Question: How are stars formed?

Susan Cartwright answered on 23 Jun 2015:

In space, there is lots of gas floating around. Most of this gas is very low density, but some of it is gravitationally bound into large clouds (called Giant Molecular Clouds or GMCs). If part of a GMC becomes denser than a certain threshold, the dense bit will start to collapse under its own gravity. This is the first stage in star formation.

Initially, the collapse is very quick, because the gravitational potential energy lost as the cloud collapses can be converted to heat and radiated away. However, as the cloud becomes denser, it becomes opaque to its own radiation. This slows the collapse down, but also makes the young star start getting hotter in the middle.

The gas cloud will almost always have some rotational motion, and as it collapses its spin gets faster and faster (this is due to conservation of angular momentum). If it spins too fast, it will break apart into two or more clumps. This is how binary and multiple star systems are formed. More than half of all stars are binaries – our Sun is mildly unusual in being a single star. The spin also causes a flat disc to develop around the young star. This will later form the star’s planetary system. We suspect that most stars have planets.

The centre of the star continues to heat up, until eventually it reaches 10 million degrees. At this point, hydrogen can start to fuse to make helium in the core of the star. The star has now officially been born and can take its place on the Main Sequence of hydrogen-fusing stars.

This process only works for objects of mass >0.08 solar masses. Anything less massive will never get hot enough to ignite hydrogen fusion. Such low-mass “failed stars” are called “brown dwarfs”.