STAR FORMATION

Star formation is the process in which dense parts of molecular clouds collapse into a ball of plasma that lead to the birth of a star. The study Star Formation is a branch of astronomy that includes the study of the interstellar medium and giant molecular clouds (GMC) as precursors to the process of star formation and the study of young stellar objects and planetary formation as its immediate products.

The formation of stars is an ongoing and relentless process that continues to the present day.Star formation can be most easily observed in the Milky Way, and these observations, have led to a working model for the formation of low mass stars like our Sun. In this picture, stars are born from the gravitational collapse of the cold, molecular interstellar material (gas and dust) that resides in structures known as Interstellar Molecular Clouds. In the dynamical collapse and accretion of in-falling matter, a circumstellar disk forms around the accreting star as a consequence of the finite Angular Momentum of the molecular cloud.

Stars obtain a significant fraction of their mass by accretion through their disks, and disk matter is a reservoir for the formation of stellar and planetary mass companions. Additional processes such as winds and magnetic accretion flows are believed to be responsible for the removal of angular momentum that allows the star to continue to grow in mass. These processes are combined in our current view of how Sun-like stars obtain their initial masses and angular momentum, i.e., the initial conditions for subsequent Stellar Evolution. The extent, to which this picture, developed to explain the formation of low mass stars, applies to the formation of more massive stars, is an active area of research.

Despite the success of the above picture, there are numerous issues that remain to be addressed, including several basic aspects of this picture. For example, it is as yet unclear theoretically whether the magnetic interaction between the star and disk is likely to be time steady or not. Another important issue is the origin of angular momentum transport in disks. Gravitational instabilities are efficient when the circumstellar disk mass is a fair fraction of the stellar mass, a situation which may be relevant at early times. Our understanding of the stellar rotational properties of young stars is still in its early stages, despite several conclusions having already been drawn from the existing data. From an observational perspective, observations of large numbers of stars will be needed to study rotation as a function of mass and age and to average over the differences that may be inherent in trying to compare stars with different formation histories (e.g. molecular cloud initial conditions, etc). Another generic issue is the extent to which this picture, developed to explain the formation of isolated, low-mass stars, applies to stars in clusters and/or to stars of higher masses. It may be necessary to consider other physical processes (e.g. fragmentation, mergers of stars and/or clouds) in order to understand the origin of stellar masses and angular momentum in dense clusters. Further research is needed to understand these processes.

Questions to Ponder

• What is the evidence supporting the nebula theory of Solar System formation?
• How does a star take mass from another star?
• Could a planetary system survive if its star merged with another, or if its star went supernova?