What happens next in the life of a star depends on its initial mass.
Whether it was a "massive" star (some 5 or more times the mass
of our Sun) or whether it was a "low or medium mass" star (about
0.4 to 3.4 times the mass of our Sun), the next steps after the red giant
phase are very, very different.
III. The End
A. The Fate of Sun-Sized Stars: Black Dwarfs
Once a medium size star (such as our Sun) has reached the red
giant phase, its outer layers continue to expand, the core contracts
inward, and helium atoms in the core fuse together to form carbon. This
fusion releases energy and the star gets a temporary reprieve. However, in
a Sun-sized star, this process might only take a few minutes! The atomic
structure of carbon is too strong to be further compressed by the mass of
the surrounding material. The core is stabilized and the end is near.
The star will now begin to shed its outer layers as a diffuse cloud
called a planetary nebula. Eventually, only about 20% of the star's
initial mass remains and the star spends the rest of its days cooling and
shrinking until it is only a few thousand miles in diameter. It has become
a white dwarf. White dwarfs are stable because the inward pull of gravity
is balanced by the electrons in the core of the star repulsing each other.
With no fuel left to burn, the hot star radiates its remaining heat into
the coldness of space for many billions of years. In the end, it will just
sit in space as a cold dark mass sometimes referred to as a black dwarf.
B. The Fate of Massive Stars: Supernovae! and Then...
Fate has something very different, and very dramatic, in store
for stars which are some 5 or more times as massive as our Sun. After the
outer layers of the star have swollen into a red supergiant (i.e., a very
big red giant), the core begins to yield to gravity and starts to shrink.
As it shrinks, it grows hotter and denser, and a new series of nuclear
reactions begin to occur, temporarily halting the collapse of the core.
However, when the core becomes essentially just iron, it has nothing left
to fuse (because of iron's nuclear structure, it does not permit its atoms
to fuse into heavier elements) and fusion ceases. In less than a second,
the star begins the final phase of its gravitational collapse. The core
temperature rises to over 100 billion degrees as the iron atoms are
crushed together. The repulsive force between the nuclei overcomes the
force of gravity, and the core recoils out from the heart of the star in
an explosive shock wave. As the shock encounters material in the star's
outer layers, the material is heated, fusing to form new elements and
radioactive isotopes. In one of the most spectacular events in the
Universe, the shock propels the material away from the star in a
tremendous explosion called a supernova. The material spews off into
interstellar space -- perhaps to collide with other cosmic debris and form
new stars, perhaps to form planets and moons, perhaps to act as the seeds
for an infinite variety of living things.
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