star of 30 solar masses will run the entire element-generating gamut
from hydrogen to iron.
If the next reaction in the series cannot be touched off, then the
star becomes a white dwarf - a stellar cinder. Our own sun is approximately
4.5 billion years old. In another 5 billion years, it will swell
to a red giant, annihilating life on Earth, and then shrink and
touch off the "helium flash" - the burning of helium to carbon.
The sun is too small to ignite the carbon reaction when helium is
depleted at the core, so in a short 30 million years after the helium
flash, it will swell again to engulf the orbit of Mars, puff off
material, then shrink and die as a white dwarf.
Something radical, and fundamentally important to the distribution
of elements through the universe, happens if the star is massive
enough to continue to synthesize heavy elements until iron is reached.
Since fusion involving iron absorbs energy rather than emitting
it, the "fire" goes out. No more fusion reactions are available
as energy sources. Paradoxically, this does not mean that the star
cools to a dwarf, but that it explodes as a supernova. When the
fuel at the core is finally and totally depleted, the center cools.
Some of the iron at the center may break down again into helium,
and by absorbing energy, make the cooling process even faster: