  Star formation is one of the least understood processes in cosmic evolution.
It is difficult to formulate a general theory for star formation in part
because of the wide range of physical processes involved. The interstellar gas
out of which stars form is a supersonically turbulent plasma governed by
magnetohydrodynamics. This is hard enough by itself, since we do not understand
even subsonic hydrodynamic turbulence very well, let alone supersonic non-ideal
MHD turbulence. However, the behavior of star-forming clouds in the ISM is also
obviously influenced by gravity, which adds complexity, and by both continuum
and line radiative processes. Finally, the behavior of star-forming clouds is
influenced by a wide variety of chemical processes, including formation and
destruction of molecules and dust grains (which changes the thermodynamic
behavior of the gas) and changes in ionization state (which alter how strongly
the gas couples to magnetic fields). As a result of these complexities, there
is nothing like a generally agreed-upon theory of star formation, as there is
for stellar structure. Instead, we are forced to take a much more
phenomenological approach. These notes provide an introduction to our current
thinking about how star formation works.
