Post main sequence stars

The fight against gravity in the cores of stars

During the main sequence lifetime of stars, the energy provided by burning hydrogen provides the pressure needed to support the weight of the outer layers.

When the hydrogen is gone in the core, the only available energy source is gravitational potential energy. The core gets smaller and hotter as gravitational potential energy is converted into heat. As this heat begins to leak out, the core shrinks more and becomes even hotter.

This process can be interupted when the core is dense enough and hot enough for a new source of fusion energy to take over.

Sources of fusion energy

4 ¹H --> ⁴He.
3 ⁴He --> ¹²C.
¹²C + ⁴He --> ¹⁶O.
¹²C + ¹²C --> ²⁴Mg.
etc. making ²⁸Si, ³²S, ...

Production of nuclei beyond C and O takes place only in the heaviest stars because of the very high temperature and density required.

H burning lasts a long time. (10 x 10⁹ years in a 1 M_sun star, less in heavier stars.)
He burning doesn't last so long. (2 x 10⁹ years in a 1 M_sun star, less in heavier stars.)
Successive burning stages in very massive stars take less and less time to complete.

The end of fusion energy

Combining lighter nuclei into heavier nuclei releases energy. But this stops at iron, ⁵⁶Fe.

Nuclei heavier than iron can be made, and are made, in very massive stars. But this uses energy. It doesn't produce energy.

HR tracks for a 1 M_sun star and a 5 M_sun star

The 1 M_sun star burns H in its core, (red) then He (red).
The 5 M_sun star burns H in its core, (red) then He (red), then C (red).
At other (briefer) times, nothing is burning in the cores, which are shrinking. Nuclear burning is occuring only in shells outside the cores.

The theoretical tracks are rather complicated, but in general, the stars are very bright, their photospheres are big, and the photospheres are cooler than they were during the main sequence lifetime of the star. This region is called the red giant region. (Often this region is subdivided into smaller regions.)

The observational evidence for this comes from looking at the HR diagrams for star clusters.

Mass loss

During these stages of stellar evolution, stars commonly lose a significant fraction of their mass by blowing away some of their surface gas in strong stellar winds.