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. For a one solar-mass star it lasts about 1,000,000 years. The
phase begins when a molecular cloud first collapses under the force of
self-gravity. It ends when the protostar blows back the infalling gas and is
revealed as an optically visible pre-main-sequence star, which later
contracts to become a main sequence star.
History The modern picture of protostars,
summarized above, was first suggested by Hayashi. In the first models, the size
of protostars was greatly overestimated. Subsequent numerical calculations
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
Star formation begins in relatively small molecular clouds called dense
cores. 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. 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. 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.
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 is not yet
undergoing nuclear fusion. Theory predicts, however, that the hydrogen
isotope deuterium is undergoing fusion, 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.
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. However, there is still
no definitive evidence for this identification.
Chemical Composition The Sun as a protostar had the same
composition as today, which is 71.1% hydrogen, 27.4% helium, and 1.5% heavier
elements, by mass. Observed Classes of Young Stars
Gallery See also
Stellar Birthline Pre-main-sequence star
Protoplanetary disk Star Formation
Stellar Evolution Notes
References External links
Planet-Forming Disks Might Put Brakes On Stars Jul 25, 2006
Planets could put the brakes on young stars Lucy Sherriff Thursday 27 July
2006 13:02 GMT Why Fast-Spinning Young Stars Don't Fly
Apart 24 July 2006 03:10 pm ET