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A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution.[1] For a low mass star (i.e. that of the Sun or lower), it lasts about 500,000 years.[2] The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main-sequence star at the onset of hydrogen fusion producing helium.

History

The modern picture of protostars, summarized above, was first suggested by Chushiro Hayashi in 1966.[3] In the first models, the size of protostars was greatly overestimated. Subsequent numerical calculations[4][5][6] clarified the issue, and showed that protostars are only modestly larger than main-sequence stars of the same mass. This basic theoretical result has been confirmed by observations, which find that the largest pre-main-sequence stars are also of modest size.
Protostellar evolution
Infant star CARMA-7 and its jets are located approximately 1400 light-years from Earth within the Serpens South star cluster.[7]
Main article: Star formation

Star formation begins in relatively small molecular clouds called dense cores.[8] Each dense core is initially in balance between self-gravity, which tends to compress the object, and both gas pressure and magnetic pressure, which tend to inflate it. As the dense core accrues mass from its larger, surrounding cloud, self-gravity begins to overwhelm pressure, and collapse begins. Theoretical modeling of an idealized spherical cloud initially supported only by gas pressure indicates that the collapse process spreads from the inside toward the outside.[9] Spectroscopic observations of dense cores that do not yet contain stars indicate that contraction indeed occurs. So far, however, the predicted outward spread of the collapse region has not been observed.[10]

The gas that collapses toward the center of the dense core first builds up a low-mass protostar, and then a protoplanetary disk orbiting the object. As the collapse continues, an increasing amount of gas impacts the disk rather than the star, a consequence of angular momentum conservation. Exactly how material in the disk spirals inward onto the protostar is not yet understood, despite a great deal of theoretical effort. This problem is illustrative of the larger issue of accretion disk theory, which plays a role in much of astrophysics.
HBC 1 is a young pre-main-sequence star.[11]

Regardless of the details, the outer surface of a protostar consists at least partially of shocked gas that has fallen from the inner edge of the disk. The surface is thus very different from the relatively quiescent photosphere of a pre-main sequence or main-sequence star. Within its deep interior, the protostar has lower temperature than an ordinary star. At its center, hydrogen-1 is not yet fusing with itself. Theory predicts, however, that the hydrogen isotope deuterium fuses with hydrogen-1, creating helium-3. The heat from this fusion reaction tends to inflate the protostar, and thereby helps determine the size of the youngest observed pre-main-sequence stars.[12]

The energy generated from ordinary stars comes from the nuclear fusion occurring at their centers. Protostars also generate energy, but it comes from the radiation liberated at the shocks on its surface and on the surface of its surrounding disk. The radiation thus created must traverse the interstellar dust in the surrounding dense core. The dust absorbs all impinging photons and reradiates them at longer wavelengths. Consequently, a protostar is not detectable at optical wavelengths, and cannot be placed in the Hertzsprung–Russell diagram, unlike the more evolved pre-main-sequence stars.

The actual radiation emanating from a protostar is predicted to be in the infrared and millimeter regimes. Point-like sources of such long-wavelength radiation are commonly seen in regions that are obscured by molecular clouds. It is commonly believed that those conventionally labeled as Class 0 or Class I sources are protostars.[13][14] However, there is still no definitive evidence for this identification.
Observed classes of young stars
For details of observational classification, see Young stellar object.
Class peak emission duration (Years)
0 submillimeter 104
I far-infrared 105
II near-infrared 106
III visible 107[15]
Gallery
File:A Young Star Flaunts its X-ray Spots.ogvPlay media
Video about the protostar V1647 Orionis and its X-ray emission (2004).
Protostar outburst - HOPS 383 (2015).
A protostar inside a Bok globule (Artist's image).
Stellar cluster RCW 38, around the young star IRS2, a system of two massive stars and protostars.
See also

Stellar birthline
Pre-main-sequence star
Protoplanetary disk
Star formation
Stellar evolution

Notes

Stahler, S. W. & Palla, F. (2004). The Formation of Stars. Weinheim: Wiley-VCH. ISBN 3-527-40559-3.
Dunham, M. M.; et al. (2014). The Evolution of Protostars in Protostars and Planets VI. University of Arizona Press.arXiv:1401.1809. doi:10.2458/azu_uapress_9780816531240-ch009. ISBN 9780816598762. S2CID 89604015.
Hayashi, C. (1966). "The Evolution of Protostars". Annual Review of Astronomy and Astrophysics. 4: 171–192. Bibcode:1966ARA&A...4..171H. doi:10.1146/annurev.aa.04.090166.001131.
Larson, R. B. (1969). "Numerical Calculations of the Dynamics of a Collapsing Protostar". Monthly Notices of the Royal Astronomical Society. 145 (3): 271–295. Bibcode:1969MNRAS.145..271L. doi:10.1093/mnras/145.3.271.
Winkler, K.-H. A. & Newman, M. J. (1980). "Formation of Solar-Type Stars in Spherical Symmetry: I. The Key Role of the Accretion Shock". Astrophysical Journal. 236: 201. Bibcode:1980ApJ...236..201W. doi:10.1086/157734.
Stahler, S. W., Shu, F. H., and Taam, R. E. (1980). "The Evolution of Protostars: I. Global Formulation and Results". Astrophysical Journal. 241: 637. Bibcode:1980ApJ...241..637S. doi:10.1086/158377.
"Infant Star's First Steps". Retrieved 10 November 2015.
Myers, P. C. & Benson, P. J. (1983). "Dense Cores in Dark Clouds: II. NH3 Observations and Star Formation". Astrophysical Journal. 266: 309. Bibcode:1983ApJ...266..309M. doi:10.1086/160780.
Shu, F. H. (1977). "Self-Similar Collapse of Isothermal Spheres and Star Formation". Astrophysical Journal. 214: 488. Bibcode:1977ApJ...214..488S. doi:10.1086/155274.
Evans, N. J., Lee, J.-E., Rawlings, J. M. C., and Choi, M. (2005). "B335 - A Laboratory for Astrochemistry in a Collapsing Cloud". Astrophysical Journal. 626 (2): 919–932. arXiv:astro-ph/0503459. Bibcode:2005ApJ...626..919E. doi:10.1086/430295. S2CID 16270619.
"A diamond in the dust". Retrieved 16 February 2016.
Stahler, S. W. (1988). "Deuterium and the Stellar Birthline". Astrophysical Journal. 332: 804. Bibcode:1988ApJ...332..804S. doi:10.1086/166694.
Adams, F. C., Lada, C. J., and Shu, F. H. (1987). "The Spectral Evolution of Young Stellar Objects". Astrophysical Journal. 312: 788. Bibcode:1987ApJ...312..788A. doi:10.1086/164924. hdl:2060/19870005633.
Andre, P, Ward-Thompson, D. and Barsony, M. (1993). "Submillimeter Continuum Observations of rho Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps". Astrophysical Journal. 406: 122. Bibcode:1993ApJ...406..122A. doi:10.1086/172425.

"IMPRS" (PDF). www.solar-system-school.de.

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References
External links

Planet-Forming Disks Might Put Brakes On Stars (SpaceDaily) July 25, 2006
Planets could put the brakes on young stars Lucy Sherriff (The Register) Thursday 27 July 2006 13:02 GMT
Why Fast-Spinning Young Stars Don't Fly Apart (SPACE.com) 24 July 2006 03:10 pm ET

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Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism

Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system

Earth-centric
observations

Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard

Lists

Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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