A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, a small number compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy. Dwarf galaxies' formation and activity are thought to be influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their composition. Current theory states that most galaxies, including dwarf galaxies, form in association with dark matter, or from gas that contains metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity; these galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo. Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies, resulting in galaxy merger.
There are many dwarf galaxies in the Local Group. A 2007 paper has suggested that many dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when their gravitational masses interact. Streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. A 2018 study suggests that some local dwarf galaxies formed early, during the Dark Ages within the first billion years after the big bang. More than 20 known dwarf galaxies orbit the Milky Way, recent observations have led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole at its centre, at some time absorbed by the Milky Way. Elliptical galaxy: dwarf elliptical galaxy Dwarf spheroidal galaxy: Once a subtype of dwarf ellipticals, now regarded as a distinct type Irregular galaxy: dwarf irregular galaxy Spiral galaxy: dwarf spiral galaxy Magellanic type dwarfs Blue compact dwarf galaxies Ultra-compact dwarf galaxies In astronomy, a blue compact dwarf galaxy is a small galaxy which contains large clusters of young, massive stars.
These stars, the brightest of which are blue, cause the galaxy. Most BCD galaxies are classified as dwarf irregular galaxies or as dwarf lenticular galaxies; because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become violent when forming. BCD galaxies cool in the process of forming new stars; the galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies. Nearby examples include NGC 1705, NGC 2915, NGC 3353 and UGCA 281. Ultra-compact dwarf galaxies are a class of compact galaxies with high stellar densities, discovered in the 2000s, they are thought to be on the order of 200 light years across. It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by tidal interactions, travelling through the hearts of rich clusters.
UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, the Coma Cluster, amongst others. In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team; the first relatively robust studies of the global properties of Virgo UCDs suggest that UCDs have distinct dynamical and structural properties from normal globular clusters. An extreme example of UCD is M60-UCD1, about 54 million light years away, which contains 200 million solar masses within a 160 light year radius. M59-UCD3 is the same size as M60-UCD1 with a half-light radius, rh, of 20 parsecs but is 40% more luminous with an apparent relative magnitude of −14.6. This makes M59-UCD3 the densest known galaxy. Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have supermassive black holes weighing 13% and 18% of the galaxies' masses. Galaxy morphological classification – System for categorizing galaxies based on appearance List of nearest galaxies Pea galaxy – Possibly a type of Luminous Blue Compact Galaxy, undergoing high rates of star formation Milky Way Satellite Galaxies SPACE.com article on "hobbit galaxies" Science article on "hobbit galaxies"
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, tasteless, non-toxic, monatomic gas, the first in the noble gas group in the periodic table, its boiling point is the lowest among all the elements. After hydrogen, helium is the second lightest and second most abundant element in the observable universe, being present at about 24% of the total elemental mass, more than 12 times the mass of all the heavier elements combined, its abundance is similar in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium; this helium-4 binding energy accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, the vast majority of, formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars. Helium is named for the Greek Titan of the Sun, Helios, it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R. Pogson, Lieutenant John Herschel, was subsequently confirmed by French astronomer Jules Janssen.
Janssen is jointly credited with detecting the element along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868 while Lockyer observed it from Britain. Lockyer was the first to propose; the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, by far the largest supplier of the gas today. Liquid helium is used in cryogenics in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers—account for half of the gas produced. A well-known but minor use is as a lifting gas in airships; as with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice.
In scientific research, the behavior of the two fluid phases of helium-4 is important to researchers studying quantum mechanics and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero. On Earth it is rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements, as the alpha particles emitted by such decays consist of helium-4 nuclei; this radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Terrestrial helium—a non-renewable resource, because once released into the atmosphere it escapes into space—was thought to be in short supply. However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases having been released by volcanic activity.
The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India; this line was assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 because it was near the known D1 and D2 Fraunhofer line lines of sodium, he concluded. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος. In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material, sublimated during a recent eruption of Mount Vesuvius. On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D3 line observed in the spectrum of the Sun.
These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, who collected enough of the gas to determine its atomic weight. Helium was isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, attributed the lines to nitrogen, his letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science. In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate the thin glass wall of
The Milky Way is the galaxy that contains our Solar System. The name describes 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 term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος. From Earth, the Milky Way appears as a band. 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 Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies; the Milky Way is a barred spiral galaxy with a diameter between 200,000 light-years. It is estimated to contain 100 -- more than 100 billion planets; the Solar System is located at a radius of 26,490 light-years 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*, assumed to be a supermassive black hole of 4.100 million solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at 220 kilometers per second; the constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter"; the rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of 600 km per second with respect to extragalactic frames of reference; the oldest stars in the Milky Way are nearly as old as the Universe itself and thus formed shortly after the Dark Ages of the Big Bang. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, itself a component of the Laniakea Supercluster.
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way, 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. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars; the area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a low surface brightness, its visibility can be 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 +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but 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 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. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga; the band continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two equal hemispheres. The galactic plane is inclined by about 60° to the ecliptic. 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° near β Comae Berenices, the south galactic pole is near α Sculptoris.
Because of this high inclination, depending on the time of night and year, the arch of the Milky Way may appear low or high in the sky. For observers from latitudes 65° north to 65° south, the Milky Way passes directly overhead twice a day; the Milky Way is the second-largest galaxy in the Local Group, with its stellar disk 100,000 ly in diameter and, on average 1,000 ly thick. The Milky Way is 1.5 trillion times the mass of the Sun. To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter, the Milky Way would be the size of the contiguous United States. There is a ring-like filament of stars rippling above and below the flat galactic plane, wrapping around the Milky Way at a diameter of 150,000–180,000 light-years, which may be part of the Milky Way itself. 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, somewhat less than that of the Andromeda Galaxy.
Measurements using the Very Long Baseline Array in 2009 found
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dark matter. The word galaxy is derived from the Greek galaxias "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers; the Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, observed as it existed just 400 million years after the Big Bang. Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested 2 trillion or more, containing more stars than all the grains of sand on planet Earth.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. For comparison, the Milky Way has a diameter of at least 30,000 parsecs and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs; the space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups and superclusters; the Milky Way is part of the Local Group, dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are arranged into sheets and filaments surrounded by immense voids; the largest structure of galaxies yet recognised is a cluster of superclusters, named Laniakea, which contains the Virgo supercluster. The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias, or kyklos galaktikos due to its appearance as a "milky" band of light in the sky.
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, it produces the faint band of light known as the Milky Way. In the astronomical literature, the capitalized word "Galaxy" is used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe; the English term Milky Way can be traced back to a story by Chaucer c. 1380: "See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt." Galaxies were discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
For this reason they were popularly called island universes, but this term fell into disuse, as the word universe implied the entirety of existence. Instead, they became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, the MCG and UGC. All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue of Messier, having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, PGC 37617; the realization that we live in a galaxy, one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae. The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World, continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger was critical of this view, arguing that if the Milky Way is sublunary it should appear different at different times and places on Earth, that it should have parallax, which it does not. In his view, the Milky Way is celestial. According to Mohani Mohamed, the Arabian astronomer Alhazen made the first attempt at observing and measuring the Milky Way's parallax, he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī
The Coma Cluster is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster, it is one of the two major clusters comprising the Coma Supercluster, it takes its name from the constellation Coma Berenices. The cluster's mean distance from Earth is 99 Mpc, its ten brightest spiral galaxies have apparent magnitudes of 12–14 that are observable with amateur telescopes larger than 20 cm. The central region is dominated by two supergiant elliptical galaxies: NGC 4874 and NGC 4889; the cluster is within a few degrees of the north galactic pole on the sky. Most of the galaxies that inhabit the central portion of the Coma Cluster are ellipticals. Both dwarf and giant ellipticals are found in abundance in the Coma Cluster; as is usual for clusters of this richness, the galaxies are overwhelmingly elliptical and S0 galaxies, with only a few spirals of younger age, many of them near the outskirts of the cluster. The full extent of the cluster was not understood until it was more studied in the 1950s by astronomers at Mount Palomar Observatory, although many of the individual galaxies in the cluster had been identified previously.
The Coma Cluster is one of the first places where observed gravitational anomalies were considered to be indicative of unobserved mass. In 1933 Fritz Zwicky showed that the galaxies of the Coma Cluster were moving too fast for the cluster to be bound together by the visible matter of its galaxies. Though the idea of dark matter would not be accepted for another fifty years, Zwicky wrote that the galaxies must be held together by some dunkle Materie. About 90% of the mass of the Coma cluster is believed to be in the form of dark matter. However, the distribution of dark matter throughout the cluster is poorly constrained. An extended X-ray source centered at 1300+28 in the direction of the Coma cluster of galaxies was reported before August 1966; this X-ray observation was performed by balloon, but the source was not detected in the sounding rocket flight launched by the X-ray astronomy group at the Naval Research Laboratory on November 25, 1964. A strong X-ray source was observed by the X-ray observatory satellite Uhuru close to the center of the Coma cluster and this source was suggested be designated Coma X-1.
The Coma cluster contains about 800 galaxies within a 100 x 100 arc-min area of the celestial sphere. The source near the center at RA 12h56m ± 2m Dec 28°6' ± 12' has a luminosity Lx = 2.6 x 1044 ergs/s. As the source is extended, with a size of about 45', this argues against the possibility that a single galaxy is responsible for the emission; the Uhuru observations indicated a source strength of no greater than ~10−3 photons cm−2s−1keV−1 at 25 keV, which disagrees with the earlier observations claiming a source strength of ~10−2 photons cm−2s−1keV−1 at 25 keV, a size of 5°. List of Abell clusters List of galaxy groups and clusters Eridanus Cluster Fornax Cluster – another nearby cluster of galaxies Norma Cluster Virgo Cluster – another nearby cluster of galaxies X-1 X-ray source The Coma Cluster of Galaxies - Astronomy Picture of the Day, NASA, 2000 August 6 The dynamical state of the Coma cluster with XMM-Newton Very Small Array observations of the Sunyaev-Zel'dovich effect in nearby galaxy clusters Hubble Space Telescope Treasury Survey of the Coma Cluster The Coma Cluster on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Large Magellanic Cloud
The Large Magellanic Cloud is a satellite galaxy of the Milky Way. At a distance of about 50 kiloparsecs, the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal and the possible dwarf irregular galaxy known as the Canis Major Overdensity. Based on visible stars and a mass of 10 billion solar masses, the diameter of the LMC is about 14,000 light-years, making it one one-hundredth as massive as the Milky Way; this makes the LMC the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy, the Milky Way, the Triangulum Galaxy. The LMC is classified as a Magellanic spiral, it contains a stellar bar, geometrically off-center, suggesting that it was a barred dwarf spiral galaxy before its spiral arms were disrupted by tidal interactions from the Small Magellanic Cloud, the Milky Way's gravity. With a declination of about -70°, the LMC is visible as a faint "cloud" only in the southern celestial hemisphere and from latitudes south of 20° N, straddling the border between the constellations of Dorado and Mensa, appears longer than 20 times the Moon's diameter from dark sites away from light pollution.
The Milky Way and the LMC are expected to collide in 2.4 billion years. Although both clouds have been visible for southern nighttime observers well back into prehistory, the first known written mention of the Large Magellanic Cloud was by the Persian astronomer'Abd al-Rahman al-Sufi Shirazi, in his Book of Fixed Stars around 964 AD; the next recorded observation was in 1503–4 by Amerigo Vespucci in a letter about his third voyage. In this letter he mentions "three Canopes, two bright and one obscure". Ferdinand Magellan sighted the LMC on his voyage in 1519, his writings brought the LMC into common Western knowledge; the galaxy now bears his name. Measurements with the Hubble Space Telescope, announced in 2006, suggest the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way; the Large Magellanic Cloud has a spiral arm. The central bar seems to be warped so that the east and west ends are nearer the Milky Way than the middle. In 2014, measurements from the Hubble Space Telescope made it possible to determine that the LMC has a rotation period of 250 million years.
The LMC was long considered to be a planar galaxy that could be assumed to lie at a single distance from the Solar System. However, in 1986, Caldwell and Coulson found that field Cepheid variables in the northeast portion of the LMC lie closer to the Milky Way than Cepheids in the southwest portion. More this inclined geometry for field stars in the LMC has been confirmed via observations of Cepheids, core helium-burning red clump stars and the tip of the red giant branch. All three of these papers find an inclination of ~35°, where a face-on galaxy has an inclination of 0°. Further work on the structure of the LMC using the kinematics of carbon stars showed that the LMC's disk is both thick and flared. Regarding the distribution of star clusters in the LMC, Schommer et al. measured velocities for ~80 clusters and found that the LMC's cluster system has kinematics consistent with the clusters moving in a disk-like distribution. These results were confirmed by Grocholski et al. who calculated distances to a number of clusters and showed that the LMC's cluster system is in fact distributed in the same plane as the field stars.
The distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular. Cepheids have been shown to have a relationship between their absolute luminosity and the period over which their brightness varies. However, Cepheids appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period–luminosity relations; the Cepheids in the Milky Way used to calibrate the period–luminosity relation are more metal rich than those found in the LMC. Modern 8-meter-class optical telescopes have discovered eclipsing binaries throughout the Local Group. Parameters of these systems can be measured without compositional assumptions; the light echoes of supernova 1987A are geometric measurements, without any stellar models or assumptions. In 2006, the Cepheid absolute luminosity was re-calibrated using Cepheid variables in the galaxy Messier 106 that cover a range of metallicities. Using this improved calibration, they find an absolute distance modulus of 48 kpc.
This distance has been confirmed by other authors. By cross-correlating different measurement methods, one can bound the distance; the results of a study using late-type eclipsing binaries to determine the distance more was published in the scientific journal Nature in March 2013. A distance of 49.97 kpc with an accuracy of 2.2% was obtained. Like many irregular galaxies, the LMC is rich in gas and dust, it is undergoing vigorous star formation activity, it is home to the Tarantula Nebula, the most active star-forming region in the Local Group. The LMC has a wide range of galactic objects and phenomena that make it aptly known as an "astronomical treasure-house, a great celestial laboratory for the study of the growth and evolution of the stars," as described by Robert Burnham Jr. Surveys of the galaxy have found 60 globular clusters, 400 planetary nebula
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process, the study of protostars and young stellar objects as its immediate products, it is related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations. A spiral galaxy like the Milky Way contains stars, stellar remnants, a diffuse interstellar medium of gas and dust; the interstellar medium consists of 10−4 to 106 particles per cm3 and is composed of 70% hydrogen by mass, with most of the remaining gas consisting of helium.
This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies. In the dense nebulae where stars are produced, much of the hydrogen is in the molecular form, so these nebulae are called molecular clouds. Observations indicate that the coldest clouds tend to form low-mass stars, observed first in the infrared inside the clouds in visible light at their surface when the clouds dissipate, while giant molecular clouds, which are warmer, produce stars of all masses; these giant molecular clouds have typical densities of 100 particles per cm3, diameters of 100 light-years, masses of up to 6 million solar masses, an average interior temperature of 10 K.
About half the total mass of the galactic ISM is found in molecular clouds and in the Milky Way there are an estimated 6,000 molecular clouds, each with more than 100,000 M☉. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula, 1,300 ly away. However, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex. A more compact site of star formation is the opaque clouds of dense gas and dust known as Bok globules, so named after the astronomer Bart Bok; these can form in association with collapsing molecular clouds or independently. The Bok globules are up to a light year across and contain a few solar masses, they can be observed as dark clouds silhouetted against bright emission background stars. Over half the known Bok globules have been found to contain newly forming stars. An interstellar cloud of gas will remain in hydrostatic equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force.
Mathematically this is expressed using the virial theorem, which states that, to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy. If a cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse; the mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud, but is thousands to tens of thousands of solar masses. During cloud collapse dozens to ten thousands of stars form more or less, observable in so-called embedded clusters; the end product of a core collapse is an open cluster of stars. In triggered star formation, one of several events might occur to compress a molecular cloud and initiate its gravitational collapse. Molecular clouds may collide with each other, or a nearby supernova explosion can be a trigger, sending shocked matter into the cloud at high speeds. Alternatively, galactic collisions can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces.
The latter mechanism may be responsible for the formation of globular clusters. A supermassive black hole at the core of a galaxy may serve to regulate the rate of star formation in a galactic nucleus. A black hole, accreting infalling matter can become active, emitting a strong wind through a collimated relativistic jet; this can limit further star formation. Massive black holes ejecting radio-frequency-emitting particles at near-light speed can block the formation of new stars in aging galaxies. However, the radio emissions around the jets may trigger star formation. A weaker jet may trigger star formation when it collides with a cloud; as it collapses, a molecular cloud breaks into smaller and smaller pieces in a hierarchical manner, until the fragments reach stellar mass. In each of these fragments, the collapsing gas radiates away the energy gained by the release of gravitational potential energy; as the density increases, the fragments become opaque and are thus less efficient at radiating away their energy.
This inhibits further fragmentation. The fragments now condense into rotating spheres of gas. Complicating this picture of a collapsing cloud are the effects of turbulence, macroscopic flows, magnetic f