What happens when stars die?

On Sat, 6 Mar 1999 you wrote:

Hi Chris,

Our sun is in this part of the universe because it formed here. There's
nothing special about where we are in the universe. The sun could be in
many different parts of the universe and we would still be able to live
around it just fine.

For most of a star's life, all stars do pretty much the same nuclear
reactions, that of burning hydrogen into helium. Hot, massive stars do
this burning in a slightly different way then cool, less-massive stars,
but the end result is the same. When stars die, they swell up and become
red giants. At this point, different nuclear reactions happen. Helium
burns into carbon, carbon into oxygen and neon, oxygen into silicon, and
silicon into iron. High-mass stars will do all of these reactions (and
more!) eventually, but low-mass stars stop when all the helium is burnt
into carbon. Brown dwarfs never do any nuclear burning at all, and so
are more like failed stars than actual stars.

The sun is what we call a "main sequence" star, meaning it is in the
prime of its life and burning hydrogen into helium. It has been that way
for 4 or 5 billion years, and will be that way for another 4 or 5
billion years.

This depends on how massive the star is. Stars that are about 8 to 10
times the mass of the sun (or even bigger) will explode in a supernova.
These are the stars that go on to make iron in their red giant stage of
life. The build-up of iron will cause the star to explode. The very
massive stars (20 times the mass of the sun or more) probably turn in to
black holes, while stars with 8 to 20 times the mass of the sun become
neutron stars. Lower mass stars (including the sun itself) will change
into a white dwarf - a star with most of the mass of the sun, but shrunk
into something the size of the Earth. The outer parts of the sun will
slowly get blown away in what is called a planetary nebula.

One thing I find interesting about stars is studying white dwarfs - the
dead, burnt-out cores of stars like the sun. You have something the mass
of the sun squeezed into something the size of the Earth. Even more
interesting is that, as these things cool, they crystallize, forming what
are essentially giant diamonds.

Thanks for writing, and good luck with your project.

Sincerely,

Kurtis Williams

"A man is like a fraction whose numerator is what he is and
whose denominator is what he thinks of himself. The larger the
denominator the smaller the fraction. " -- Lev Tolstoy

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