within a radius of about 39.5 kpc (130,000 ly), over twice as much as was determined in earlier studies, suggesting that about 90% of the mass of the galaxy is dark matter.[7][8]
In September 2023, astronomers reported that the virial mass of the Milky Way Galaxy is only 2.06 1011 solar masses, only a 10th of the mass of previous studies. The mass was determined from data of the Gaia spacecraft.[149]
Contents
The Milky Way contains between 100 and 400 billion stars[12][13] and at least that many planets.[150] An exact figure would depend on counting the number of very-low-mass stars, which are difficult to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[151] The Milky Way may contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes.[f][154][155] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[156] whereas the thickness of the gas layer ranges from hundreds of light-years for the colder gas to thousands of light-years for warmer gas.[157][158]
The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way. Beyond a radius of roughly 40,000 light years (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius.[120] Surrounding the galactic disk is a spherical galactic halo of stars and globular clusters that extends farther outward, but is limited in size by the orbits of two Milky Way satellites, the Large and Small Magellanic Clouds, whose closest approach to the Galactic Center is about 180,000 ly (55 kpc).[159] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be around −20.9.[160][161][g]
Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way,[32][162] and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[163][164] The Milky Way contains an average of at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 by the Kepler space observatory.[33] A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way.[165]
In November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.[166][167][168] 11 billion of these estimated planets may be orbiting Sun-like stars.[169] The nearest exoplanet may be 4.2 light-years away, orbiting the red dwarf Proxima Centauri, according to a 2016 study.[170] Such Earth-sized planets may be more numerous than gas giants,[32] though harder to detect at great distances given their small size. Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way.[171] More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy.[172]
When compared to other more distant galaxies in the universe, the Milky Way galaxy has a below average amount of neutrino luminosity making our galaxy a "neutrino desert".[173]
Structure
Overview of different elements of the overall structure of the Milky Way
The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust and stars.[174][175] The mass distribution within the Milky Way closely resembles the type Sbc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms.[5] Astronomers first began to conjecture that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1960s.[176][177][178] These conjectures were confirmed by the Spitzer Space Telescope observations in 2005 that showed the Milky Way's central bar to be larger than previously thought.[179]
Galactic Center
Main articles: Galactic Center and Sagittarius A*
A dark spot surrounded by doughnut shaped orange-yellow ring
Supermassive black hole Sagittarius A* imaged by the Event Horizon Telescope in radio waves. The central dark spot is the black hole's shadow, which is larger than the event horizon.
Bright X-ray flares from Sagittarius A* (inset) in the center of the Milky Way, as detected by the Chandra X-ray Observatory.[180]
The Sun is 25,000–28,000 ly (7.7–8.6 kpc) from the Galactic Center. This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range.[181][103][34][182][183][184] In the inner few kiloparsecs (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[185] It has been proposed that the Milky Way lacks a bulge due to a collision and merger between previous galaxies, and that instead it only has a pseudobulge formed by its central bar.[186] However, confusion in the literature between the (peanut shell)-shaped structure created by instabilities in the bar, versus a possible bulge with an expected half-light radius of 0.5 kpc, abounds.[187]
The Galactic Center is marked by an intense radio source named Sagittarius A* (pronounced Sagittarius A-star). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.[188] This concentration of mass is best explained as a supermassive black hole[h][181][189] (SMBH) with an estimated mass of 4.1–4.5 million times the mass of the Sun.[189] The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at 1×10−5 M☉ per year.[190] Observations indicate that there are SMBHs located near the center of most normal galaxies.[191][192]
The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1 to 5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[183][184][193] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[194] However, RR Lyrae-type stars do not trace a prominent Galactic bar.[184][195][196] The bar may be surrounded by a ring called the "5 kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[197] X-ray emission from the core is aligned with the massive stars surrounding the central bar[190] and the Galactic ridge.[198]
In June 2023, astronomers led by Naoko Kurahashi Neilson reported using a new cascade neutrino technique[199] to detect, for the first time, the release of neutrinos from the galactic plane of the Milky Way galaxy, creating the first neutrino view of the Milky Way.[200][201]
Gamma rays and x-rays
All-sky x-ray image
Since 1970, various gamma-ray detection missions have discovered 511-keV gamma rays coming from the general direction of the Galactic Center. These gamma rays are produced by positrons (antielectrons) annihilating with electrons. In 2008 it was found that the distribution of the sources of the gamma rays resembles the distribution of low-mass X-ray binaries, seeming to indicate that these X-ray binaries are sending positrons (and electrons) into interstellar space where they slow down and annihilate.[202][203][204] The observations were both made by NASA and ESA's satellites. In 1970 gamma ray detectors found that the emitting region was about 10,000 light-years across with a luminosity of about 10,000 suns.[203]
Illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet) of the Milky Way (center)
In 2010, two gigantic spherical bubbles of high energy gamma-emission were detected to the north and the south of the Milky Way core, using data from the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc) (or about 1/4 of the galaxy's estimated diameter); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere.[205][206] Subsequently, observations with the Parkes Telescope at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central 640 ly (200 pc) of the Milky Way.[207]
Later, on January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*.[180]
Spiral arms
Further information: Spiral galaxy
Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms of the Milky Way, viewed from north of the galaxy – the galaxy rotates clockwise in this view. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations
Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms.[208] Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by H II regions[209][210] and molecular clouds.[211]
The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's arms.[212] Perfect logarithmic spiral patterns only crudely describe features near the Sun,[210][213] because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[184][213][214] The possible scenario of the Sun within a spur / Local arm[210] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way.[213] Estimates of the pitch angle of the arms range from about 7° to 25°.[156][215] There are thought to be four spiral arms that all start near the Milky Way Galaxy's center.[216] These are named as follows, with the positions of the arms shown in the image:
Color Arm(s)
turquoise Near 3 kpc and Perseus Arm
blue Norma and Outer arm (Along with extension discovered in 2004[217])
green Far 3 kpc and Scutum–Centaurus Arm
red Carina–Sagittarius Arm
There are at least two smaller arms or spurs, including:
orange Orion–Cygnus Arm (which contains the Sun and Solar System)
Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum–Centaurus Arm contains approximately 30% more red giants than would be expected in the absence of a spiral arm.[215][218]
This observation suggests that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars.[212] In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.[219][220][221] Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.[221]
The Near 3 kpc Arm (also called the Expanding 3 kpc Arm or simply the 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21 centimeter radio measurements of HI (atomic hydrogen).[222][223] It was found to be expanding away from the central bulge at more than 50 km/s. It is located in the fourth galactic quadrant at a distance of about 5.2 kpc from the Sun and 3.3 kpc from the Galactic Center. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Center for Astrophysics | Harvard & Smithsonian). It is located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center.[223][224]
A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.[225]
It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring,[226] and the two patterns would be connected by the Cygnus arm.[227]
Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way.[228] The structure of the Milky Way's disk is warped along an "S" curve.[229]
Halo
Main article: Galactic halo
The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center.[230] However, a few globular clusters have been found farther, such as PAL 4 and AM 1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[231] The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star.[232]
Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little cool gas to collapse into stars.[112] Open clusters are also located primarily in the disk.[233]
Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much farther than previously thought,[234] the possibility of the disk of the Milky Way extending farther is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm[217][235] and of a similar extension of the Scutum–Centaurus Arm.[236] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.[citation needed]
The Sloan Digital Sky Survey of the northern sky shows a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.[237]
Gaseous halo
In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is also a gaseous halo containing a large amount of hot gas. This halo extends for hundreds of thousands of light-years, much farther than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself.[238][239][240] The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million °F).[241]
Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.[242] If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way.[242]
Galactic rotation
Galaxy rotation curve for the Milky Way – vertical axis is speed of rotation about the galactic center; horizontal axis is distance from the galactic center in kpcs; the sun is marked with a yellow ball; the observed curve of speed of rotation is blue; the predicted curve based upon stellar mass and gas in the Milky Way is red; scatter in observations roughly indicated by gray bars, the difference is due to dark matter[243][244][245]
The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 ± 10 km/s (470,000 ± 22,000 mph).[246] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate, and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.[citation needed]
If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotational speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[243] The rotation curve of the Milky Way agrees with the universal rotation curve of spiral galaxies, the best evidence for the existence of dark matter in galaxies. Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.[247]
Formation
Main article: Galaxy formation and evolution
History
A galaxy color–magnitude diagram showing the red sequence (old galaxies, typically elliptical galaxies), the green valley (where the Milky Way is believed to be in), and the blue cloud (young galaxies, typically spiral galaxies).
The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang 13.61 billion years ago.[248][249][250] Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. Nearly half the matter in the Milky Way may have come from other distant galaxies.[248] These stars and clusters now comprise the stellar halo of the Milky Way. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.[251][252]
Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo.[252] The Milky Way is currently accreting material from several small galaxies, including two of its largest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high-velocity clouds like the Smith Cloud.[253][254]
Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the Kraken.[255][256] Properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies. Its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.[257][258]
According to recent studies, the Milky Way as well as the Andromeda Galaxy lie in what in the galaxy color–magnitude diagram is known as the "green valley", a region populated by galaxies in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy.[259] Measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.[260]
Age and cosmological history
Comparison of the night sky with the night sky of a hypothetical planet within the Milky Way 10 billion years ago, at an age of about 3.6 billion years and 5 billion years before the Sun formed.[261]
Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way. The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology. These yield values of about 12.5 ± 3 billion years for CS 31082-001[262] and 13.8 ± 4 billion years for BD +17° 3248.[263]
Once a white dwarf is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[264]
In November 2018, astronomers reported the discovery of one of the oldest stars in the universe. About 13.5 billion-years-old, 2MASS J18082002-5104378 B is a tiny ultra metal-poor (UMP) star made almost entirely of materials released from the Big Bang, and is possibly one of the first stars. The discovery of the star in the Milky Way Galaxy suggests that the galaxy may be at least 3 billion years older than previously thought.[265][266][267]
Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year age of the Universe. In 2007, a star in the galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.[268] This estimate was made using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.[268] Another star, HD 140283, has been estimated at 14.5 ± 0.7 billion years old.[37][269][contradictory]
According to observations utilizing adaptive optics to correct for Earth's atmospheric distortion, stars in the galaxy's bulge date to about 12.8 billion years old.[270]
The age of stars in the galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the galactic halo and the thin disk.[271] Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before.[272]
The satellite galaxies surrounding the Milky Way are not randomly distributed but seem to be the result of a breakup of some larger system producing a ring structure 500,000 light-years in diameter and 50,000 light-years wide.[273] Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy, can rip off huge tails of gas, which, over time can coalesce to form dwarf galaxies in a ring at an arbitrary angle to the main disc.[274]
Intergalactic neighbourhood
A diagram of the galaxies in the Local Group relative to the Milky Way
The position of the Local Group within the Laniakea Supercluster
Main article: Local Group
The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, surrounded by a Local Void, itself being part of the Local Sheet[275] and in turn the Virgo Supercluster. Surrounding the Virgo Supercluster are a number of voids, devoid of many galaxies, the Microscopium Void to the "north", the Sculptor Void to the "left", the Boötes Void to the "right" and the Canes-Major Void to the "south". These voids change shape over time, creating filamentous structures of galaxies. The Virgo Supercluster, for instance, is being drawn towards the Great Attractor,[276] which in turn forms part of a greater structure, called Laniakea.[277]
Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 32,200 light-years.[278] It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a stream of neutral hydrogen gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way.[279] Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf.[280]
The smallest dwarf galaxies of the Milky Way are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies that are dynamically bound to the Milky Way, which is supported by the detection of nine new satellites of the Milky Way in a relatively small patch of the night sky in 2015.[280] There are some dwarf galaxies that have already been absorbed by the Milky Way, such as the progenitor of Omega Centauri.[281]
In 2005[282] with further confirmation in 2012[283] researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction. This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not explained.[284]
In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way.[285]
Current measurements suggest the Andromeda Galaxy is approaching the Milky Way at 100 to 140 km/s (220,000 to 310,000 mph). In 4.3 billion years, there may be an Andromeda–Milky Way collision, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, the chance of individual stars colliding with each other is extremely low,[286] but instead the two galaxies will merge to form a single elliptical galaxy or perhaps a large disk galaxy[287] over the course of about six billion years.[288]
Velocity
Although special relativity states that there is no "preferred" inertial frame of reference in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological frames of reference.[citation needed]
One such frame of reference is the Hubble flow, the apparent motions of galaxy clusters due to the expansion of space. Individual galaxies, including the Milky Way, have peculiar velocities relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately 630 km/s (1,400,000 mph) with respect to this local co-moving frame of reference.[289][290]
The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley Supercluster, behind it.[291] The Local Group, a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy, is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s (2,160,000 mph) as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.[292]
Another reference frame is provided by the cosmic microwave background (CMB), in which the CMB temperature is least distorted by Doppler shift (zero dipole moment). The Milky Way is moving at 552 ± 6 km/s (1,235,000 ± 13,000 mph)[19] with respect to this frame, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[19]
See also
solar system portal
outer space portal
astronomy portal
Baade's Window
Galactic astronomy
Galactic Center GeV excess
Oort constants
Notes
^ The distance towards its center (Sagittarius A*).
^ This is the diameter measured using the D25 standard. It has been recently suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.[11]
^ Some authors use the term Milky Way to refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy receives the full name Milky Way Galaxy. See for example Laustsen et al.,[21] Pasachoff,[22] Jones,[23] van der Kruit,[24] and Hodge et al.[25]
^ See also Bortle Dark-Sky Scale.
^ The bright center of the galaxy is located in the constellation Sagittarius. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, to the galactic anticenter in Auriga. From there, it passes through Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius.
^ These estimates are very uncertain, as most non-star objects are difficult to detect; for example, black hole estimates range from ten million to one billion.[152][153]
^ Karachentsev et al. give a blue absolute magnitude of −20.8. Combined with a color index of 0.55 estimated here, an absolute visual magnitude of −21.35 (−20.8 − 0.55 = −21.35) is obtained. Determining the absolute magnitude of the Milky Way is very difficult, because Earth is inside it.
^ For a photo see: "Sagittarius A*: Milky Way monster stars in cosmic reality show". Chandra X-ray Observatory. Center for Astrophysics | Harvard & Smithsonian. January 6, 2003. Archived from the original on March 17, 2008. Retrieved May 20, 2012.
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On page 57 Archived November 20, 2016, at the Wayback Machine, Wright stated that despite their mutual gravitational attraction, the stars in the constellations do not collide because they are in orbit, so centrifugal force keeps them separated: "centrifugal force, which not only preserves them in their orbits, but prevents them from rushing all together, by the common universal law of gravity, ..."
On page 48 Archived November 20, 2016, at the Wayback Machine, Wright stated that the form of the Milky Way is a ring: "the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space, without order or design, ... this phænomenon [is] no other than a certain effect arising from the observer's situation, ... To a spectator placed in an indefinite space, ... it [i.e. the Milky Way (Via Lactea)] [is] a vast ring of stars ..."
On page 65 Archived November 20, 2016, at the Wayback Machine, Wright speculated that the central body of the Milky Way, around which the rest of the galaxy revolves, might not be visible to us: "the central body A, being supposed as incognitum [i.e. an unknown], without [i.e. outside of] the finite view; ..."
On page 73 Archived November 20, 2016, at the Wayback Machine, Wright called the Milky Way the Vortex Magnus (the great whirlpool) and estimated its diameter to be 8.64×1012 miles (13.9×1012 km).
On page 33 Archived November 20, 2016, at the Wayback Machine, Wright speculated that there are a vast number of inhabited planets in the galaxy: "therefore we may justly suppose, that so many radiant bodies [i.e. stars] were not created barely to enlighten an infinite void, but to ... display an infinite shapeless universe, crowded with myriads of glorious worlds, all variously revolving round them; and ... with an inconceivable variety of beings and states, animate ..."
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^ Evans, J. C. (November 24, 1998). "Our Galaxy". George Mason University. Archived from the original on June 30, 2012. Retrieved January 4, 2007.
^ The term Weltinsel (world island) appears nowhere in Kant's book of 1755. The term first appeared in 1850, in the third volume of von Humboldt's Kosmos: Alexander von Humboldt, Kosmos, vol. 3 (Stuttgart & Tübingen, (Germany): J. G. Cotta, 1850), pp. 187, 189. From p. 187: Archived November 20, 2016, at the Wayback Machine "Thomas Wright von Durham, Kant, Lambert und zuerst auch William Herschel waren geneigt die Gestalt der Milchstraße und die scheinbare Anhäufung der Sterne in derselben als eine Folge der abgeplatteten Gestalt und ungleichen Dimensionen der Weltinsel (Sternschict) zu betrachten, in welche unser Sonnensystem eingeschlossen ist." ("Thomas Wright of Durham, Kant, Lambert and at first also William Herschel were inclined to regard the shape of the Milky Way and the apparent clustering of stars in it as a consequence of the oblate shape and unequal dimensions of the world island (star stratum), in which our solar system is included.)
In the English translation – Alexander von Humboldt with E. C. Otté, trans., Cosmos ... (New York City: Harper & Brothers, 1897), vols. 3–5. see p. 147 Archived November 6, 2018, at the Wayback Machine.
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^ See:
Rosse revealed the spiral structure of Whirlpool Galaxy (M51) at the 1845 meeting of the British Association for the Advancement of Science. Rosse's illustration of M51 was reproduced in J. P. Nichol's book of 1846.
Rosse, Earl of (1846). "On the nebula 25 Herschel, or 61 [should read: 51] of Messier's catalogue". Report of the Fifteenth Meeting of the British Association for the Advancement of Science; Held at Cambridge in June 1845 § Notices and Abstracts of Miscellaneous Communications to the Sections. Report of the ... Meeting of the British Association for the Advancement of Science (1833): 4. Archived from the original on March 10, 2021. Retrieved February 17, 2020.
Nichol, John Pringle (1846). Thoughts on Some Important Points Relating to the System of the World. Edinburgh, Scotland: William Tait. p. 23. Archived from the original on March 8, 2021. Retrieved February 17, 2020. Rosse's illustration of the Whirlpool Galaxy appears on the plate that immediately precedes p. 23.
South, James (1846). "Auszug aus einem Berichte über Lord Rosse's grosses Telescop, den Sir James South in The Times, Nr. 18899, 1845 April 16 bekannt gemacht hat" [Excerpt from a report about Lord Rosse's great telescope, which Sir James South made known in The Times [of London], no. 18,899, 1845 April 16]. Astronomische Nachrichten (in German). 23 (536): 113–118. doi:10.1002/asna.18460230802. Archived from the original on March 8, 2021. Retrieved February 17, 2020. On March 5, 1845, Rosse observed M51, the Whirlpool Galaxy. From column 115: "The most popularly known nebulæ observed this night were the ring nebulæ in the Canes Venatici, or the 51st of Messier's catalogue, which was resolved into stars with a magnifying power of 548".
Robinson, T. R. (1845). "On Lord Rosse's telescope". Proceedings of the Royal Irish Academy. 3 (50): 114–133. Archived from the original on June 10, 2020. Retrieved February 17, 2020. Rosse's early observations of nebulae and galaxies are discussed on pp. 127–130.
Rosse, The Earl of (1850). "Observations on the nebulae". Philosophical Transactions of the Royal Society of London. 140: 499–514. doi:10.1098/rstl.1850.0026. Archived from the original on March 26, 2023. Retrieved February 17, 2020. Rosse's illustrations of nebulae and galaxies appear on the plates that immediately precede the article.
Bailey, M. E.; Butler, C. J.; McFarland, J. (April 2005). "Unwinding the discovery of spiral nebulae". Astronomy & Geophysics. 46 (2): 2.26 – 2.28. doi:10.1111/j.1468-4004.2005.46226.x.
^ See:
Kapteyn, Jacobus Cornelius (1906). "Statistical methods in stellar astronomy". In Rogers, Howard J. (ed.). Congress of Arts and Science, Universal Exposition, St. Louis, 1904. Vol. 4. Boston and New York: Houghton, Mifflin and Co. pp. 396–425. Archived from the original on March 8, 2021. Retrieved February 6, 2020. From pp. 419–420: "It follows that the one set of the stars must have a systematic motion relative to the other. ... these two main directions of motion must be in reality diametrically opposite."
Kapteyn, J. C. (1905). "Star streaming". Report of the Seventy-fifth Meeting of the British Association for the Advancement of Science, South Africa. Report of the ... Meeting of the British Association for the Advancement of Science (1833): 257–265. Archived from the original on March 8, 2021. Retrieved February 6, 2020.
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Further reading
Dambeck, Thorsten (March 2008). "Gaia's Mission to the Milky Way". Sky & Telescope: 36–39.
Chiappini, Cristina (November–December 2001). "The Formation and Evolution of the Milky Way" (PDF). American Scientist. 89 (6): 506–515. doi:10.1511/2001.40.745.
McTier, Moiya (August 16, 2022). The Milky Way. Grand Central Publishing. ISBN 978-1-5387-5415-3.
Plait, Phil, "The Milky Way's Secrets: Our galaxy's night-sky spectacle sparked scientific revolutions", Scientific American, vol. 329, no. 4 (November 2023), pp. 86–87.
External links
প্রবেশ কর প্রবেশ কর অনুসন্ধান করুন প্রবেশ কর ভাষা নির্বাচন কর English العربية বাংলা فارسی Français Indonesia Italiano Dutch Português русский Shqip ภาษาไทย Türkçe اردو 简体中文 Melayu Español Kiswahili Tiếng Việt কুরআন অনুসন্ধান করুন... কুরআন নেভিগেট করুন জনপ্রিয় পড়া চালিয়ে যান আমার কুরআন 001 ১. Al-Fatihah সূচনা আয়াত ১ 🕋 সেরা দিনগুলোর পুরস্কারগুলো হাতছাড়া করবেন না! মাসিক দাতা হন মাসিক অনুদান আমাদের Quran.com-এর উন্নতি এবং কার্যক্রম টিকিয়ে রাখতে সাহায্য করে, তাই আমরা তহবিল সংগ্রহের উপর কম এবং প্রভাব তৈরির উপর বেশি মনোযোগ দিই। আরও জানুন এখনই দান করুন কুরআন বৃদ্ধির যাত্রা তোমার কুরআনের লক্ষ্য অর্জন করো স্ট্রিক ট্র্যাক করুন, কাস্টম লক্ষ্য তৈরি করুন, ধারাবাহিক থাকুন বিষয়গুলি অন্বেষণ করুন রমজান কি? কুরআন সম্পর্কে সুন্নাহ সম্পর্কে আয়াত এক বছরে কুরআন ক্যালেন্ডার واذكروا نعمة الله عليكم وميثاقه الذي واثقكم به اذ قلتم سمعنا واطعنا واتقوا الله ان الله عليم بذات الصدور ٧ وَٱذْكُرُوا۟ نِعْمَةَ ٱللَّهِ عَلَيْكُمْ وَمِيثَـٰقَهُ ٱلَّذِى وَاثَقَكُم بِهِۦٓ إِذْ قُلْتُمْ سَمِعْنَا وَأَطَعْنَا ۖ وَٱتَّقُوا۟ ٱللَّهَ ۚ إِنَّ ٱللَّهَ عَلِيمٌۢ بِذَاتِ ٱلصُّدُورِ ٧ এই সপ্তাহের পাঠের একটি পদ وَٱذۡكُرُواْ نِعۡمَةَ ٱللَّهِ عَلَيۡكُمۡ وَمِيثَٰقَهُ ٱلَّذِي وَاثَقَكُم بِهِۦٓ إِذۡ قُلۡتُمۡ سَمِعۡنَا وَأَطَعۡنَاۖ وَٱتَّقُواْ ٱللَّهَۚ إِنَّ ٱللَّهَ عَلِيمُۢ بِذَاتِ ٱلصُّدُورِ ٧ "তোমাদের প্রতি আল্লাহর নিআমতের কথা স্মরণ কর আর তাঁর অঙ্গীকারের কথা যা তিনি তোমাদের নিকট থেকে গ্রহণ করেছিলেন যখন তোমরা বলেছিলে- আমরা শুনলাম ও মেনে নিলাম। আল্লাহকে ভয় কর, অন্তরে যা আছে সে সম্পর্কে আল্লাহ খুব ভালভাবেই অবগত আছেন।" Al-Ma'idah ৫:৭ এই সপ্তাহের পঠন শেখা শুরু করুন আরও দেখুন Answering Allah’s Call to Hajjনতুন! Where Allah’s Names Interlock: A Deep Quranic Journey Through Divine Pairings The Path to Paradise: Small Consistent Actions That Lead to Jannah The Nearest Ties: Quranic Rights of Family Why Allah Tests Us: Finding Strength and Wisdom in Trials Little Hearts, Big Prayers: Raising Children Who Love Salah From Wandering Eyes to a Watchful Heart: The Road Back to Modesty The Heart of the Quran: Lessons from Surah Yā Sīn on Faith, Mercy, and Accountability The Opening of the Heart: Lessons From Surah Al-Fatihah 30 Transformative Days with Surah Al-Mulk: Learn, Reflect, Memorize Preparing our Hearts for Ramadan When the Stars Prostrated: Reflections on Surah Yusuf 4 Life-Changing Stories: Lessons From Surah Al-Kahf Created in Pairs: Mercy and Forgiveness in Marriage The Islam of Ibrahim: Walking in the Footsteps of the Friend of Allah Tawakkul: Trusting Allah in an Age of Anxiety Standing Tall: A Faith‑Based Response to Hate SWT: Allah’s Absolute Glory and Perfection What is Ihsan? Discover the quality that elevates how you live, work, and worship "Am I Sincere?": The Importance of Doing Everything for the Sake of Allah How To Cultivate Barakah in a Busy Routine From Recitation to Transformation: A 7‑Day Path to Living the Quran Guarding the Tongue: Protecting the Heart The Rescuer: Powerful Lessons in Surah Al-Mulk Quiet the Storm: A Quranic Guide for Anxious Hearts Among Those Allah Loves: Qualities Loved By The Most Merciful Do Not Despair: Journeying from the Weight of Guilt to the Doors of Mercy Screen Addiction: When Screens Steal the Heart Transforming How You Approach the Salah: Praying on time, with Focus Mindful Fasting Five Lenses to Reflect on the Quran The Iron Healing: Lessons & Reflections from Surah al-Hadid Maintaining Your Momentum: Avoiding the Post-Ramadan Slump How to Explore the Quran اللہ ہمیں کیوں آزماتا ہے: آزمائشوں میں قوت اور حکمت تلاش کرنا مستقل مزاجی سے چھوٹے اعمال: جنت کا راستہ دلوں کی کشادگی: سورۃ الفاتحہ سے اسباق قریبی رشتے: خاندان کے قرآنی حقوق ننھے دل، بڑی دعائیں: ایسے بچوں کی تربیت جو نماز سے محبت کریں سورہ یٰسین: ایمان، رحمت اور احتساب کے ابدی اسباق সম্প্রদায় যুল-হিজ্জাহ মাসের ফজিলত সম্পর্কে জানুন কুরআনের আলোচনা, স্মরণীয় বিষয় এবং শেখার উপকরণের মাধ্যমে যুল-হিজ্জাহ মাস সম্পর্কে আপনার জ্ঞানকে আরও গভীর করুন। আজকের আলোচিত পদ এবং প্রতিফলন পড়ুন কুরআন অ্যাপস কুরআনের সাথে আপনার বিকাশকে আরও এগিয়ে নিতে সংযুক্ত অ্যাপস। বৈশিষ্ট্যযুক্ত অ্যাপস আরও দেখুন কুরআন প্রতিফলন কুরআন প্রতিফলন প্রতিফলন পড়ুন এবং শেয়ার করুন কারিয়াহ কারিয়াহ মহিলা কুরআন তিলাওয়াতকারীরা কুরআন স্পেস কুরআন স্পেস লাইভ স্টাডি সার্কেল সূরা জুজ উদ্ঘাটন আদেশ ক্রমানুসার: আরোহী ১ Al-Fatihah সূচনা 001 ৭ আয়াত ২ Al-Baqarah বকনা-বাছুর 002 ২৮৬ আয়াত ৩ Ali 'Imran ইমরানের পরিবার 003 ২০০ আয়াত ৪ An-Nisa নারী 004 ১৭৬ আয়াত ৫ Al-Ma'idah খাদ্য পরিবেশিত টেবিল 005 ১২০ আয়াত ৬ Al-An'am গৃহপালিত পশু 006 ১৬৫ আয়াত ৭ Al-A'raf উচু স্থান 007 ২০৬ আয়াত ৮ Al-Anfal যুদ্ধ-লব্ধ ধনসম্পদ 008 ৭৫ আয়াত ৯ At-Tawbah অনুশোচনা 009 ১২৯ আয়াত ১০ Yunus নবী ইউনুস 010 ১০৯ আয়াত ১১ Hud নবী হুদ 011 ১২৩ আয়াত ১২ Yusuf নবী ইউসুফ 012 ১১১ আয়াত ১৩ Ar-Ra'd বজ্রপাত 013 ৪৩ আয়াত ১৪ Ibrahim নবী ইবরাহীম 014 ৫২ আয়াত ১৫ Al-Hijr পাথুরে পাহাড় 015 ৯৯ আয়াত ১৬ An-Nahl মৌমাছি 016 ১২৮ আয়াত ১৭ Al-Isra ইহুদি জাতি 017 ১১১ আয়াত ১৮ Al-Kahf গুহা 018 ১১০ আয়াত ১৯ Maryam মারইয়াম (ঈসা নবীর মা) 019 ৯৮ আয়াত ২০ Taha ত়া হা 020 ১৩৫ আয়াত ২১ Al-Anbiya নবীগণ 021 ১১২ আয়াত ২২ Al-Hajj হ়াজ্জ 022 ৭৮ আয়াত ২৩ Al-Mu'minun বিশ্বাসী 023 ১১৮ আয়াত ২৪ An-Nur আলো 024 ৬৪ আয়াত ২৫ Al-Furqan মানদণ্ড 025 ৭৭ আয়াত ২৬ Ash-Shu'ara কবি 026 ২২৭ আয়াত ২৭ An-Naml পিঁপড়া 027 ৯৩ আয়াত ২৮ Al-Qasas কাহিনি 028 ৮৮ আয়াত ২৯ Al-'Ankabut মাকড়শা 029 ৬৯ আয়াত ৩০ Ar-Rum রোমান জাতি 030 ৬০ আয়াত ৩১ Luqman এক জ্ঞানী ব্যাক্তি 031 ৩৪ আয়াত ৩২ As-Sajdah সিজদা 032 ৩০ আয়াত ৩৩ Al-Ahzab জোট 033 ৭৩ আয়াত ৩৪ Saba রানী সাবা/শেবা 034 ৫৪ আয়াত ৩৫ Fatir আদি স্রষ্টা 035 ৪৫ আয়াত ৩৬ Ya-Sin ইয়াসীন 036 ৮৩ আয়াত ৩৭ As-Saffat সারিবদ্ধভাবে দাঁড়ানো 037 ১৮২ আয়াত ৩৮ Sad আরবি বর্ণ সাদ 038 ৮৮ আয়াত ৩৯ Az-Zumar দলবদ্ধ জনতা 039 ৭৫ আয়াত ৪০ Ghafir ক্ষমাকারী 040 ৮৫ আয়াত ৪১ Fussilat সুস্পষ্ট বিবরণ 041 ৫৪ আয়াত ৪২ Ash-Shuraa পরামর্শ 042 ৫৩ আয়াত ৪৩ Az-Zukhruf সোনাদানা 043 ৮৯ আয়াত ৪৪ Ad-Dukhan ধোঁয়া 044 ৫৯ আয়াত ৪৫ Al-Jathiyah নতজানু 045 ৩৭ আয়াত ৪৬ Al-Ahqaf বালুর পাহাড় 046 ৩৫ আয়াত ৪৭ Muhammad নবী মুহাম্মদ 047 ৩৮ আয়াত ৪৮ Al-Fath বিজয় 048 ২৯ আয়াত ৪৯ Al-Hujurat আবাস 049 ১৮ আয়াত ৫০ Qaf কাফ 050 ৪৫ আয়াত ৫১ Adh-Dhariyat বিক্ষেপকারী বাতাস 051 ৬০ আয়াত ৫২ At-Tur পাহাড় 052 ৪৯ আয়াত ৫৩ An-Najm তারা 053 ৬২ আয়াত ৫৪ Al-Qamar চাঁদ 054 ৫৫ আয়াত ৫৫ Ar-Rahman পরম করুণাময় 055 ৭৮ আয়াত ৫৬ Al-Waqi'ah নিশ্চিত ঘটনা 056 ৯৬ আয়াত ৫৭ Al-Hadid লোহা 057 ২৯ আয়াত ৫৮ Al-Mujadila অনুযোগকারিণী 058 ২২ আয়াত ৫৯ Al-Hashr সমাবেশ 059 ২৪ আয়াত ৬০ Al-Mumtahanah নারী, যাকে পরীক্ষা করা হবে 060 ১৩ আয়াত ৬১ As-Saf সারিবদ্ধ সৈন্যদল 061 ১৪ আয়াত ৬২ Al-Jumu'ah সম্মেলন/শুক্রবার 062 ১১ আয়াত ৬৩ Al-Munafiqun ভণ্ড বিশ্বাসী 063 ১১ আয়াত ৬৪ At-Taghabun মোহ অপসারণ 064 ১৮ আয়াত ৬৫ At-Talaq তালাক 065 ১২ আয়াত ৬৬ At-Tahrim নিষিদ্ধকরণ 066 ১২ আয়াত ৬৭ Al-Mulk সার্বভৌম কর্তৃত্ব 067 ৩০ আয়াত ৬৮ Al-Qalam কলম 068 ৫২ আয়াত ৬৯ Al-Haqqah নিশ্চিত সত্য 069 ৫২ আয়াত ৭০ Al-Ma'arij উন্নয়নের সোপান 070 ৪৪ আয়াত ৭১ Nuh নবী নূহ 071 ২৮ আয়াত ৭২ Al-Jinn জিন সম্প্রদায় 072 ২৮ আয়াত ৭৩ Al-Muzzammil বস্ত্রাচ্ছাদনকারী 073 ২০ আয়াত ৭৪ Al-Muddaththir পোশাক পরিহিত 074 ৫৬ আয়াত ৭৫ Al-Qiyamah পুনরুত্থান 075 ৪০ আয়াত ৭৬ Al-Insan মানবজাতি 076 ৩১ আয়াত ৭৭ Al-Mursalat প্রেরিত পুরুষ 077 ৫০ আয়াত ৭৮ An-Naba মহাসংবাদ 078 ৪০ আয়াত ৭৯ An-Nazi'at প্রচেষ্টাকারী 079 ৪৬ আয়াত ৮০ 'Abasa সে ভ্রু কুঁচকালো 080 ৪২ আয়াত ৮১ At-Takwir অন্ধকারাচ্ছন্ন 081 ২৯ আয়াত ৮২ Al-Infitar বিদীর্ণ করা 082 ১৯ আয়াত ৮৩ Al-Mutaffifin প্রতারণা করা 083 ৩৬ আয়াত ৮৪ Al-Inshiqaq খণ্ড-বিখণ্ডকরণ 084 ২৫ আয়াত ৮৫ Al-Buruj নক্ষত্রপুঞ্জ 085 ২২ আয়াত ৮৬ At-Tariq রাতের আগন্তুক 086 ১৭ আয়াত ৮৭ Al-A'la সর্বোন্নত 087 ১৯ আয়াত ৮৮ Al-Ghashiyah বিহ্বলকর ঘটনা 088 ২৬ আয়াত ৮৯ Al-Fajr ভোরবেলা 089 ৩০ আয়াত ৯০ Al-Balad নগর 090 ২০ আয়াত ৯১ Ash-Shams সূর্য 091 ১৫ আয়াত ৯২ Al-Layl রাত 092 ২১ আয়াত ৯৩ Ad-Duhaa পূর্বাহ্নের সুর্যকিরণ 093 ১১ আয়াত ৯৪ Ash-Sharh বক্ষ প্রশস্তকরণ 094 ৮ আয়াত ৯৫ At-Tin ডুমুর 095 ৮ আয়াত ৯৬ Al-'Alaq রক্তপিণ্ড 096 ১৯ আয়াত ৯৭ Al-Qadr মহিমান্বিত 097 ৫ আয়াত ৯৮ Al-Bayyinah প্রমাণ 098 ৮ আয়াত ৯৯ Az-Zalzalah ভূমিকম্প 099 ৮ আয়াত ১০০ Al-'Adiyat অভিযানকারী 100 ১১ আয়াত ১০১ Al-Qari'ah মহাসংকট 101 ১১ আয়াত ১০২ At-Takathur প্রাচুর্যের প্রতিযোগিতা 102 ৮ আয়াত ১০৩ Al-'Asr সময় 103 ৩ আয়াত ১০৪ Al-Humazah পরনিন্দাকারী 104 ৯ আয়াত ১০৫ Al-Fil হাতি 105 ৫ আয়াত ১০৬ Quraysh কুরাইশ গোত্র 106 ৪ আয়াত ১০৭ Al-Ma'un সাহায্য সহায়তা 107 ৭ আয়াত ১০৮ Al-Kawthar কাউসার/প্রাচুর্য 108 ৩ আয়াত ১০৯ Al-Kafirun অবিশ্বাসী 109 ৬ আয়াত ১১০ An-Nasr সাহায্য 110 ৩ আয়াত ১১১ Al-Masad খেজুরের পাকানো (রশি) 111 ৫ আয়াত ১১২ Al-Ikhlas আন্তরিকতা 112 ৪ আয়াত ১১৩ Al-Falaq নিশিভোর 113 ৫ আয়াত ১১৪ An-Nas মানুষ জাতি 114 ৬ আয়াত মাসিক দাতা হন মাসিক অনুদান আমাদের Quran.com-এর উন্নতি এবং কার্যক্রম টিকিয়ে রাখতে সাহায্য করে, তাই আমরা তহবিল সংগ্রহের উপর কম এবং প্রভাব তৈরির উপর বেশি মনোযোগ দিই। আরও জানুন এখনই দান করুন কুরআন পড়ুন, শুনুন, অনুসন্ধান করুন এবং চিন্তা করুন Quran.com হল একটি বিশ্বস্ত প্ল্যাটফর্ম যা বিশ্বব্যাপী লক্ষ লক্ষ মানুষ বিভিন্ন ভাষায় কুরআন পড়তে, অনুসন্ধান করতে, শুনতে এবং তার উপর চিন্তাভাবনা করার জন্য ব্যবহার করে। এটি অনুবাদ, তাফসির, তেলাওয়াত, শব্দে শব্দ অনুবাদ এবং গভীর অধ্যয়নের জন্য সরঞ্জাম সরবরাহ করে, যা সকলের কাছে কুরআনকে সহজলভ্য করে তোলে। সাদাকাহ জারিয়াহ হিসেবে, Quran.com মানুষকে কুরআনের সাথে গভীরভাবে সংযুক্ত হতে সাহায্য করার জন্য নিবেদিতপ্রাণ। Quran.Foundation দ্বারা সমর্থিত, একটি 501(c)(3) অলাভজনক সংস্থা, Quran.com সকলের জন্য একটি বিনামূল্যের এবং মূল্যবান সম্পদ হিসেবে বেড়ে চলেছে, আলহামদুলিল্লাহ. নেভিগেট করুন বাড়ি কোরআন রেডিও আবৃত্তিকারী আমাদের সম্পর্কে বিকাশকারীরা পণ্য আপডেট প্রতিক্রিয়া সাহায্য দান করুন আমাদের প্রকল্পগুলি Quran.com Quran For Android Quran iOS QuranReflect.com Quran.AI Sunnah.com Nuqayah.com Legacy.Quran.com Corpus.Quran.com Quran.Foundation এর মালিকানাধীন, পরিচালিত, অথবা স্পন্সরকৃত অলাভজনক প্রকল্প। জনপ্রিয় লিঙ্ক Ayatul Kursi Surah Yaseen Surah Al Mulk Surah Ar-Rahman Surah Al Waqi'ah Surah Al Kahf Surah Al Muzzammil সাইটম্যাপগোপনীয়তাশর্তাবলী © ২০২৬ Quran.com. সমস্ত অধিকার সংরক্ষিত অটো বাংলা সূরা Al-Fatihah - ১-৭ - Quran.com # Star A star is a luminous spheroid of plasma held together by self-gravity.[1] The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy.[2] A star's life begins with the gravitational collapse of a gaseous nebula of material largely comprising hydrogen, helium, and trace heavier elements. Its total mass mainly determines its evolution and eventual fate. A star shines for most of its active life due to the thermonuclear fusion of hydrogen into helium in its core. This process releases energy that traverses the star's interior and radiates into outer space. At the end of a star's lifetime as a fusor, its core becomes a stellar remnant: a white dwarf, a neutron star, or—if it is sufficiently massive—a black hole. Stellar nucleosynthesis in stars or their remnants creates almost all naturally occurring chemical elements heavier than lithium. Stellar mass loss or supernova explosions return chemically enriched material to the interstellar medium. These elements are then recycled into new stars. Astronomers can determine stellar properties—including mass, age, metallicity (chemical composition), variability, distance, and motion through space—by carrying out observations of a star's apparent brightness, spectrum, and changes in its position in the sky over time. Stars can form orbital systems with other astronomical objects, as in planetary systems and star systems with two or more stars. When two such stars orbit closely, their gravitational interaction can significantly impact their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. # Etymology The word "star" ultimately derives from the Proto-Indo-European root "h₂stḗr" also meaning star, but further analyzable as h₂eh₁s- ("to burn", also the source of the word "ash") + -tēr (agentive suffix). Compare Latin stella, Greek aster, German Stern. Some scholars[who?] believe the word is a borrowing from Akkadian "istar" (Venus). "Star" is cognate (shares the same root) with the following words: asterisk, asteroid, astral, constellation, Esther.[3] Observation history See also: Stars in astrology People have interpreted patterns and images in the stars since ancient times.[4] This 1690 depiction of the constellation of Leo, the lion, is by Johannes Hevelius.[5] Historically, stars have been important to civilizations throughout the world. They have been part of religious practices, divination rituals, mythology, used for celestial navigation and orientation, to mark the passage of seasons, and to define calendars. Early astronomers recognized a difference between "fixed stars", whose position on the celestial sphere does not change, and "wandering stars" (planets), which move noticeably relative to the fixed stars over days or weeks.[6] Many ancient astronomers believed that the stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped prominent stars into asterisms and constellations and used them to track the motions of the planets and the inferred position of the Sun.[4] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[7] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun. The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[8] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531 BC – c. 1155 BC).[9] Alternative text Stars in the night sky The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[10] The star catalog of Hipparchus (2nd century BC) included 1,020 stars, and was used to assemble Ptolemy's star catalogue.[11] Hipparchus is known for the discovery of the first recorded nova (new star).[12] Many of the constellations and star names in use today derive from Greek astronomy. Despite the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[13] In 185 AD, they were the first to observe and write about a supernova, now known as SN 185.[14] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[15] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[16][17][18] Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly to produce Zij star catalogues.[19] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[20] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and gave the latitudes of various stars during a lunar eclipse in 1019.[21] According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[22] Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[23] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[24] and by medieval Islamic cosmologists[25] such as Fakhr al-Din al-Razi.[26] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[27] The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[23] William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this, he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[28] In addition to his other accomplishments, William Herschel is noted for his discovery that some stars do not merely lie along the same line of sight, but are physical companions that form binary star systems.[29] The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[30] The modern version of the stellar classification scheme was developed by Annie J. Cannon during the early 1900s.[31] The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[23] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32] The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33] Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35] Infrared image from NASA's Spitzer Space Telescope showing hundreds of thousands of stars in the Milky Way galaxy With the exception of rare events such as supernovae and supernova impostors, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for the Milky Way galaxy) and its satellites.[37] Individual stars such as Cepheid variables have been observed in the M87[38] and M100 galaxies of the Virgo Cluster,[39] as well as luminous stars in some other relatively nearby galaxies.[40] With the aid of gravitational lensing, a single star (named Icarus) has been observed at 9 billion light-years away.[41][42] Designations Main articles: Stellar designation, Astronomical naming conventions, and Star catalogue The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[43] Many of the more prominent individual stars were given names, particularly with Arabic or Latin designations. As well as certain constellations and the Sun itself, individual stars have their own myths.[44] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[44] (Uranus and Neptune were Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.) Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[45][46] The internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[47] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[48] which catalogs and standardizes proper names for stars.[49] A number of private companies sell names of stars which are not recognized by the IAU, professional astronomers, or the amateur astronomy community.[50] The British Library calls this an unregulated commercial enterprise,[51][52] and the New York City Department of Consumer and Worker Protection issued a violation against one such star-naming company for engaging in a deceptive trade practice.[53][54] # Units of measurement Although stellar parameters can be expressed in SI units or Gaussian units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters: nominal solar luminosity L☉ = 3.828×1026 W [55] nominal solar radius R☉ = 6.957×108 m [55] The solar mass M☉ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian constant of gravitation G. Since the product of the Newtonian constant of gravitation and solar mass together (GM☉) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be: nominal solar mass parameter: GM☉ = 1.3271244×1020 m3/s2 [55] The nominal solar mass parameter can be combined with the most recent (2014) CODATA estimate of the Newtonian constant of gravitation G to derive the solar mass to be approximately 1.9885×1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters. Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[55] Formation and evolution Main article: Stellar evolution Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics Size comparison (radius and mass) of a red dwarf, the Sun, a supermassive blue supergiant, and a red giant. Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[56] Most stars form in groups of dozens to hundreds of thousands of stars.[57] Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.[58] All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[59] Very low mass stars, with masses below 0.5 M☉, are fully convective and distribute helium evenly throughout the whole star while on the main sequence. Therefore, they never undergo shell burning and never become red giants. After exhausting their hydrogen they become helium white dwarfs and slowly cool.[60] As the lifetime of 0.5 M☉ stars is longer than the age of the universe, no such star has yet reached the white dwarf stage. Low mass stars (including the Sun), with a mass between 0.5 M☉ and ~2.25 M☉ depending on composition, do become red giants as their core hydrogen is depleted and they begin to burn helium in core in a helium flash; they develop a degenerate carbon-oxygen core later on the asymptotic giant branch; they finally blow off their outer shell as a planetary nebula and leave behind their core in the form of a white dwarf.[61][62] Intermediate-mass stars, between ~2.25 M☉ and ~8 M☉, pass through evolutionary stages similar to low mass stars, but after a relatively short period on the red-giant branch they ignite helium without a flash and spend an extended period in the red clump before forming a degenerate carbon-oxygen core.[61][62] Massive stars generally have a minimum mass of ~8 M☉.[63] After exhausting the hydrogen at the core these stars become supergiants and go on to fuse elements heavier than helium. Many end their lives when their cores collapse and they explode as supernovae.[61][64] Star formation Main article: Star formation Artist's conception of the birth of a star within a dense molecular cloud A cluster of approximately 500 young stars lies within the nearby W40 stellar nursery. The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67] As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 million years for a star like the sun, up to 100 million years for a red dwarf.[69] Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[70][71] These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[72] Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.[73] Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[74] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.[75] Main sequence Main article: Main sequence Stars spend about 90% of their lifetimes fusing hydrogen into helium in high-temperature-and-pressure reactions in their cores. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[76] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6×109) years ago.[77] Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[78] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[79] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[80] An example of a Hertzsprung–Russell diagram for a set of stars that includes the Sun (center) (see Classification) The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (10×1012) years; the most extreme of 0.08 M☉ will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[60] Since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[81] are expected to have moved off the main sequence. Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[82] which affects the strength of its stellar wind.[83] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.[84] Post–main sequence Main articles: Subgiant, Red giant, Horizontal branch, Red clump, and Asymptotic giant branch Betelgeuse as seen by ALMA. This is the first time that ALMA has observed the surface of a star and resulted in the highest-resolution image of Betelgeuse available. As stars of at least 0.4 M☉[85] exhaust the supply of hydrogen at their core, they start to fuse hydrogen in a shell surrounding the helium core. The outer layers of the star expand and cool greatly as they transition into a red giant. In some cases, they will fuse heavier elements at the core or in shells around the core. As the stars expand, they throw part of their mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[86] In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[77][87] As the hydrogen-burning shell produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, core helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[88] After a star has fused the helium of its core, it begins fusing helium along a shell surrounding the hot carbon core. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red-giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate. During the AGB phase, stars undergo thermal pulses due to instabilities in the core of the star. In these thermal pulses, the luminosity of the star varies and matter is ejected from the star's atmosphere, ultimately forming a planetary nebula. As much as 50 to 70% of a star's mass can be ejected in this mass loss process. Because energy transport in an AGB star is primarily by convection, this ejected material is enriched with the fusion products dredged up from the core. Therefore, the planetary nebula is enriched with elements like carbon and oxygen. Ultimately, the planetary nebula disperses, enriching the general interstellar medium.[89] Therefore, future generations of stars are made of the "star stuff" from past stars.[90] Massive stars Main articles: Supergiant star, Hypergiant, and Wolf–Rayet star Onion-like layers at the core of a massive, evolved star just before core collapses During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue supergiant and then a red supergiant. Particularly massive stars (exceeding 40 solar masses, like Alnilam, the central blue supergiant of Orion's Belt)[91] do not become red supergiants due to high mass loss.[92] These may instead evolve to a Wolf–Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss, or from stripping of the outer layers.[93] When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[94] The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[95] Some massive stars, particularly luminous blue variables, are very unstable to the extent that they violently shed their mass into space in events known as supernova impostors, becoming significantly brighter in the process. Eta Carinae is known for having underwent a supernova impostor event, the Great Eruption, in the 19th century. Collapse As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[96] The electron-degenerate matter inside a white dwarf is no longer a plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.[97] The Crab Nebula, remnants of a supernova that was first observed around 1050 AD In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[98] A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[98] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[99] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core.[100] The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[98] Binary stars Binary stars' evolution may significantly differ from that of single stars of the same mass. For example, when any star expands to become a red giant, it may overflow its Roche lobe, the surrounding region where material is gravitationally bound to it; if stars in a binary system are close enough, some of that material may overflow to the other star, yielding phenomena including contact binaries, common-envelope binaries, cataclysmic variables, blue stragglers,[101] and type Ia supernovae. Mass transfer leads to cases such as the Algol paradox, where the most-evolved star in a system is the least massive.[102] The evolution of binary star and higher-order star systems is intensely researched since so many stars have been found to be members of binary systems. Around half of Sun-like stars, and an even higher proportion of more massive stars, form in multiple systems, and this may greatly influence such phenomena as novae and supernovae, the formation of certain types of star, and the enrichment of space with nucleosynthesis products.[103] The influence of binary star evolution on the formation of evolved massive stars such as luminous blue variables, Wolf–Rayet stars, and the progenitors of certain classes of core collapse supernova is still disputed. Single massive stars may be unable to expel their outer layers fast enough to form the types and numbers of evolved stars that are observed, or to produce progenitors that would explode as the supernovae that are observed. Mass transfer through gravitational stripping in binary systems is seen by some astronomers as the solution to that problem.[104][105][106] Distribution Artist's impression of the Sirius system, a white dwarf star in orbit around an A-type main-sequence star Stars are not spread uniformly across the universe but are normally grouped into galaxies along with interstellar gas and dust. A typical large galaxy like the Milky Way contains hundreds of billions of stars. There are more than 2 trillion (1012) galaxies, though most are less than 10% the mass of the Milky Way.[107] Overall, there are likely to be between 1022 and 1024 stars[108][109] (more stars than all the grains of sand on planet Earth).[110][111][112] Most stars are within galaxies, but between 10 and 50% of the starlight in large galaxy clusters may come from stars outside of any galaxy.[113][114][115] A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars exist. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[116] Larger groups are called star clusters. These range from loose stellar associations with only a few stars to open clusters with dozens to thousands of stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy. The stars in an open or globular cluster all formed from the same giant molecular cloud, so all members normally have similar ages and compositions.[89] Many stars are observed, and most or all may have originally formed in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, 80% of which are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, more than two thirds of stars in the Milky Way are likely single red dwarfs.[117] In a 2017 study of the Perseus molecular cloud, astronomers found that most of the newly formed stars are in binary systems. In the model that best explained the data, all stars initially formed as binaries, though some binaries later split up and leave single stars behind.[118][119] This view of NGC 6397 includes stars known as blue stragglers for their location on the Hertzsprung–Russell diagram. The nearest star to the Earth, apart from the Sun, is Proxima Centauri, 4.2465 light-years (40.175 trillion kilometres) away. Travelling at the orbital speed of the Space Shuttle, 8 kilometres per second (29,000 kilometres per hour), it would take about 150,000 years to arrive.[120] This is typical of stellar separations in galactic discs.[121] Stars can be much closer to each other in the centres of galaxies[122] and in globular clusters,[123] or much farther apart in galactic halos.[124] Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[125] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature and thus are bluer than stars at the main sequence turnoff in the cluster to which they belong; in standard stellar evolution, blue stragglers would already have evolved off the main sequence and thus would not be seen in the cluster.[126] Characteristics Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate. Age Main article: Stellar age estimation Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[127] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the universe, determined by the Planck satellite as 13.799 ± 0.021).[127][128] The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[129][130] Lifetimes of stages of stellar evolution in billions of years[131] Initial Mass (M☉) Main Sequence Subgiant First Red Giant Core He Burning 1.0 9.33 2.57 0.76 0.13 1.6 2.28 0.03 0.12 0.13 2.0 1.20 0.01 0.02 0.28 5.0 0.10 0.0004 0.0003 0.02 Chemical composition See also: Metallicity and Molecules in stars When stars form in the present Milky Way galaxy, they are composed of about 71% hydrogen and 27% helium,[132] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[133] As of 2005 the star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[134] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[135] Chemically peculiar stars show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[136] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[137] Size comparison of some well-known supergiant and hypergiant stars, featuring Cygnus OB2-12, V382 Carinae, Betelgeuse, VV Cephei, and VY Canis Majoris Diameter Main articles: List of largest known stars, List of smallest stars, and Solar radius Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[138] The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[139] Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 640 times that of the Sun[140] with a much lower density.[141] Kinematics Main article: Stellar kinematics The Pleiades, an open cluster of stars in the constellation of Taurus. These stars share a common motion through space.[142] The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.[143] The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.[144] Radial velocity is measured by the doppler shift of the star's spectral lines and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[145] When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[146] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds; such groups with common points of origin are referred to as stellar associations.[147] Magnetic field Main article: Stellar magnetic field Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed by means of Zeeman–Doppler imaging The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[148] Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[149] During the Maunder Minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.[150] Mass Main article: Stellar mass Stars have masses ranging from less than half the solar mass to over 200 solar masses (see List of most massive stars). One of the most massive stars known is Eta Carinae,[151] which, with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as a rough upper limit for stars in the current era of the universe.[152] This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[153] but it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[154] The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis. The black patch of sky is a vast hole of empty space and not a dark nebula as previously thought. The first stars to form after the Big Bang may have been larger, up to 300 M☉,[155] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[156][157] With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[158] For stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[159][160] When the metallicity is very low, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[160][161] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.[159][160] The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35] Rotation Main article: Stellar rotation The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[162] By contrast, the Sun rotates once every 25–35 days depending on latitude,[163] with an equatorial velocity of 1.93 km/s.[164] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[165] Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[166] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[167] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[168] Temperature The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[169] The temperature is normally given in terms of an effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. The effective temperature is only representative of the surface, as the temperature increases toward the core.[170] The temperature in the core region of a star is several million kelvins.[171] The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35] Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they have a high luminosity due to their large exterior surface area.[172] Radiation Eta Carinae is an unstable blue hypergiant star, roughly 100 times more massive than the Sun, over 700 times wider, and 4 million times more luminous. In a 19th century event termed the Great Eruption, Eta Carinae brightened and violently ejected mass to form the surrounding Homunculus Nebula (pictured). The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[173] which streams from the outer layers as electrically charged protons and alpha and beta particles. A steady stream of almost massless neutrinos emanate directly from the star's core.[174] The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.[175] The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[176] Besides visible light, stars emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.[174] Using the stellar spectrum, astronomers can determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[177]) With these parameters, astronomers can estimate the age of the star.[178] Luminosity The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[179] Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Giant stars have much larger, more obvious starspots,[180] and they exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[181] Red dwarf flare stars such as UV Ceti may possess prominent starspot features.[182] Magnitude Main articles: Apparent magnitude and Absolute magnitude The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).[183] Number of stars brighter than magnitude Apparent magnitude Number of stars[184] 0 4 1 15 2 48 3 171 4 513 5 1,602 6 4,800 7 14,000 Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[185] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.[186] On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say: {\displaystyle \Delta {m}=m_{\mathrm {f} }-m_{\mathrm {b} }} {\displaystyle 2.512^{\Delta {m}}=\Delta {L}} Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[185] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41. The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, the latter star appears the brighter of the two. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.[187] The most luminous known stars have absolute magnitudes of roughly −12, corresponding to 6 million times the luminosity of the Sun.[188] Theoretically, the least luminous stars are at the lower limit of mass at which stars are capable of supporting nuclear fusion of hydrogen in the core; stars just above this limit have been located in the NGC 6397 cluster. The faintest red dwarfs in the cluster are absolute magnitude 15, while a 17th absolute magnitude white dwarf has been discovered.[189][190] Classification Main article: Stellar classification Surface temperature ranges for different stellar classes[191] Class Temperature Sample star O 33,000 K or more Zeta Ophiuchi B 10,500–30,000 K Rigel A 7,500–10,000 K Altair F 6,000–7,200 K Procyon A G 5,500–6,000 K Sun K 4,000–5,250 K Epsilon Indi M 2,600–3,850 K Proxima Centauri The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[192] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[193] Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[194] In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[194] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.[195] There is additional nomenclature in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[194] White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[196] Variable stars Main article: Variable star Mira, an oscillating variable star on the asymptotic giant branch, is a red giant nearing the end of its life, noted for its asymmetrical appearance. Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups. During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[197] Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[197] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars. Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[88] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[198] Some novae are recurrent, having periodic outbursts of moderate amplitude.[197] Stars can vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[197] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[199] Structure Main article: Stellar structure Internal structures of main sequence stars with masses indicated in solar masses, convection zones with arrowed cycles, and radiative zones with red flashes. Left to right, a red dwarf, a yellow dwarf, and a blue-white main sequence star The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[200][201] As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[202] In addition to hydrostatic equilibrium, the interior of a stable star will maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.[203] The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. Where this is not the case, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[201] The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[204] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[85] For most stars the convective zones will vary over time as the star ages and the constitution of the interior is modified.[201] A cross-section of the Sun The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.[205] Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[206] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[204] Despite its high temperature, the corona emits very little light, due to its low gas density.[207] The corona region of the Sun is normally only visible during a solar eclipse. From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[208] Nuclear fusion reaction pathways Main article: Stellar nucleosynthesis Overview of the proton–proton chain The carbon-nitrogen-oxygen cycle When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship {\displaystyle E=mc^{2}}.[209] A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[171] In the Sun, with a 16-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[210] 41H → 22H + 2e+ + 2νe(2 x 0.4 MeV) 2e+ + 2e− → 2γ (2 x 1.0 MeV) 21H + 22H → 23He + 2γ (2 x 5.5 MeV) 23He → 4He + 21H (12.9 MeV) There are a couple other paths, in which 3He and 4He combine to form 7Be, which eventually (with the addition of another proton) yields two 4He, a gain of one. All these reactions result in the overall reaction: 41H → 4He + 2γ + 2νe (26.7 MeV) where γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts. Each individual reaction produces only a tiny amount of energy, but because enormous numbers of these reactions occur constantly, they produce all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV. In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[210] In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[210] 4He + 4He + 92 keV → 8*Be 4He + 8*Be + 67 keV → 12*C 12*C → 12C + γ + 7.4 MeV For an overall reaction of: Overview of consecutive fusion processes in massive stars 34He → 12C + γ + 7.2 MeV In massive stars, heavier elements can be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[210] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse. Duration of the main phases of fusion for a 20 M☉ star[211] Fuel material Temperature (million kelvins) Density (kg/cm3) Burn duration (τ in years) H 37 0.0045 8.1 million He 188 0.97 1.2 million C 870 170 976 Ne 1,570 3,100 0.6 O 1,980 5,550 1.25 S/Si 3,340 33,400 0.0315 (~11.5 days) # [About](a.md)