Star formation is the process by which dense regions withinmolecular clouds ininterstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—collapse and formstars.[1] As a branch ofastronomy, star formation includes the study of theinterstellar medium (ISM) andgiant molecular clouds (GMC) as precursors to the star formation process, and the study ofprotostars andyoung stellar objects as its immediate products. It is closely related toplanet formation, another branch ofastronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics ofbinary stars and theinitial mass function. Most stars do not form in isolation but as part of a group of stars referred asstar clusters orstellar associations.[2]
The first stars were believed to be formed approximately 12-13 billion years ago following theBig Bang. Over intervals of time, stars have fused helium to form a series ofchemical elements.
Spiral galaxies like the Milky Way containstars,stellar remnants, and a diffuseinterstellar medium (ISM) of gas and dust. The interstellar medium consists of 104 to 106 particles per cm3, and is typically composed of roughly 70%hydrogen, 28%helium, and 1.5%heavier elements by mass. The trace amounts of heavier elements were and are produced within stars viastellar nucleosynthesis and ejected as the stars pass beyond the end of theirmain sequence lifetime. Higher density regions of the interstellar medium form clouds, ordiffuse nebulae,[3] where star formation takes place.[4] In contrast to spiral galaxies,elliptical galaxies lose the cold component[definition needed] of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies.[5]
In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H2) form, so these nebulae are calledmolecular clouds.[4] TheHerschel Space Observatory has revealed that filaments, or elongated dense gas structures, are truly ubiquitous in molecular clouds and central to the star formation process. They fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending[definition needed], and magnetic fields may control the detailed manner in which the filaments are fragmented. Observations of supercritical filaments have revealed quasi-periodic chains of dense cores with spacing comparable to the filament inner width, and embedded protostars with outflows.[jargon][6]
Observations indicate that the coldest clouds tend to form low-mass stars, which are first observed via the infrared light they emit inside the clouds, and then as visible light when the clouds dissipate. Giant molecular clouds, which are generally warmer, produce stars of all masses.[7] These giant molecular clouds have typical densities of 100 particles per cm3, diameters of 100light-years (9.5×1014 km), masses of up to 6 millionsolar masses (M☉), or six million times the mass of Earth's sun.[8] The average interior temperature is 10 K (−441.7 °F).
About half the total mass of theMilky Way's galactic ISM is found in molecular clouds[9] and the galaxy includes an estimated 6,000 molecular clouds, each with more than 100,000 M☉.[10] The nebula nearest to theSun where massive stars are being formed is theOrion Nebula, 1,300 light-years (1.2×1016 km) away.[11] However, lower mass star formation is occurring about 400–450 light-years distant in theρ Ophiuchi cloud complex.[12]
A more compact site of star formation is the opaque clouds of dense gas and dust known asBok globules, so named after the astronomerBart Bok. These can form in association with collapsing molecular clouds or possibly independently.[13] The Bok globules are typically up to a light-year across and contain a fewsolar masses.[14] They can be observed as dark clouds silhouetted against brightemission nebulae or background stars. Over half the known Bok globules have been found to contain newly forming stars.[15]
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An interstellar cloud of gas will remain inhydrostatic equilibrium as long as thekinetic energy of the gaspressure is in balance with thepotential energy of the internalgravitational force. Mathematically this is expressed using thevirial theorem, which states that, to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy.[17] If a cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergogravitational collapse. The mass above which a cloud will undergo such collapse is called theJeans mass. The Jeans mass depends on the temperature and density of the cloud, but is typically thousands to tens of thousands of solar masses.[4] During cloud collapse dozens to tens of thousands of stars form more or less simultaneously which is observable in so-calledembedded clusters. The end product of a core collapse is anopen cluster of stars.[18]
Intriggered star formation, one of several events might occur to compress a molecular cloud and initiate itsgravitational collapse. Molecular clouds may collide with each other, or a nearbysupernova explosion can be a trigger, sendingshocked matter into the cloud at very high speeds.[4] (The resulting new stars may themselves soon produce supernovae, producingself-propagating star formation.) Alternatively,galactic collisions can trigger massivestarbursts of star formation as the gas clouds in each galaxy are compressed and agitated bytidal forces.[20] The latter mechanism may be responsible for the formation ofglobular clusters.[21]
Asupermassive black hole at the core of a galaxy may serve to regulate the rate of star formation in a galactic nucleus. A black hole that is accreting infalling matter can becomeactive, emitting a strong wind through a collimatedrelativistic jet. This can limit further star formation. Massive black holes ejecting radio-frequency-emitting particles at near-light speed can also block the formation of new stars in aging galaxies.[22] However, the radio emissions around the jets may also trigger star formation. Likewise, a weaker jet may trigger star formation when it collides with a cloud.[23]
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 ofgravitationalpotential energy. As the density increases, the fragments become opaque and are thus less efficient at radiating away their energy. This raises the temperature of the cloud and inhibits further fragmentation. The fragments now condense into rotating spheres of gas that serve as stellar embryos.[25]
Complicating this picture of a collapsing cloud are the effects ofturbulence, macroscopic flows,rotation,magnetic fields and the cloud geometry. Both rotation and magnetic fields can hinder the collapse of a cloud.[26][27] Turbulence is instrumental in causing fragmentation of the cloud, and on the smallest scales it promotes collapse.[28]
A protostellar cloud will continue to collapse as long as the gravitational binding energy can be eliminated. This excess energy is primarily lost through radiation. However, the collapsing cloud will eventually become opaque to its own radiation, and the energy must be removed through some other means. The dust within the cloud becomes heated to temperatures of60–100 K, and these particles radiate at wavelengths in the far infrared where the cloud is transparent. Thus the dust mediates the further collapse of the cloud.[29]
During the collapse, the density of the cloud increases towards the center and thus the middle region becomes optically opaque first. This occurs when the density is about10−13 g / cm3. A core region, called the first hydrostatic core, forms where the collapse is essentially halted. It continues to increase in temperature as determined by the virial theorem. The gas falling toward this opaque region collides with it and creates shock waves that further heat the core.[30]
When the core temperature reaches about2000 K, the thermal energy dissociates the H2 molecules.[30] This is followed by the ionization of the hydrogen and helium atoms. These processes absorb the energy of the contraction, allowing it to continue on timescales comparable to the period of collapse at free fall velocities.[31] After the density of infalling material has reached about 10−8 g / cm3, that material is sufficiently transparent to allow energy radiated by the protostar to escape. The combination of convection within the protostar and radiation from its exterior allow the star to contract further.[30] This continues until the gas is hot enough for the internalpressure to support the protostar against further gravitational collapse—a state calledhydrostatic equilibrium. When this accretion phase is nearly complete, the resulting object is known as aprotostar.[4]
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Accretion of material onto the protostar continues partially from the newly formedcircumstellar disc. When the density and temperature are high enough,deuterium fusion begins, and the outwardpressure of the resultant radiation slows (but does not stop) the collapse. Material comprising the cloud continues to "rain" onto theprotostar. In this stage bipolar jets are produced calledHerbig–Haro objects. This is probably the means by which excessangular momentum of the infalling material is expelled, allowing the star to continue to form.
When the surrounding gas and dust envelope disperses and accretion process stops, the star is considered apre-main-sequence star (PMS star). The energy source of these objects is (gravitational contraction)Kelvin–Helmholtz mechanism, as opposed to hydrogen burning in main sequence stars. The PMS star follows aHayashi track on theHertzsprung–Russell (H–R) diagram.[33] The contraction will proceed until theHayashi limit is reached, and thereafter contraction will continue on aKelvin–Helmholtz timescale with the temperature remaining stable. Stars with less than 0.5 M☉ thereafter join the main sequence. For more massive PMS stars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following theHenyey track.[34]
Finally,hydrogen begins to fuse in the core of the star, and the rest of the enveloping material is cleared away. This ends the protostellar phase and begins the star'smain sequence phase on the H–R diagram.
The stages of the process are well defined in stars with masses around 1 M☉ or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined. The later evolution of stars is studied instellar evolution.
| Protostar |
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 Protostar outburst -HOPS 383 (2015). |
Key elements of star formation are only available by observing inwavelengths other than theoptical. The protostellar stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from theGMC. Often, these star-forming cocoons known asBok globules, can be seen insilhouette against bright emission from surrounding gas.[35] Early stages of a star's life can be seen ininfrared light, which penetrates the dust more easily thanvisible light.[36] Observations from theWide-field Infrared Survey Explorer (WISE) have thus been especially important for unveiling numerous galactic protostars and their parentstar clusters.[37][38] Examples of such embedded star clusters are FSR 1184, FSR 1190, Camargo 14, Camargo 74, Majaess 64, and Majaess 98.[39]
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The structure of the molecular cloud and the effects of the protostar can be observed in near-IRextinction maps (where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky), continuum dust emission androtational transitions ofCO and other molecules; these last two are observed in the millimeter andsubmillimeter range. The radiation from the protostar and early star has to be observed ininfrared astronomy wavelengths, as theextinction caused by the rest of the cloud in which the star is forming is usually too big to allow us to observe it in the visual part of the spectrum. This presents considerable difficulties as the Earth's atmosphere is almost entirely opaque from 20μm to 850μm, with narrow windows at 200μm and 450μm. Even outside this range, atmospheric subtraction techniques must be used.
X-ray observations have proven useful for studying young stars, since X-ray emission from these objects is about 100–100,000 times stronger than X-ray emission from main-sequence stars.[41] The earliest detections of X-rays from T Tauri stars were made by theEinstein X-ray Observatory.[42][43] For low-mass stars X-rays are generated by the heating of the stellar corona throughmagnetic reconnection, while for high-massO and early B-type stars X-rays are generated through supersonic shocks in the stellar winds. Photons in the soft X-ray energy range covered by theChandra X-ray Observatory andXMM-Newton may penetrate the interstellar medium with only moderate absorption due to gas, making the X-ray a useful wavelength for seeing the stellar populations within molecular clouds. X-ray emission as evidence of stellar youth makes this band particularly useful for performing censuses of stars in star-forming regions, given that not all young stars have infrared excesses.[44] X-ray observations have provided near-complete censuses of all stellar-mass objects in theOrion Nebula Cluster andTaurus Molecular Cloud.[45][46]
The formation of individual stars can only be directly observed in theMilky Way Galaxy, but in distant galaxies star formation has been detected through its uniquespectral signature.
Initial research indicates star-forming clumps start as giant, dense areas in turbulent gas-rich matter in young galaxies, live about 500 million years, and may migrate to the center of a galaxy, creating the central bulge of a galaxy.[47]
On February 21, 2014,NASA announced agreatly upgraded database for trackingpolycyclic aromatic hydrocarbons (PAHs) in theuniverse. According to scientists, more than 20% of thecarbon in the universe may be associated with PAHs, possiblestarting materials for theformation oflife. PAHs seem to have been formed shortly after theBig Bang, are widespread throughout the universe, and are associated with new stars andexoplanets.[48]
In February 2018, astronomers reported, for the first time, a signal of thereionization epoch, an indirect detection of light from the earliest stars formed - about 180 million years after theBig Bang.[49]
An article published on October 22, 2019, reported on the detection of3MM-1, a massive star-forming galaxy about 12.5 billion light-years away that is obscured by clouds ofdust.[50] At a mass of about 1010.8solar masses, it showed a star formation rate about 100 times as high as in theMilky Way.[51]
Stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by observation, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. As described above, the collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channeled onto a central protostar. For stars with masses higher than about 8 M☉, however, the mechanism of star formation is not well understood.
Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that thisradiation pressure might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses.[57] Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the radiation from a massive protostar can escape without hindering accretion through the disk and onto the protostar.[58][59] Present thinking is that massive stars may therefore be able to form by a mechanism similar to that by which low mass stars form.
There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks.[60] Disk accretion in high-mass protostars, similar to their low-mass counterparts, is expected to exhibit bursts of episodic accretion as a result of a gravitationally instability leading to clumpy and in-continuous accretion rates. Recent evidence of accretion bursts in high-mass protostars has indeed been confirmed observationally.[60][61][62] Several other theories of massive star formation remain to be tested observationally. Of these, perhaps the most prominent is the theory of competitive accretion, which suggests that massive protostars are "seeded" by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region.[63][64]
Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.[65]
Recent studies have emphasized the role of filamentary structures in molecular clouds as the initial conditions for star formation. Findings from the Herschel Space Observatory highlight the ubiquitous nature of these filaments in the cold interstellar medium (ISM). The spatial relationship between cores and filaments indicates that the majority of prestellar cores are located within 0.1 pc of supercritical filaments. This supports the hypothesis that filamentary structures act as pathways for the accumulation of gas and dust, leading to core formation.[66]
Both the core mass function (CMF) and filament line mass function (FLMF) observed in the California GMC follow power-law distributions at the high-mass end, consistent with the Salpeter initial mass function (IMF). Current results strongly support the existence of a connection between the FLMF and the CMF/IMF, demonstrating that this connection holds at the level of an individual cloud, specifically the California GMC.[66] The FLMF presented is a distribution of local line masses for a complete, homogeneous sample of filaments within the same cloud. It is the local line mass of a filament that defines its ability to fragment at a particular location along its spine, not the average line mass of the filament. This connection is more direct and provides tighter constraints on the origin of the CMF/IMF.[66]
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