1 Stellar evolution after the main sequence
During the main part of their lifetime on the main sequence most stars are relatively stable, with nuclear reactions in their cores converting hydrogen into helium at a steady rate. As long as there is sufficient hydrogen available to sustain these reactions a star will remain in an equilibrium state with the energy produced in the core supporting the star against the inward pull of gravity. On reaching the surface, the energy radiates out into space maintaining the star’s luminosity.
This flow of energy keeps the star in equilibrium; while the rate of energy production in the core matches the rate of energy leaving the surface, then conditions such as temperature and pressure within the star will remain stable.
However, gravity is always at work waiting and sooner or later the supply of hydrogen in the core will run out. As we have seen for our own Sun, this will not happen for several billion years, but for heavier stars the supply of hydrogen is used up more quickly. Eventually, all stars will run out of hydrogen and when this happens, things will start to change – energy production and gravity will no longer be in equilibrium and the star will no longer be stable.
What happens next depends on the mass of the star and its position on the main sequence of the HR diagram.