The Onset of Star Formation

Abstract

Stars form out of interstellar matter. With modern telescopes and instruments this can nowadays be observed directly and in many phases of the formation process. 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 Chap. 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 Chap. 25 holds only if the free-fall time is much shorter than the timescale of thermal adjustment. As we will see later, just the opposite is the case here, during the earliest phases of star formation.