Maturity

   At the end of a million to several tens of billions of years (depending on the initial mass) star exhausts hydrogen nuclei resources. In large and hot stars that is much faster than in smaller and colder. Depletion of hydrogen leads to a shutdown of thermonuclear reactions.

   Without pressure, that produced these reactions and the balance due to the gravitational attraction of its own star, the outer layers begin to shrink to the nucleus. Temperature and pressure are increased during the formation of protozvezdy, but this time to a much higher level. The collapse continues until at a temperature of about 100 million by the start thermonuclear reactions involving helium.

   Very hot nucleus causes horrendous expansion of the star. Its size increases to approximately 100 times. Thus becomes a red giant star and the helium burning phase lasts for about a few million years. Almost all the red giants are variable stars.

   What is happening in the future, again depends on the mass of the star.