Clusters, Galaxies, Black Holes and Stars 287
The Death of Stars
After the hydrogen fuel at the core of a main sequence star has been
exhausted the star undergoes dramatic evolutionary changes, passing into
a red giant stage. The exact manner in which this change takes place
depends on the mass and the chemical composition of the star in
question. The general pattern, however, is more or less the same.
Variable Stars
Some stars in passing from the main sequence stage to the red giant stage
pass through a highly unstable phase in which the luminosity and the
radius of the star vary in a periodic fashion. During this stage of periodic
variation the interior structure of the star is changing. These variable
stars known as Cepheid variables were mentioned earlier because of
their use in measuring astronomical distances. The period of variation
and the absolute magnitude of these stars are related to each other. The
period of variation of the Cepheid variables lies somewhere between
1 day and 5 months. Stars do not remain long in the variable phase of
their evolution but pass on rapidly to the red giant phase. Other types of
variable stars are also observed. Some are known to correspond to later
phases of stellar evolution. The exact role others play in stellar evolution
is not yet known. Not all stars that leave the stable main sequence to
become red giants pass through a variable phase. This depends on their
mass.
Red Giants
Once the hydrogen burning at the core of a star ceases and energy is no
longer being generated the helium core that remains begins to collapse.
The outer core of the star expands and becomes cooler. The helium core
of a star of one solar mass will shrink to about the size of the Earth. This
core containing about 1/10 to ¼ the mass of the star will have a density
100,000 times that of the Earth. The temperatures inside the helium core
will eventually become so high that a shell of hydrogen surrounding the
helium core will undergo nuclear ignition and the star will once again
generate new energy. The luminosity of this star will increase to a value
of 1000 times its main sequence luminosity. The radius of its outer
envelope increases to about 100 times its main sequence radius.