Download The death of a star

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 out 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 6000o of its main lifetime to some 3000o 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.
© Keith Gibbs 2007
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