Main Sequence Stars and their Formation

There are stars which come in all sizes from low mass to high mass.

Our sun is an example of a star with a mass above low but lower than medium. When characteristics are plotted of temperature and brightness for various normal stars they fall on an orderly line called the main sequence. They stay on this line for billions of years until their hydrogen nuclear fuel is mostly used up.

By the term “main sequence” we mean those stars which are fusing hydrogen to helium in its core. However main sequence stars can be quite different to each other. Their masses vary from little red dwarfs which are cool and dim to quite massive O-type stars which are hot and bright.

Although there is variation between stars, any individual star stays at a roughly constant temperature and luminosity as long as it’s on the main sequence.

Confusingly, all stars on the main sequence are called dwarfs. (There are historical reasons for this.) Giants or supergiants are stars that have left the main sequence and are carrying out a different type of fusion in their cores.

Massive stars use up all their hydrogen quickly and leave the main sequence. Red dwarfs stay on the main sequence for so long that no red dwarfs are thought to have left the main sequence since the beginning of the Universe.

How do they form

Astronomers have given good ideas of how stars form, some of them are current issues of star-formation astrophysics. First of all, we can consider a collapsing cloud of gas, yet no-one can say for sure how many stars will form from it and what their masses will be. Nor can we say at the start how many will form in binary systems. 

Stars form when a cloud of gas becomes unstable to gravitational collapse. A cloud of gas has some density of particles (usually only a few per cubic centimetre), which are moving around and thus exert some pressure. As long as the pressure is enough to balance the force of gravity, the cloud can keep being a cloud. There is, however, a critical size beyond which a cloud becomes unstable. People usually use the Jeans' mass: the smallest mass that is unstable to collapse because of density fluctuations in a uniform cloud. The instability can be excited because of anything that might compress the cloud. Nearby supernovae and galaxy collisions are example mechanisms.

Now, suppose we have a cloud that has become unstable and starts to collapse. As the cloud shrinks, it heats up and becomes denser. It's initial temperature is very, very cold, like tens of Kelvin. As long as the density is low enough for radiation to escape, then gas can keep cooling and contracting. Somewhere along the line, the density becomes too great and the radiation becomes trapped in the gas and heats it up. We say that the gas is optically thick. This happens at the centre in a core first because that's the densest part. Here, the pressure now comes back into mechanical equilibrium: pressure balances gravity. But the source of the energy being used to support the core is the contraction itself so, although its now much slower, the whole thing is still collapsing. This core is a protostar.

The protostar is really small, maybe one-tenth as massive as the Sun (and potentially even smaller). To continue growing, the cloud has to keep collapsing onto it. But as it heats up, more and more of the cloud becomes optically thick and the collapse slows down. Somewhere along the line, the angular momentum of the cloud becomes important. The presence of angular momentum means that the gas is basically swirling around the protostar as it collapses. The gas collides with itself along the axis of rotation and this roughly cancels out so that the gas keeps its rotation speed but loses its up/down component. It forms an accretion disk. From the disk, material can get to the protostar without being pushed away because it pushes gas away along the poles, instead. The accretion disk can itself become unstable to form smaller fragments. This is ultimately where planets will form.

This individual collapsing cloud will be one of many, many such clouds embedded a much larger cloud. Perhaps think of it like this. A big cloud becomes unstable and starts to collapse. It isn't perfectly smooth so, as it collapses, subcloud become unstable too. The pace of collapse increases slightly as you trigger a slightly smaller cloud. The overall result is that you form a cluster of stars, rather than just one star. Now, the Sun clearly isn't in a cluster now but it once was. So what happens to the cluster? It turns out that the gas doesn't hang around long in astronomical terms. Only a few millions or tens of millions of years. The biggest stars have already died, the gas is driven off by all the stellar radiation and the smaller stars go on their merry ways over hundreds of millions of years.

Still, as said above, the process of star formation is an active area of Astrophysical research.