Theories of star formation

Abstract

A well defined theory of star formation does not yet exist. A serious deficiency therefore remains in current theories of the structure and evolution of stars. Since stars must be forming at the present phase of Galactic evolution, it is pertinent to investigate what conditions favour star formation. Observational evidence for the pre-main sequence phase of stellar evolution is entirely absent apart from that obtained about a single class of proto-stellar objects - the T-Tauri stars. Most theories of star formation require stars to form from the interstellar gas. The existing knowledge of the interstellar gas is not sufficient to define the initial conditions for star formation precisely. However, recent theoretical and observational studies are forcing a radical revision of interstellar concepts. Theoretical studies suggest that interstellar gas clouds may well be transient phenomena and that continuous variation of interstellar conditions may be expected. The observational studies have shown that the previously accepted enumeration of interstellar species may well be far from complete.Four major groups of star formation theory are identified. These deal with formation by collapse under gravity, by random accretion, by condensation, and by processes associated with the activity of galactic nuclei. Considerable attention is given to the theory of collapse under gravity since this is regarded as the major theory of star formation on the ground that its conceptional framework is better defined than that of the others. It is also a theory capable of further specific investigation. It is, however, pointed out that no theory of star formation can be regarded as definitive and that each group of theories has significant contributions to make to a subject where the essential problems - though simple in conception - may not be well posed.The recent numerical investigations of star formation within the context of star formation through collapse under gravity are summarized. These new investigations have shown that one of the basic problems of collapse under gravity (namely the separation of a collapsing region of stellar mass from a very much larger parent cloud or fragmentation) may no longer be a problem in that fragmentation may be inevitable within non-uniform gas clouds collapsing under gravity. The numerical studies have shown that collapse under gravity takes a characteristic form. Such studies have enhanced the prospects for a renewed attack on the problem of collapse under gravity, in view of the possibility of relaxation of the severe constraint imposed by the assumption of spherical symmetry. Numerical studies, by taking account of atomic processes and the detailed structures of gas clouds, seem likely to lead to a clearer definition of the conditions necessary for star formation and to give form to a detailed theory of star formation replacing the more speculative attempts to do so which are reviewed here.