In their later stages of evolution as red giants or supergiants, stars
often become variables -- their light output changes with time. These stars change in both
size and temperature which causes a change in brightness:
- In these variable stars, the core will increase its energy output causing
the outer layers to heat up and expand.
- The outer layers over-react, expand too far, and begin to cool off.
- The outer layers contract because they have cooled off.
- The contraction causes the core to heat up again and increase its energy
output which causes the whole cycle to start over. Pulsing red giant from http://www.lcse.umn.edu/MOVIES/ (reload to restart animations)
|The star alternates between expanding and contracting,
between heating and cooling. Finally, its core cools out of sight -- it is about to
collapse into a white dwarf/planetary nebula (to be discussed soon).