The Onset of Star Formation

Abstract

Observational evidence favours the picture that stars form out of interstellar matter. Indeed a homogeneous cloud of compressible gas can become gravitationally unstable and collapse. In this section we shall deal with gravitational instability and then discuss some of its consequences. But before we do so it may be worth comparing this instability with those discussed in §25. For gravitational instability the inertia terms are important as well as heat exchange of the collapsing mass with its surroundings. But it is not a vibrational instability, since the classification scheme of §25 holds only if the free-fall time is much shorter than the time-scale of thermal adjustment. As we will see later, just the opposite is the case here.