Our first insight into the birth of stars came from the discovery
at radio wavelengths of cold (typically 10 K) clouds of gas and dust
in interstellar space. Sizes and masses of these ``molecular clouds''
range from the Dark Cloud Complexes, which are about 10-20 pc in
extent and have masses of about 10
-10
M
, to Giant
Molecular Clouds, about 20-80 pc in extent with masses of
10
- 10
M.
Within these clouds, denser ``cores'' were observed, believed to be
the earliest stages of stellar formation. The cores result from the
gravitational collapse of regions within the cloud. Low-mass stars
generally form in the Dark Cloud Complexes, while high-mass stars are
found in the Giant Molecular Clouds. The cores from which solar-type
stars form have sizes ranging between roughly 0.05-0.2 pc, and masses
of perhaps 0.3-10 M.
The visual extinction in molecular clouds is enormous, so that observations of these earliest stages of a star's life are only possible at radio, and perhaps infrared, wavelengths. Such observations have only been feasible for the past 15-20 years. Detailed observations are still difficult due to the large obscuration and confusion with ambient material, but sufficient observational information has been obtained to enable theorists to begin to model the collapse of cores and the formation of stars.
Thermal and turbulent motions within the cloud support against collapse. Magnetic fields also play a role by inhibiting a spherically symmetric collapse of charged particles. Eventually, a temperature decrease or density enhancement allows a gravitational collapse to commence. If even a small amount of overall angular momentum is initially present in the collapsing region, conservation of angular momentum will create a flattened structure upon collapse. The most likely scenarios produce multiple star systems, circumstellar disks, or rings, depending on initial conditions. As it is highly probable there will be some small amount of angular momentum in the region of the cloud undergoing collapse, the formation of disk or ring systems (or multiple systems) is expected to be a common occurrence. This dissertation focuses on stars having circumstellar disks (or rings), from which planetary systems may evolve. For reference purposes, the initial protosun and associated ``cocoon'' of gas and dust is commonly referred to as the ``solar nebula.''
The above scenario is a simplistic view of a complicated process. The mass of the collapsing region may be several times larger than the final star-disk system, requiring a means of removing mass from the system. In addition, when a region of size 0.05-0.2 pc collapses to the size of a planetary system, conservation of angular momentum causes the orbital speed of the material to increase drastically. In fact, conservation of angular momentum should cause the star to rotate at its breakup speed! Since stars do form, there must be some mechanism to shed angular momentum. The details of this mechanism are currently one of the great mysteries of star formation.