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119-554: The cluster is following an orbit through the Milky Way that has a period of 227.4 million years with an ellipticity of 0.06, carrying it as far as 28 kly (8.6 kpc) from the Galactic Center , and as close as 25 kly (7.7 kpc). The inclination of the orbit carries it up to 260 light-years (80 parsecs) away from the galactic plane . On average it crosses the plane every 33.3 million years. The star cluster

238-401: A {\displaystyle a} is the scale parameter . The Hubble parameter, however, is proportional to a − 2 {\displaystyle a^{-2}} . Going back in time and higher in energy, and assuming no new physics at these energies, a careful estimate gives that thermalization was first possible when the temperature was: approximately 10 seconds after

357-489: 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

476-666: 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

595-619: 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

714-533: 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

833-460: 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

952-474: 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

1071-421: A phase transition of this kind, when gravitation separated from the universal combined gauge force . This caused two forces to now exist: gravity , and an electrostrong interaction . There is no hard evidence yet that such a combined force existed, but many physicists believe it did. The physics of this electrostrong interaction would be described by a Grand Unified Theory. The grand unification epoch ended with

1190-454: A phase transition. For example, in a later epoch, a side effect of one phase transition is that suddenly, many particles that had no mass at all acquire a mass (they begin to interact differently with the Higgs field ), and a single force begins to manifest as two separate forces. Assuming that nature is described by a so-called Grand Unified Theory (GUT), the grand unification epoch began with

1309-432: A phenomenon of quantum fields called " symmetry breaking ". In everyday terms, as the universe cools, it becomes possible for the quantum fields that create the forces and particles around us, to settle at lower energy levels and with higher levels of stability. In doing so, they completely shift how they interact. Forces and interactions arise due to these fields, so the universe can behave very differently above and below

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1428-410: 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

1547-401: A redshift of z=13.2, from 13.4 billion years ago. The JWST was designed to observe as far as z≈20 (180 million years cosmic time). To derive the age of the universe from redshift, numeric integration or its closed-form solution involving the special Gaussian hypergeometric function 2 F 1 may be used: Lookback time is the age of the observation subtracted from the present age of

1666-517: A result the universe also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium —quickly reach their lowest energy state ( ground state ) by releasing photons (" photon decoupling "), and these photons can still be detected today as the cosmic microwave background (CMB). This is the oldest direct observation we currently have of the universe. This period measures from 370,000 years until about 1 billion years. After recombination and decoupling ,

1785-432: A second phase transition, as the electrostrong interaction in turn separated, and began to manifest as two separate interactions, called the strong and the electroweak interactions. Depending on how epochs are defined, and the model being followed, the electroweak epoch may be considered to start before or after the inflationary epoch. In some models, it is described as including the inflationary epoch. In other models,

1904-402: A small excess of matter in the universe. At about one second, neutrinos decouple ; these neutrinos form the cosmic neutrino background (CνB). If primordial black holes exist, they are also formed at about one second of cosmic time. Composite subatomic particles emerge—including protons and neutrons —and from about 2 minutes, conditions are suitable for nucleosynthesis : around 25% of

2023-415: 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

2142-428: 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

2261-405: 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

2380-599: 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 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

2499-548: 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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2618-453: 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

2737-530: 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

2856-477: Is estimated to contain 100–400 billion stars and at least that number of planets . The Solar System 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

2975-472: 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

3094-408: Is known about the details of these processes. The number density of each particle species was, by a similar analysis to Stefan–Boltzmann law : which is roughly just ( k B T / ℏ c ) 3 {\displaystyle (k_{B}T/\hbar c)^{3}} . Since the interaction was strong, the cross-section σ {\displaystyle \sigma }

3213-880: Is known as the cosmic Dark Ages . At some point around 200 to 500 million years, the earliest generations of stars and galaxies form (exact timings are still being researched), and early large structures gradually emerge, drawn to the foam-like dark matter filaments which have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been very massive (100–300 solar masses ) and non-metallic , with very short lifetimes compared to most stars we see today , so they commonly finish burning their hydrogen fuel and explode as highly energetic pair-instability supernovae after mere millions of years. Other theories suggest that they may have included small stars, some perhaps still burning today. In either case, these early generations of supernovae created most of

3332-509: Is much better understood, and the energies involved in the Quark epoch are directly accessible in particle physics experiments and other detectors. Sometime after inflation, the created particles went through thermalization , where mutual interactions lead to thermal equilibrium . The earliest stage that we are confident about is some time before the electroweak symmetry breaking , at a temperature of around 10 K, approximately 10 seconds after

3451-401: Is no lower than 1 TeV , the electroweak scale. The masses of particles and their superpartners would then no longer be equal. This very high energy could explain why no superpartners of known particles have ever been observed. After cosmic inflation ends, the universe is filled with a hot quark–gluon plasma , the remains of reheating. From this point onwards the physics of the early universe

3570-464: Is often considered to mark the start of the electroweak epoch, and some theories, such as warm inflation , avoid a reheating phase entirely. In non-traditional versions of Big Bang theory (known as "inflationary" models), inflation ended at a temperature corresponding to roughly 10 seconds after the Big Bang, but this does not imply that the inflationary era lasted less than 10 seconds. To explain

3689-518: Is one of the possibilities.) This provides a model of the universe which matches all current physical observations extremely closely. This initial period of the universe's chronology is called the " Big Bang ". The Standard Model of cosmology attempts to explain how the universe physically developed once that moment happened. The singularity from the FLRW metric is interpreted to mean that current theories are inadequate to describe what actually happened at

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3808-421: Is said that the Big Bang "happened everywhere". During the earliest moments of cosmic time, the energies and conditions were so extreme that current knowledge can only suggest possibilities, which may turn out to be incorrect. To give one example, eternal inflation theories propose that inflation lasts forever throughout most of the universe, making the notion of "N seconds since Big Bang" ill-defined. Therefore,

3927-434: 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

4046-664: 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

4165-405: 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 ") 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

4284-486: 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

4403-578: Is young and contains a large number of low-mass stars , some T Tauri stars and brown dwarfs . One notable member is LOri167 , which is a wide binary consisting of a potential planetary-mass object and a brown dwarf. Observations of the star cluster with the Spitzer Space Telescope have shown that 25% of the low-mass stars and 40% of the substellar objects are surrounded by a circumstellar disk . Two of these being actively photoevaporated by Meissa. The cluster might have formed in

4522-458: The Planck epoch , during which currently established laws of physics may not have applied; the emergence in stages of the four known fundamental interactions or forces —first gravitation , and later the electromagnetic , weak and strong interactions; and the accelerated expansion of the universe due to cosmic inflation . Tiny ripples in the universe at this stage are believed to be

4641-499: 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,

4760-457: The electromagnetic and weak interactions.) The exact point where electrostrong symmetry was broken is not certain, owing to speculative and as yet incomplete theoretical knowledge. At this point of the very early universe, the universe is thought to have expanded by a factor of at least 10 in volume. This is equivalent to a linear increase of at least 10 times in every spatial dimension—equivalent to an object 1 nanometre (10 m , about half

4879-413: The fields which define the universe's fundamental forces and particles also completely change their behaviors and structures when the temperature/energy falls below a certain point. This is not apparent in everyday life, because it only happens at far higher temperatures than usually seen in the present-day universe. These phase transitions in the universe's fundamental forces are believed to be caused by

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4998-533: 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

5117-614: 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

5236-440: 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

5355-419: The separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field . As this field settled into its lowest energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of the universe. Inflation explains several observed properties of

5474-467: 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

5593-518: 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 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,

5712-660: The Big Bang. The electromagnetic and weak interaction have not yet separated , and the gauge bosons and fermions have not yet gained mass through the Higgs mechanism . However exotic massive particle-like entities, sphalerons , are thought to have existed. This epoch ended with electroweak symmetry breaking , potentially through a phase transition . In some extensions of the Standard Model of particle physics , baryogenesis also happened at this stage, creating an imbalance between matter and anti-matter (though in extensions to this model, this may have happened earlier). Little

5831-571: 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

5950-530: 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

6069-399: 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

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6188-584: 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 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 ,

6307-405: 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

6426-475: 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. Chronology of the universe#Dark Ages The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. Research published in 2015 estimates

6545-446: 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

6664-449: 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

6783-428: 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

6902-404: 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

7021-514: 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

7140-530: 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

7259-438: 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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7378-437: 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

7497-501: 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

7616-411: 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

7735-412: 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

7854-404: The ability of gravity to decelerate the expansion of the universe; in contrast, dark energy (believed to be a constant scalar field throughout the visible universe) is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an inflection point about five or six billion years ago when the universe entered the modern "dark-energy-dominated era" where

7973-562: The basis of large-scale structures that formed much later. Different stages of the very early universe are understood to different extents. The earlier parts are beyond the grasp of practical experiments in particle physics but can be explored through the extrapolation of known physical laws to extremely high temperatures. This period lasted around 370,000 years. Initially, various kinds of subatomic particles are formed in stages. These particles include almost equal amounts of matter and antimatter , so most of it quickly annihilates, leaving

8092-482: 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

8211-475: The central region of an elongated cloud, which is supported by the distribution of pre-main-sequence star candidates, which are concentrated in the cluster and nearby regions in an elongated shape. Massive OB stars and low-mass stars formed in the central regions of these clouds. The low-mass stars closest to the massive stars likely lost their circumstellar disks due to photoevaporation . Many low-mass stars parsecs away were unaffected by this and represent

8330-577: The constellation Orion: Milky Way The Milky Way 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 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

8449-479: The current population of low-mass stars with a circumstellar disk. The cluster is surrounded by a large molecular ring , called the Lambda Orionis ring . This was interpreted as a remnant of a supernova that exploded one million years ago. The supernova blast encountered the clouds and gas in the region and the blast dispersed the parent core, creating the molecular ring. Other celestial bodies included in

8568-471: The current universe that are otherwise difficult to account for, including explaining how today's universe has ended up so exceedingly homogeneous (spatially uniform) on a very large scale, even though it was highly disordered in its earliest stages. It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10 and 10 seconds after the Big Bang. The rapid expansion of space meant that elementary particles remaining from

8687-471: 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

8806-417: The earliest stages are an active area of research and based on ideas that are still speculative and subject to modification as scientific knowledge improves. Although a specific "inflationary epoch" is highlighted at around 10 seconds, observations and theories both suggest that distances between objects in space have been increasing at all times since the moment of the Big Bang, and are still increasing (with

8925-469: The earliest stages of the universe's existence as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level. For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated , into five parts. It is generally considered meaningless or unclear whether time existed before this chronology: The first picosecond  (10 seconds) of cosmic time includes

9044-469: 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

9163-461: The electroweak epoch is said to begin after the inflationary epoch ended, at roughly 10 seconds. According to traditional Big Bang cosmology, the electroweak epoch began 10 seconds after the Big Bang, when the temperature of the universe was low enough (10 K) for the electronuclear force to begin to manifest as two separate interactions, the strong and the electroweak interactions. (The electroweak interaction will also separate later, dividing into

9282-586: The end of inflation (roughly 10 seconds after the Big Bang) do not follow the same timeline as in traditional Big Bang cosmology. Models that aim to describe the universe and physics during the Planck epoch are generally speculative and fall under the umbrella of " New Physics ". Examples include the Hartle–Hawking initial state , string theory landscape , string gas cosmology , and the ekpyrotic universe . As

9401-482: The every day elements we see around us today, and seeded the universe with them. Galaxy clusters and superclusters emerge over time. At some point, high-energy photons from the earliest stars, dwarf galaxies and perhaps quasars lead to a period of reionization that commences gradually between about 250–500 million years and finishes by about 1 billion years (exact timings still being researched). The Dark Ages only fully came to an end at about 1 billion years as

9520-407: The exception of gravitationally bound objects such as galaxies and most clusters , once the rate of expansion had greatly slowed). The inflationary period marks a specific period when a very rapid change in scale occurred, but does not mean that it stayed the same at other times. More precisely, during inflation, the expansion accelerated. After inflation, and for about 9.8 billion years, the expansion

9639-664: The far future and ultimate fate of the universe . More exact knowledge of the present-day universe may allow these to be better understood. The Standard Model of cosmology is based on a model of spacetime called the Friedmann–Lemaître–Robertson–Walker (FLRW) metric . A metric provides a measure of distance between objects, and the FLRW metric is the exact solution of Einstein field equations (EFE) if some key properties of space such as homogeneity and isotropy are assumed to be true. The FLRW metric very closely matches overwhelming other evidence, showing that

9758-410: The grand unification epoch was now distributed very thinly across the universe. However, the huge potential energy of the inflaton field was released at the end of the inflationary epoch, as the inflaton field decayed into other particles, known as "reheating". This heating effect led to the universe being repopulated with a dense, hot mixture of quarks, anti-quarks and gluons . In other models, reheating

9877-409: 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

9996-527: 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. Llys Dôn (literally "The Court of Dôn ")

10115-436: 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

10234-526: 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

10353-531: 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

10472-589: 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

10591-405: 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

10710-455: The observed homogeneity of the universe, the duration in these models must be longer than 10 seconds. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation. After inflation ended, the universe continued to expand, but at a decelerating rate. About 4 billion years ago the expansion gradually began to speed up again. This is believed to be due to dark energy becoming dominant in

10829-433: 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

10948-499: 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

11067-528: 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

11186-481: The protons and all the neutrons fuse into heavier elements , initially deuterium which itself quickly fuses into mainly helium-4 . By 20 minutes, the universe is no longer hot enough for nuclear fusion , but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaque plasma . The recombination epoch begins at around 18,000 years, as electrons are combining with helium nuclei to form He . At around 47,000 years, as

11305-400: 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

11424-593: 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

11543-516: 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

11662-542: 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

11781-544: 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). 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

11900-420: The start of the Big Bang itself. It is widely believed that a correct theory of quantum gravity may allow a more correct description of that event, but no such theory has yet been developed. After that moment, all distances throughout the universe began to increase from (perhaps) zero because the FLRW metric itself changed over time, affecting distances between all non-bound objects everywhere. For this reason, it

12019-524: 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

12138-407: The universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule . Much later, hydrogen and helium hydride react to form molecular hydrogen (H 2 ) the fuel needed for the first stars . At about 370,000 years, neutral hydrogen atoms finish forming ("recombination"), and as

12257-413: The universe expanded and cooled, it crossed transition temperatures at which forces separated from each other. These cosmological phase transitions can be visualized as similar to condensation and freezing phase transitions of ordinary matter. At certain temperatures/energies, water molecules change their behavior and structure, and they will behave completely differently. Like steam turning to water,

12376-516: The universe gradually transitioned into the universe we see around us today, but denser, hotter, more intense in star formation, and richer in smaller (particularly unbarred) spiral and irregular galaxies, as opposed to giant elliptical galaxies. While early stars have not been observed, galaxies have been observed from 329 million years since the Big Bang, with JADES-GS-z13-0 which the James Webb Space Telescope observed with

12495-409: The universe has expanded since the Big Bang. If the FLRW metric equations are assumed to be valid all the way back to the beginning of the universe, they can be followed back in time, to a point where the equations suggest all distances between objects in the universe were zero or infinitesimally small. (This does not necessarily mean that the universe was physically small at the Big Bang, although that

12614-469: The universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies , so there were few sources of light and the emission from these sources was immediately absorbed by hydrogen atoms. The only photons (electromagnetic radiation, or "light") in the universe were those released during decoupling (visible today as the cosmic microwave background) and 21 cm radio emissions occasionally emitted by hydrogen atoms. This period

12733-556: The universe's expansion is now accelerating rather than decelerating. The present-day universe is quite well understood, but beyond about 100 billion years of cosmic time (about 86 billion years in the future), we are less sure which path the universe will take. At some time, the Stelliferous Era will end as stars are no longer being born, and the expansion of the universe will mean that the observable universe becomes limited to local galaxies. There are various scenarios for

12852-530: The universe's large-scale behavior. It is still expanding (and, accelerating), today. On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-modes power spectrum which was interpreted as clear experimental evidence for the theory of inflation. However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings

12971-508: 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

13090-494: The universe: From 1 billion years, and for about 12.8 billion years, the universe has looked much as it does today and it will continue to appear very similar for many billions of years into the future. The thin disk of our galaxy began to form at about 5 billion years (8.8 Gya ), and the Solar System formed at about 9.2 billion years (4.6 Gya), with the earliest evidence of life on Earth emerging by about 10 billion years (3.8 Gya). The thinning of matter over time reduces

13209-515: The universe— gravitation , electromagnetism , the weak nuclear force , and the strong nuclear force —comprised a single fundamental force. Little is understood about physics in this environment. Traditional big bang cosmology predicts a gravitational singularity —a condition in which spacetime breaks down—before this time, but the theory relies on the theory of general relativity , which is thought to break down for this epoch due to quantum effects . In inflationary models of cosmology, times before

13328-399: 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

13447-478: 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

13566-409: The width of a molecule of DNA ) in length, expanding to one approximately 10.6 light-years (100 trillion kilometres) long in a tiny fraction of a second. This phase of the cosmic expansion history is known as inflation . The mechanism that drove inflation remains unknown, although many models have been put forward. In several of the more prominent models, it is thought to have been triggered by

13685-404: Was approximately the particle wavelength squared, which is roughly n − 2 / 3 {\displaystyle n^{-2/3}} . The rate of collisions per particle species can thus be calculated from the mean free path , giving approximately: For comparison, since the cosmological constant was negligible at this stage, the Hubble parameter was: where x ~ 10

13804-515: 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

13923-453: Was much slower and became slower yet over time (although it never reversed). About 4 billion years ago, it began slightly speeding up again. The Planck epoch is an era in traditional (non-inflationary) Big Bang cosmology immediately after the event that began the known universe. During this epoch, the temperature and average energies within the universe were so high that subatomic particles could not form. The four fundamental forces that shape

14042-521: Was reported and finally, on 2 February 2015, a joint analysis of data from BICEP2/Keck and the European Space Agency 's Planck microwave space telescope concluded that the statistical "significance [of the data] is too low to be interpreted as a detection of primordial B-modes" and can be attributed mainly to polarized dust in the Milky Way. If supersymmetry is a property of the universe, then it must be broken at an energy that

14161-395: Was the number of available particle species. Thus H is orders of magnitude lower than the rate of collisions per particle species. This means there was plenty of time for thermalization at this stage. At this epoch, the collision rate is proportional to the third root of the number density, and thus to a − 1 {\displaystyle a^{-1}} , where

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