Protostar

The modern picture of protostars, summarized above, was first suggested by Chushiro Hayashi in 1966.{{Cite journal |title=The Evolution of Protostars |last=Hayashi |first=Chushiro |date=1966 |journal=Annual Review of Astronomy and Astrophysics |volume=4 |pages=171–192 |doi=10.1146/annurev.aa.04.090166.001131 |bibcode=1966ARA&A...4..171H }} In the first models, the size of protostars was greatly overestimated. Subsequent numerical calculations{{Cite journal|title = Numerical Calculations of the Dynamics of a Collapsing Protostar|last = Larson, R. B.|date = 1969|journal = Monthly Notices of the Royal Astronomical Society |volume=145 |issue = 3|pages=271–295|doi = 10.1093/mnras/145.3.271|bibcode = 1969MNRAS.145..271L |doi-access = free}}{{Cite journal|title = Formation of Solar-Type Stars in Spherical Symmetry: I. The Key Role of the Accretion Shock|author1=Winkler, K.-H. A. |author2=Newman, M. J. |name-list-style=amp |date = 1980|journal = Astrophysical Journal |volume=236 |pages=201|doi = 10.1086/157734|bibcode = 1980ApJ...236..201W |doi-access=free }}{{Cite journal|title = The Evolution of Protostars: I. Global Formulation and Results|last = Stahler, S. W., Shu, F. H., and Taam, R. E.|date = 1980|journal = Astrophysical Journal |volume=241 |pages=637|doi = 10.1086/158377|bibcode = 1980ApJ...241..637S }} 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.

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Star formation begins in relatively small molecular clouds called dense cores.{{Cite journal|title = Dense Cores in Dark Clouds: II. NH3 Observation and Star Formation|author1=Myers, P. C. |author2=Benson, P. J. |name-list-style=amp |date = 1983|journal = Astrophysical Journal |volume=266 |pages=309|doi = 10.1086/160780|bibcode = 1983ApJ...266..309M }} 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.{{Cite journal|title = Self-Similar Collapse of Isothermal Spheres and Star Formation|last = Shu, F. H.|date = 1977|journal = Astrophysical Journal |volume=214 |pages=488|doi = 10.1086/155274|bibcode = 1977ApJ...214..488S }} 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.{{Cite journal|title = B335 - A Laboratory for Astrochemistry in a Collapsing Cloud|last = Evans, N. J., Lee, J.-E., Rawlings, J. M. C., and Choi, M.|date = 2005|journal = Astrophysical Journal |volume=626 |issue= 2|pages=919–932|doi = 10.1086/430295|arxiv = astro-ph/0503459 |bibcode = 2005ApJ...626..919E |s2cid = 16270619}}

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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.

File:A diamond in the dust.jpg.{{cite web|title=A diamond in the dust|url=http://www.spacetelescope.org/images/potw1607a/|access-date=16 February 2016}}]]

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 (hydrogen-2) 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.{{Cite journal|title = Deuterium and the Stellar Birthline|last = Stahler, S. W.|date = 1988|journal = Astrophysical Journal |volume=332 |pages=804|doi = 10.1086/166694|bibcode = 1988ApJ...332..804S }}

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.{{Cite journal|title = The Spectral Evolution of Young Stellar Objects|last = Adams, F. C., Lada, C. J., and Shu, F. H.|date = 1987|journal = Astrophysical Journal |volume=312 |pages=788|doi = 10.1086/164924|bibcode = 1987ApJ...312..788A |hdl = 2060/19870005633|hdl-access = free}}{{Cite journal|title = Submillimeter Continuum Observations of rho Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps|last = Andre, P, Ward-Thompson, D. and Barsony, M.|date = 1993|journal = Astrophysical Journal |volume=406 |pages=122|doi = 10.1086/172425|bibcode = 1993ApJ...406..122A |doi-access = free}} However, there is still no definitive evidence for this identification.

{{for|details of observational classification|Young stellar object}}

A Young Star Flaunts its X-ray Spots.ogv|Video about the protostar V1647 Orionis and its X-ray emission (2004).

PIA18928-Protostar-HOPS383-20150323.jpg|Protostar outburst - HOPS 383 (2015).

Protostar Herbig-Haro 46 47.jpg|Protostar in Herbig-Haro 46/47.

Witness the Birth of a Star.jpg|A protostar inside a Bok globule (Artist's image).

ESO-IRS2.jpg|Stellar cluster RCW 38, around the young star IRS2, a system of two massive stars and protostars.

L1527.tif|Structures of dust forming around a protostar (in the central pink line) in nebula L1527 (2022).

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

{{reflist}}

{{commons category|Protostars}}

• [http://www.spacedaily.com/reports/Planet_Forming_Disks_Might_Put_Brakes_On_Stars_999.html Planet-Forming Disks Might Put Brakes On Stars] (SpaceDaily) July 25, 2006
• [http://www.theregister.com/2006/07/27/planets_brake/ Planets could put the brakes on young stars] Lucy Sherriff (The Register) Thursday 27 July 2006 13:02 GMT
• [http://www.space.com/scienceastronomy/060724_star_spin.html Why Fast-Spinning Young Stars Don't Fly Apart] (SPACE.com) 24 July 2006 03:10 pm ET

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