The death of a star

QUESTION: When a star is towards the end of its life and it begins to cool as fuel runs out, why does it expand and not contract?

Answer:

I will answer it by the following shortened version of the life of a star such as our Sun after it has reached the 'normal' star state. As far as expansion and contraction are concerned it all depends which part of the stellar cycle you are interested in. Both occur between the 'normal' state and the black dwarf stage.

Initially the star shines in the 'normal' state, like our Sun, for many millions of years. In fact a star like our Sun will fuse hydrogen for some 10¹⁰ years. The central core finally "runs out" of hydrogen and begins to cool. This means that the radiation from it is reduced, the radiation pressure is less and so gravity takes over.

The core begins to collapse.

The layers of the star outside the core are still fusing hydrogen. They now fall inwards and actually heat up. The rate of fusion in this shell increases and the radiation emitted by this shell increases. The layer of the star outside this is therefore heated up by this increased radiation.

It expands.

This rate of expansion continues and the star expands to produce a Red Giant the size of the Earth's orbit. This expansion is so rapid that the outer layers will cool, their temperature falling from the 6000^o of its main lifetime to some 3000^o in the Red Giant phase.

The core is still hot and continuing to collapse due to gravitational forces.

It heats up further, the hydrogen runs out and the fusion of helium begins. Known as the triple alpha process helium is fused to give beryllium and finally carbon. For stars with a mass less than 3 solar masses this occurs rapidly and is called a helium flash.

The layers of the star outside the core will be heated and explosions will take place, blowing away an outer shell of gas. In stars like the Sun between 25 and 60% of the mass is blown away to form a planetary nebula. The process repeats and the star flashes as layers of its matter are puffed away into space.

As time goes by the radiation from the fusion in the core gets less. The star shrinks and cools a little, although it is still hot enough to continue fusing hydrogen and becomes a small hot star – a white dwarf. The core has the size of the Earth and therefore the density of a white dwarf is enormous – 10⁸ to 10⁹ kgm^-3. A teaspoon of this matter would have a mass of several tons!

Eventually all fusion ceases and a black dwarf is formed, a dark, cold ball of oxygen and carbon– the star has died.