The Stellar Birthrate

Abstract

In spite of the fact that the Milky Way has an age of more than 10 Gyr it contains stars of spectral type O which have lifetimes of the order of no more than 3 million years. Therefore, star formation is still an ongoing process in the Galaxy. Such a conclusion is reinforced by the fact that OB stars are almost always surrounded by dense clouds of gas and dust. The first appearance of a hot and brilliant star in a cloud of molecular hydrogen produces immediately an HII region in the area surrounding the star. This HII region has a temperature and density much larger than the ISM and therefore soon or later the larger thermal pressure of the HII region will cause its expansion into the surrounding gas at velocities comparable with the thermal velocity of the HII gas. In this way, the ISM can reach velocities of the order of 10 km sec⁻¹. The expansion of the HII region can induce gravitational instabilities in the nearby regions thus producing the formation of new OB stars. The process of star formation is therefore a self-propagating one. Optical and infrared observations have shown that also the low mass stars appear in giant molecular complexes although they are found preferentially in dark nebulae. A difference between the low and high mass stars is that the former are generally isolated thus indicating that they do not form by means of a hierarchical fragmentation process. On the other hand, massive stars form only in groups and inside the giant molecular clouds. Although most of the physics concerning star formation is still unknown, there is a general consensus in thinking that the formation of low mass stars is a primary process in the sense that it implies the fragmentation of clouds into pieces and then the direct collapse of these pieces into stars.