Re: Why are some stars bigger than the others?

This is quite a hard question to answer!

Let's consider the question in two different ways. Firstly, let's ask why some stars are more massive than others? In other words what determines the amount of material in a star? Stars seem to be formed during the gravitational collapse of very large clouds of gas, and usually many stars are formed in the cloud at the same time. Once some stars have begun to shine, their radiation (especially ultra-violet light) blows the gas cloud apart, eventually preventing it from forming more stars.

When we look at such stellar nurseries we find a wide range of masses, which most stars being rather less massive than the Sun (between 10% and 50% the mass of the Sun), but some being much more massive, up to 50 or even 100 times the mass of the Sun!

What sets the upper limit seems to be that very massive stars begin to collapse they generate so much energy in their cores, even as they are forming, that they blow themselves apart before they can become a stable star. At the other end of the scale, very low mass bodies fail to achieve high enough temperature and pressure in their cores to trigger significant nuclear reactions, and so simply become "brown dwarfs" or massive planets.

Now let's ask the question in another way: what determines the volume of a star? This is much more complicated since stars change their volume considerably during their lifetimes, even though their masses hardly change. The Sun, for example, is currently about 1.4 million km in diameter, but towards the end of its life it is expected to swell up to become a red giant, with a diameter more like 200 million km. This is an increase in volume of more than a million! After that phase, once all the nuclear fuel is used up in the core of the Sun, it is expected it will contract down to a "white dwarf" only a few thousand km across, losing some of its outer material in the process. In other words, an even more dramatic contraction in volume!

These predictions are based on our observations of billions of other stars in our galaxy, and also on theoretical modelling of how stars should behave. In the theory, stars are pictured as having relatively small, dense "cores" at their centres, which is where all the nuclear energy is produced. Surrounding the core is a much larger, but less dense envelope of gas which is generally not involved in energy generation, but instead acts like a thick blanket which ensures that the heat from the centre only comes very slowly out towards the surface of the star. The balance between the heat produced and the insulation provided by this outer blanket, determines the size of the star at any given time. The more insulating it is, for example, the more it traps the heat, and the more the star is forced to expand. This is a bit like a hot-air balloon which expands as it is heated, and contracts as it cools down.