Introduction to Astrophysics Lecture 10: Star formation

Star formation Understanding star formation is one of the most pressing problems in contemporary astrophysics. As well as trying to understand how our own Sun might have formed, we want to understand the whole range of possible stellar properties.

The Initial Mass Function (IMF) One goal of star formation studies is to predict the Initial Mass Function of stars, usually known as the IMF. This tells us the number of stars forming with a given mass. There is some dispute over the observational form of the IMF, with popular examples being the Salpeter mass function and the Scalo mass function, which are quite different at the high-mass end.

Gravitational collapse Stars are thought to be born in the gravitational collapse of large clouds of gas and dust. The problem is complex because it is believed stars form in clusters, rather than individually. Gravitational collapse is possible if the gravitational potential energy of the cloud exceeds the kinetic energy, i.e. <v2> is the mean square velocity Putting M=4R3/3 gives the relation where the last equality uses typical interstellar densities and velocities.

Gravitational collapse As the cloud begins to collapse, the density  increases, which means that the minimum collapse mass decreases. We therefore expect that the initial cloud to collapse is about one thousand solar masses or more, and as it collapses it is able to fragment into smaller masses, so that the original cloud breaks up into a cluster of stars.

Movies can be viewed via link from course WWW page. Computer simulation of star cluster formation on the UK Astrophysical Fluids Facility (UKAFF). The initial cloud has a mass of fifty solar masses. Credit: Matthew Bate (Exeter)

Science from star formation simulations The computer simulations confirm the basic picture of fragmentation and collapse. They indicate: Many stars do form in the collapse of a single dust cloud. The collapse is very complex due to fragmentation. The stars interact frequently with each other after formation, through their gravitational attraction. The presence of stars can also induce new star formation. Some stars get thrown out of the gas cloud by these interactions.

From protostars to stars The cloud fragments into separate collapsing regions which become protostars. A protostar is a hot ball of gas, whose heat has been derived from the gravitational collapse of the original gas cloud. So far it is not undergoing nuclear burning. As it radiates heat it collapses further. Artist’s impression A star like the Sun is thought to spend 50 million years as a protostar.

From protostars to stars Eventually the density at the centre is sufficient to ignite the star, and nuclear burning begins. This happens when the temperature reaches several million degrees. By now any planet formation should also have taken place.

From protostars to stars During its early stages, our star will have a very strong stellar wind which will blow away all the remaining gas. During this phase it is known as a T-Tauri star, after the particular star that was the first known example. The T-Tauri phase is quite brief, and after it the star becomes a main sequence star.

Overview Star formation is a highly complex process, and a detailed understanding requires computer simulations which are just becoming feasible. Stars are expected to form in clusters rather than individually. Current theories have not been able to predict the Initial Mass Function of stars, which plays a crucial role in understanding the evolution of galaxies.