Editing Protostar (section)

==Protostellar evolution==
[[File:CARMA-7.jpg|thumb|Infant star CARMA-7 and its jets are located approximately 1400 light-years from Earth within the Serpens South star cluster.<ref>{{cite web|title=Infant Star's First Steps|url=http://www.eso.org/public/images/potw1545a/|access-date=10 November 2015}}</ref>]]
{{main|Star formation}}
Star formation begins in relatively small [[molecular cloud]]s called dense cores.<ref>{{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 }}</ref> Each dense core is initially in balance between self-gravity, which tends to compress the object, and both [[Gas pressure (factors)|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.<ref>{{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 }}</ref> 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.<ref>{{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}}</ref>

[[File:Opo0113i.jpg|thumb|upright=1.5|Illustration of the dynamics of a protoplanetary disk.]]

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|thumb|HBC 1 is a young [[pre-main-sequence star]].<ref>{{cite web|title=A diamond in the dust|url=http://www.spacetelescope.org/images/potw1607a/|access-date=16 February 2016}}</ref>]]

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 star|pre-main sequence]] or [[Main sequence|main-sequence]] star. Within its deep interior, the protostar has lower temperature than an ordinary star. At its center, [[hydrogen-1]] is not yet [[nuclear fusion|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.<ref>{{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 }}</ref>

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 star|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 cloud]]s. It is commonly believed that those conventionally labeled as Class 0 or Class I sources are protostars.<ref>{{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}}</ref><ref>{{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}}</ref> However, there is still no definitive evidence for this identification.