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C. The main sequence stage
This is the stage in which a star will spend most of its life. At this
stage the star is still fusing hydrogen to helium and releasing energy
as EM radiation. How long this stage lasts really depends on the mass of
that star. The greater the mass, the greater the rate at which Hydrogen
is fused, and the shorter the life of a star. A star of 50 solar masses
lasts perhaps one million years on the main sequence: 10 solar masses:
10 million; 5 solar masses: 100 million; 1 solar mass (our sun): 10 billion;
0.1 solar masses: 1 trillion years.
D. The giant stage
 When
Betelgeuse, a red giant in the constellation Orion
When most of the hydrogen at the core of any star is converted to helium,
H fusion in the core of that star almost ceases. As there is little outward
fusion pressure to counterbalance the gravitational force, the star contracts.
As gravitational contraction occurs, the interior heats up and two things
will happen. First, the layers that surround the core of the original main
sequence star heat up enough so that hydrogen begins to fuse in them. Second,
as temperatures in the heart of that star reach 100 million degrees K,
helium fuses to heavier elements in what was the original core. The combination
of these two processes releases much more energy than was released in the
main sequence stage, and therefore much greater fusion pressure.
With the increased fusion pressure, the outer layers of this star expand
to hundreds of times its main sequence size. This expansion leads to two
things: first of all a much greater size (the star has become a giant);
and second, a lowered surface temperature. This happens 1) because as the
outer layers of this star expand they cool; and 2) even though the star
is giving off more energy, this energy is being released over a much greater
surface area (the peak emission shifts towards longer wavelengths). Over
time a new equilibrium between gravity and fusion pressure is established
and the giant stage continues. Stars with less than 8 solar masses will
become giants. Some 5 billion years from now, our sun (with its 1 solar
mass) will expand to a radius of perhaps a hundred fifty million miles
in the giant stage. More massive stars will be bigger and expand to hundreds
of million miles and more and become supergiants. Life in the giant stage
may take billions of years for a star like the sun, but only a few million
years for a star of 10 solar masses. |
