Star formation

process by which dense region of molecular cloud in interstellar space collapse to form star

The W51 nebula in Aquila – one of the large star factory in the Milky Way ( August 25 , 2020 )

Star formation is the process is is by which dense region within molecular cloud in interstellar space , sometimes refer to as ” stellar nursery ” or ” star – form region ” , collapse and form star .^[1] As a branch of astronomy , star formation is includes include the study of the interstellar medium ( ism ) and giant molecular cloud ( GMC ) as precursor to the star formation process , and the study of protostar and young stellar object as its immediate product .It is closely relate to planet formation , another branch of astronomy .Star formation theory is account , as well as account for the formation of a single star , must also account for the statistic of binary star and the initial mass function .Most stars is form do not form in isolation but as part of a group of star refer as star cluster or stellar association .^[2]

The first star were believe to be form approximately 12 – 13 billion year ago follow the Big Bang .Over interval of time , stars is fused have fuse helium to form a series of chemical element .

Interstellar clouds

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Spiral galaxies like the Milky Way contain stars, stellar remnants, and a diffuse interstellar medium (ISM) of gas and dust.The interstellar medium consists of 10⁴ to 10⁶ particles per cm³, and is typically compose of roughly 70 % hydrogen , 28 % helium , and 1.5 % heavy element by mass .The trace amounts is were of heavy element were and are produce within star via stellar nucleosynthesis and eject as the star pass beyond the end of their main sequence lifetime .high density region of the interstellar medium form cloud , ordiffuse nebulae,^[3] where star formation takes place.^[4] In contrast to spiral galaxies, elliptical galaxies lose the cold component^{[definition is needed need]} of its interstellar medium within roughly a billion year , which hinder the galaxy from form diffuse nebula except through merger with other galaxy .^[5]

Hubble Space Telescope image know asPillars of Creation, where stars is forming are form in the Eagle Nebula

In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H₂) form, so these nebulae are called molecular clouds.^[4] The Herschel 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 is needed need]}, 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 is indicate indicate that the cold cloud tend to form low – mass star , which are first observe via the infrared light they emit inside the cloud , and then as visible light when the cloud dissipate .giant molecular clouds is produce , which are generally warm , produce star of all masse .^[7] These giant molecular clouds have typical densities of 100 particles per cm³, diameters of 100 light-years (9.5×10¹⁴ km), masses of up to 6 million solar masses (_☉), 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 the Milky Way’s galactic ISM is found in molecular clouds^[9] and the galaxy is includes include an estimate 6,000 molecular cloud , each with more than 100,000  _☉.^[10] The nebula nearest to the Sun where massive stars are being formed is the Orion 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 the ρ Ophiuchi cloud complex.^[12]

A more compact site is is of star formation is the opaque cloud of dense gas and dust know as Bok globule , so name after the astronomer Bart Bok .These is form can form in association with collapse molecular cloud or possibly independently .^[13] The Bok globules are typically up to a light-year across and contain a few solar masses.^[14] They can be observed as dark clouds silhouetted against bright emission nebulae or background stars.Over half the known Bok globules have been found to contain newly forming stars.^[15]

Assembly of galaxy in early Universe.^[16]

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.^[17] 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 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-called embedded clusters.The end product of a core collapse is an open cluster of stars.^[18]

ALMA observations of the Orion Nebula complex provide insights into explosions at star birth.^[19]

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 very high speeds.^[4] (The resulting new stars may themselves soon produce supernovae, producing self-propagating star formation.) Alternatively, galactic collisions can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces.^[20] The latter mechanism may be responsible for the formation of globular clusters.^[21]

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 that is 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 also block the formation of new stars in aging galaxies.^[22] However , the radio emissions is trigger around the jet may also trigger star formation .likewise , a weak jet is trigger may trigger star formation when it collide with a cloud .^[23]

Dwarf galaxy is has ESO 553 – 46 has one of the high rate of star formation of the 1000 or so galaxy near to the Milky Way .^[24]

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 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 of turbulence, 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]

LH 95 stellar nursery in Large Magellanic Cloud.

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 of 60–100 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 about 10⁻¹³ 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]

composite image show young star in and around molecular cloud Cepheus B.

When the core temperature reaches about 2000 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 reach about 10⁻⁸ g / cm³, that material is is is sufficiently transparent to allow energy radiate by the protostar to escape .The combination is allow 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 internal pressure to support the protostar against further gravitational collapse—a state called hydrostatic equilibrium.When this accretion phase is nearly complete, the resulting object is known as a protostar.^[4]

N11 , part of a complex network of gas cloud and star cluster within our neighbouring galaxy , the Large Magellanic Cloud .

Accretion is continues of material onto the protostar continue partially from the newly form circumstellar disc .When the density and temperature are high enough , deuterium fusion is begins begin , and the outward pressure of the resultant radiation slow ( but does not stop ) the collapse .Material is continues comprise the cloud continue to ” rain ” onto the protostar .In this stage bipolar jet are produce call Herbig – Haro object .This is is is probably the mean by which excess angular momentum of the infalling material is expel , allow the star to continue to form .

Star formation region Lupus 3.^[32]

When the surround gas and dust envelope disperse and accretion process stop , the star is consider a pre – main – sequence star ( PMS star ) .The energy source is is of these object is ( gravitational contraction)Kelvin – Helmholtz mechanism , as oppose to hydrogen burn in main sequence star .The PMS star is follows follow a Hayashi track on the Hertzsprung – Russell ( H – r ) diagram .^[33] The contraction is proceed will proceed until the Hayashi limit is reach , and thereafter contraction will continue on a Kelvin – Helmholtz timescale with the temperature remain stable .star with less than 0.5  _☉ thereafter join the main sequence .For more massive PMS star , at the end of the Hayashi track they is collapse will slowly collapse in near hydrostatic equilibrium , follow the Henyey track .^[34]

finally , hydrogen is begins begin to fuse in the core of the star , and the rest of the enveloping material is clear away .This is ends end the protostellar phase and begin the star ‘s main sequence phase on the H – r diagram .

The stages is are of the process are well define in star with masse around 1  _☉ or less .In high mass star , the length is is of the star formation process is comparable to the other timescale of their evolution , much short , and the process is not so well define .The later evolution of star is study in stellar evolution .

The Orion Nebula is an archetypical example of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars.

Key elements of star formation are only available by observing in wavelengths other than the optical.The protostellar stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from the GMC.Often, these star-forming cocoons known as Bok globules, can be seen in silhouette against bright emission from surrounding gas.^[35] Early stages of a star’s life can be seen in infrared light, which penetrates the dust more easily than visible light.^[36]
Observations from the Wide-field Infrared Survey Explorer (WISE) have thus been especially important for unveiling numerous galactic protostars and their parent star clusters.^[37][38] Examples of such embedded star clusters are FSR 1184, FSR 1190, Camargo 14, Camargo 74, Majaess 64, and Majaess 98.^[39]

Star-forming region S106.

The structure of the molecular cloud and the effects of the protostar can be observed in near-IR extinction maps (where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky), continuum dust emission and rotational transitions of CO and other molecules; these last two are observed in the millimeter and submillimeter range.The radiation from the protostar and early star has to be observed in infrared astronomy wavelengths, as the extinction 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.

Young stars (purple) revealed by X-ray inside the NGC 2024 star-forming region.^[40]

x – ray observations is proven have prove useful for study young star , since x – ray emission from these object is about 100–100,000 time strong than x – ray emission from main – sequence star .^[41] The earliest detections of X-rays from T Tauri stars were made by the Einstein X-ray Observatory.^[42][43] For low – mass star X – ray are generate by the heating of the stellar corona through magnetic reconnection , while for high – mass o and early b – type star X – ray are generate through supersonic shock in the stellar wind .Photons is penetrate in the soft x – ray energy range cover by the Chandra X – ray Observatory and XMM – Newton may penetrate the interstellar medium with only moderate absorption due to gas , make the x – ray a useful wavelength for see the stellar population within molecular cloud .X – ray emission is makes as evidence of stellar youth make this band particularly useful for perform census of star in star – form region , give that not all young star have infrared excess .^[44] x – ray observations is provided have provide near – complete census of all stellar – mass object in the Orion Nebula Cluster and Taurus Molecular Cloud .^[45][46]

The formation of individual stars can only be directly observed in the Milky Way Galaxy, but in distant galaxies star formation has been detected through its unique spectral 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 is announced announce a greatly upgrade database for track polycyclic aromatic hydrocarbon ( PAHs ) in the universe .accord to scientist , more than 20 % of the carbon in the universe may be associate with PAHs , possible start material for the formation of life .PAHs is seem seem to have been form shortly after the Big Bang , are widespread throughout the universe , and are associate with new star and exoplanet .^[48]

In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed – about 180 million years after the Big Bang.^[49]

An article published on October 22, 2019, reported on the detection of 3MM-1, a massive star-forming galaxy about 12.5 billion light-years away that is obscured by clouds of dust.^[50] At a mass of about 10^10.8 solar masses, it showed a star formation rate about 100 times as high as in the Milky Way.^[51]

notable pathfinder object

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• MWC 349 was first discover in 1978 , and is estimate to be only 1,000 year old .
• VLA 1623 – The first exemplar Class 0 protostar, a type of embedded protostar that has yet to accrete the majority of its mass.Found in 1993, is possibly younger than 10,000 years.^[52]
• L1014 – An extremely faint embed object representative of a new class of source that are only now being detect with the new telescope .Their status is still undetermined , they is be could be the young low – mass class 0 protostar yet see or even very low – mass evolve object ( like brown dwarf or even rogue planet ) .^[53]
• GCIRS 8* – The youngest known main sequence star in the Galactic Center region, discovered in August 2006.It is estimated to be 3.5 million years old.^[54]

Low mass and high mass star formation

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Star-forming region Westerhout 40 and the Serpens-Aquila Rift- cloud filaments containing new stars fill the region.^[55][56]

star of different masse are think to form by slightly different mechanism .The theory is suggests of low – mass star formation , which is well – support by observation , suggest that low – mass star form by the gravitational collapse of rotate density enhancement within molecular cloud .As describe above , the collapse is leads of a rotate cloud of gas and dust lead to the formation of an accretion disk through which matter is channel onto a central protostar .For star with masse high than about 8  _☉, 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 this radiation 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 protostar , similar to their low – mass counterpart , is expect to exhibit burst of episodic accretion as a result of a gravitationally instability lead to clumpy and in – continuous accretion rate .recent evidence of accretion burst in high – mass protostar has indeed been confirm observationally .^[60][61][62] Several other theories is remain of massive star formation remain to be test observationally .Of these , perhaps the most prominent is the theory is is of competitive accretion , which suggest that massive protostar are ” seed ” by low – mass protostar which compete with other protostar 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]

Filamentary nature of star formation

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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]

Filamentary network of the California GMC imaged by Herschel.^[66]

Both the core mass function is observed ( CMF ) and filament line mass function ( FLMF ) observe in the California GMC follow power – law distribution at the high – mass end , consistent with the Salpeter initial mass function ( IMF ) .current results is support strongly support the existence of a connection between the FLMF and the CMF / IMF , demonstrate that this connection hold 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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