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They are believed to be stars of solar mass or higher which have exhausted the supply of hydrogen in their cores and started burning hydrogen in a shell outside the core. Since the source of energy is closer to the surface, the star begins to expand. This makes the star more luminous but, counterintuitively, also reduces the effective temperature . This is because the radius (and hence surface area) of the star increases by a larger amount than the luminosity of the star. As a result, the star becomes larger, but cooler and redder -- hence red giant.
If the star is less than 2.5 solar masses, the addition of helium to the core by shell hydrogen burning will cause a helium flash -- a rapid burst of helium burning in the core, after which the star will commence a brief period of helium burning before beginning another ascent of the red giant branch. Stars more massive than 2.5 solar masses enter the helium burning phase of their lives much more smoothly. The core helium burning phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the Hertzsprung-Russell diagram of many star clusters. Metal-rich helium-burning stars do not lie on a horizontal branch, but instead lie in a clump (the red clump ) in the Hertzsprung-Russell diagram.
Stars may enter a red giant phase several times if they are capable of burning elements more massive than helium in their cores. In these cases, the star is then on the asymptotic giant branch.
As earth's Sun is of one solar mass, it is expected to become a red giant in about six billion years. It will become sufficiently large to engulf the solar system's inner planetsA planet (from the Greek , planetes or "wanderers") is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 3, including EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla.