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82-484: Galaxies with luminosities above 10  L ☉ are ultraluminous infrared galaxies (ULIRGs). Galaxies exceeding 10  L ☉ are characterised as hyper-luminous infrared galaxies (HyLIRGs). Those exceeding 10  L ☉ are extremely luminous infrared galaxies (ELIRGs). Many of the LIRGs and ULIRGs are showing interactions and disruptions. Many of these types of galaxies spawn about 100 new stars

164-485: A conjunction of Jupiter and Mars in 1106 or 1107 as evidence. 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." Ibn Qayyim al-Jawziyya (1292–1350) proposed that

246-617: A dark matter area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years (613 kpc). The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster , which is itself a component of the Laniakea Supercluster . It is estimated to contain 100–400 billion stars and at least that number of planets . The Solar System

328-654: 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 . In September 2023, astronomers reported that the virial mass of the Milky Way Galaxy is only 2.06 10 solar masses , only a 10th of the mass of previous studies. The mass was determined from data of the Gaia spacecraft . The Milky Way contains between 100 and 400 billion stars and at least that many planets. An exact figure would depend on counting

410-603: A 2016 study. Such Earth-sized planets may be more numerous than gas giants, 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. More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy. When compared to other more distant galaxies in

492-405: A ULIRG is just part of an evolutionary galaxy merger scenario. In essence, two or more spiral galaxies , galaxies that consist of a flat, rotating disk containing stars , gas and dust and a central concentration of stars known as the bulge , merge to form an early stage merger. An early stage merger in this case can also be identified as a LIRG. After that, it becomes a late stage merger, which

574-467: A foreground elliptical galaxy , was considered to be one of the most luminous objects in the Universe having an intrinsic luminosity of ~ 2 × 10 L ⊙ . It is believed that the bolometric luminosity of this HLIRG is likely amplified by a factor of ~30 as a result of the gravitational lensing. The majority (~80%) of the mid-infrared spectrum of these objects is found to be dominated by AGN emission. However,

656-456: A great deal of detail at +6.1. 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. 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. As viewed from Earth, the visible region of

738-406: 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 D 25 isophotal diameter estimated at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years ), but only about 1,000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that

820-547: A longer time. Twenty new ELIRGs, including the most luminous galaxy found to date, have been discovered. These galaxies were not found earlier because of their distance, and because dust converts their visible light into infrared light. One has been observed to have three star-forming areas. The Infrared Astronomical Satellite (IRAS) was the first all-sky survey which used far-infrared wavelengths, in 1983. In that survey, tens of thousands of galaxies were detected, many of which would not have been recorded in previous surveys. It

902-466: A measurement of the radial velocity of halo stars found that the mass enclosed within 80 kilo parsecs is 7 × 10   M ☉ . In a 2014 study, the mass of the entire Milky Way is estimated to be 8.5 × 10   M ☉ , but this is only half the mass of the Andromeda Galaxy. A recent 2019 mass estimate for the Milky Way is 1.29 × 10   M ☉ . Much of the mass of

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984-568: A new class of objects discovered by WISE, extremely luminous infrared galaxies, or ELIRGs. Light from the WISE J224607.57-052635.0 galaxy has traveled 12.5 billion years. The black hole at its center was billions of times the mass of the Sun when the universe was a tenth (1.3 billion years) of its present age of 13.8 billion years. There are three reasons the black holes in the ELIRGs could be massive. First,

1066-410: A range in mass, as large as 4.5 × 10   M ☉ and as small as 8 × 10   M ☉ . By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6 × 10   M ☉ and 6.43 × 10   M ☉ . In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass, with two thirds of

1148-415: A treatise in 1755, Immanuel Kant , drawing on earlier work by Thomas Wright , 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. 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

1230-484: A year as compared to the Milky Way which spawns one a year; this helps create the high level of luminosity. Infrared galaxies appear to be single, gas-rich spirals whose infrared luminosity is created largely by the formation of stars within them. These types of galaxies were discovered in 1983 with IRAS . A LIRG's excess infrared luminosity may also come from the presence of an active galactic nucleus (AGN) residing at

1312-408: Is a ULIRG. It then becomes a quasar and in the final stage of the evolution it becomes an elliptical galaxy . This can be evidenced by the fact that stars are much older in elliptical galaxies than those found in the earlier stages of the evolution. Hyper luminous Infrared Galaxies (HyLIRG), also referred to as HiLIRGs and HLIRGs, are considered to be some of the most luminous persistent objects in

1394-428: Is a disk of gas and dust called the interstellar medium . This disk has at least a comparable extent in radius to the stars, 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. 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

1476-500: Is a multi-wavelength study of luminous infrared galaxies, incorporating observations with NASA's Great Observatories and other ground and space-based telescopes. Using information from NASA's Spitzer , Hubble , Chandra and Galex observations in a study over 200 of the most luminous infrared selected galaxies in the local universe. Approximately 180 LIRGs were identified along with over 20 ULIRGs. The LIRGs and ULIRGs targeted in GOALS span

1558-405: 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), which may be part of the Milky Way's outer disk itself, hence making

1640-582: Is also the root of "galaxy", the name for our, and later all such, collections of stars. 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. The Milky Way

1722-537: Is approximately 890 billion to 1.54 trillion times the mass of the Sun in total (8.9 × 10 to 1.54 × 10 solar masses), 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 × 10   solar masses ( M ☉ ), somewhat less than that of the Andromeda Galaxy . Measurements using

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1804-449: Is called the Zone of Avoidance . 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. It should be visible if the limiting magnitude is approximately +5.1 or better and shows

1886-522: Is celestial. This idea would be influential later in the Muslim world . The Persian astronomer Al-Biruni (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars". 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

1968-472: 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 12 49 , declination +27.4° ( B1950 ) near β Comae Berenices , and

2050-570: Is located at a radius of about 27,000 light-years (8.3 kpc) from the Galactic Center , 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 . The oldest stars in

2132-678: Is now clear that the reason the number of detections has risen is that the majority of LIRGs in the universe emitted the bulk of their energy in the far infrared . Using the IRAS, scientists were able to determine the luminosity of the galactic objects discovered. The telescope was a joint project of the United States ( NASA ), Netherlands (NIVR) , and the United Kingdom (SERC) . Over 250,000 infrared sources were observed during this 10-month mission. The Great Observatories All-sky LIRG Survey (GOALS)

2214-480: Is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities. 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

2296-434: 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). 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. To compare

2378-637: Is the Large Sagittarius Star Cloud , a portion of the central bulge of the galaxy. 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 . The area of sky that the Milky Way obscures

2460-737: Is the traditional Welsh name for the Milky Way, and Caer Arianrhod ("The Fortress of Arianrhod ") being the constellation of Corona Borealis . 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) –

2542-486: Is visible as a hazy band of white light, some 30° wide, arching the night sky . 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. 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

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2624-489: 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. 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 × 10   M ☉ within 160,000 ly (49 kpc) of its center. In 2010,

2706-476: 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

2788-597: The magnetic fields of the Milky Way were reported. 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 ). 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) from

2870-440: 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. 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

2952-460: 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

3034-501: The starburst (SB) activity is known to be significant in all known sources with a mean SB contribution of ~30%. Star formation rates in HLIRGs have been shown to reach ~ 3×10 – 3×10 M ⊙ yr. The Extremely Luminous Infrared Galaxy WISE J224607.57-052635.0 , with a luminosity of 300 trillion suns was discovered by NASA's Wide-field Infrared Survey Explorer (WISE), and as of May 2015 is the most luminous galaxy found. The galaxy belongs to

3116-563: The Galactic Center is about 180,000 ly (55 kpc). 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. 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

3198-519: 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. The Sun is currently 5–30 parsecs (16–98 ly) above, or north of, the central plane of the Galactic disk. The distance between the local arm and the next arm out, the Perseus Arm , is about 2,000 parsecs (6,500 ly). The Sun, and thus

3280-399: The Galactic spiral arms and non-uniform mass distributions. In addition, the Sun passes through the Galactic plane approximately 2.7 times per orbit. 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. A reanalysis of the effects of

3362-688: The Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang . 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 . Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis , observations by Edwin Hubble showed that

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3444-405: The Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars". 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. In

3526-540: The Milky Way is a barred spiral galaxy , rather than an ordinary spiral galaxy , in the 1960s. 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. WISE J224607.57-052635.0 Too Many Requests If you report this error to the Wikimedia System Administrators, please include

3608-446: 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 ). The Solar System is headed in the direction of the zodiacal constellation Scorpius , which follows

3690-513: The Milky Way is just one of many galaxies. 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. This story was once thought to have been based on an older Sumerian version in which Tiamat is instead slain by Enlil of Nippur , but

3772-449: 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. 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

3854-428: 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 × 10   M ☉ . 2013 and 2014 studies indicate

3936-404: The Milky Way". 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: 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

4018-510: The Milky Way's galactic plane occupies an area of the sky that includes 30 constellations . 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 . The galactic plane

4100-526: The Milky Way, and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars. 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. A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in

4182-438: The Milky Way. 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. 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

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4264-437: The Milky Way. 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. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest exoplanet may be 4.2 light-years away, orbiting the red dwarf Proxima Centauri , according to

4346-501: The Solar System, is located in the Milky Way's galactic habitable zone . 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

4428-411: The Sun's transit through the spiral structure based on CO data has failed to find a correlation. It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a galactic year ), 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

4510-412: 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. The apex of the Sun's way, or

4592-468: The Universe, exhibiting extremely high star formation rates , and most of which are known to harbour Active Galactic Nuclei (AGN). They are defined as galaxies with luminosities above 10 L ⊙ , as distinct from the less luminous population of ULIRGs (L = 10 – 10 L ⊙ ). HLIRGs were first identified through follow-up observations of the IRAS mission. IRAS F10214+4724, a HyLIRG being gravitationally lensed by

4674-487: 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. Llys Dôn (literally "The Court of Dôn ") is the traditional Welsh name for the constellation Cassiopeia . At least three of Dôn's children also have astronomical associations: Caer Gwydion ("The fortress of Gwydion ")

4756-456: The black hole in the study would have had to repeatedly break the limit to grow this large. Third, the black holes might just be bending this limit, absorbing gas faster than thought possible, if the black hole is not spinning fast. If a black hole spins slowly, it will not repel its gas absorption as much. A slow-spinning black hole can absorb more matter than a fast-spinning black hole. The massive black holes in ELIRGs could be absorbing matter for

4838-478: 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. 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

4920-419: The center. These galaxies emit more energy in the infrared portion of the spectrum, not visible to the naked eye. The energy given off by LIRGs is comparable to that of a quasar (a type of AGN), which formerly was known as the most energetic object in the universe. LIRGs are brighter in the infrared than in the optical spectrum because the visible light is absorbed by the high amounts of gas and dust, and

5002-412: 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 ). 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

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5084-495: The dust re-emits thermal energy in the infrared spectrum. LIRGs are known to exist in denser parts of the universe than non-LIRGs. LIRGs are also capable of becoming Ultra Luminous Infrared Galaxys (ULIRGs) but there is no perfect timetable because not all LIRGs turn into ULIRGs, Newtonian mechanics is used in the calculations and because the constraints are not quite approximate. Studies have shown that ULIRGs are more likely to contain an AGN than LIRGs According to one study

5166-469: The ecliptic. 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 . Quadrants are described using ordinals  – for example, "1st galactic quadrant", "second galactic quadrant", or "third quadrant of

5248-511: The embryonic black holes might be bigger than thought possible. Second, the Eddington limit was exceeded. When a black hole feeds, gas falls in and heats, emitting light. The pressure of the emitted light forces the gas outward, creating a limit to how fast the black hole can continuously absorb matter. If a black hole broke this limit, it could theoretically increase in size at a fast rate. Black holes have previously been observed breaking this limit;

5330-466: The full range of nuclear spectral types (type-1 and type 2 Active Galactic Nuclei, LINERS's , and starbursts) and interaction stages (major mergers, minor mergers, and isolated galaxies). Some examples of extremely notable LIRGs, ULIRGs, HyLIRGs, ELIRGs Milky Way The Milky Way is the galaxy that includes the Solar System , with the name describing the galaxy's appearance from Earth :

5412-409: The hydrogen found in the atomic form and the remaining one-third as molecular hydrogen . The mass of the Milky Way's interstellar gas is equal to between 10% and 15% of the total mass of its stars. Interstellar dust accounts for an additional 1% of the total mass of the gas. In March 2019, astronomers reported that the virial mass of the Milky Way Galaxy is 1.54 trillion solar masses within

5494-436: The isophotal diameter is not well understood. 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. There are several methods being used in astronomy in defining

5576-526: The measurable volume of space by a factor of 100 in radius and a factor of 1,000 in precision. 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". In April 2024, initial studies (and related maps) involving

5658-518: The milky appearance of the Milky Way Galaxy is due to the refraction of the Earth's atmosphere. 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

5740-573: 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 . 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

5822-405: 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 (10 ) stars. The Milky Way may contain ten billion white dwarfs , a billion neutron stars , and a hundred million stellar black holes . Filling the space between the stars

5904-433: The old population of the galactic halo. 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 ). The Milky Way's stellar disk is also estimated to be approximately up to 1.35 kpc (4,000 ly) thick. The Milky Way

5986-499: 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 (μ 0 ) of 22.1 ± 0.3 B -mag/arcsec and a disk scale length ( h ) of 5.0 ± 0.5 kpc (16,300 ± 1,600 ly). This

6068-522: 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. In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning

6150-400: 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 . An even older study from 1978 gave a lower diameter for Milky Way about 23 kpc (75,000 ly). A 2015 paper discovered that there

6232-512: 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 D 25 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/arcsec . An estimate from 1997 by Goodwin and others compared the distribution of Cepheid variable stars in 17 other spiral galaxies to

6314-538: 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. In Meteorologica , Aristotle (384–322 BC) states that the Greek philosophers Anaxagoras ( c.  500 –428 BC) and Democritus (460–370 BC) proposed that

6396-512: The stellar disk larger by increasing to this size. 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. Another 2018 study revealed

6478-504: The universe, the Milky Way galaxy has a below average amount of neutrino luminosity making our galaxy a " neutrino desert ". The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust and stars. 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. Astronomers first began to conjecture that

6560-399: 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

6642-478: The visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center. 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. In 1904, studying the proper motions of stars, Jacobus Kapteyn reported that these were not random, as it

6724-505: Was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of our galaxy, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort . In 1917, Heber Doust Curtis had observed the nova S Andromedae within the Great Andromeda Nebula ( Messier object 31). Searching

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