According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. Examples include Aldebaran and Arcturus.

They are believed to be stars of near solar mass or higher which have exhausted the supply of hydrogen in their cores and started fusing 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 (from 1,000 – 10,000 times brighter) 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. Stars of spectral types O through K are believed to become red giants (or supergiants in the case of O and B stars). Red dwarfs (of spectral type M), and possibly even the cooler of the K orange dwarfs as well may not have enough mass to initiate helium fusion, and therefore rather than become red giants, may simply burn out as they die.

If the star is less than 2.57 solar masses, the addition of helium to the core by shell hydrogen fusing will cause a helium flash—a rapid burst of helium fusing in the core, after which the star will commence a brief period of helium fusing before beginning another ascent of the red giant branch. Stars more massive than 2.5 solar masses enter the helium fusing phase of their lives much more smoothly. The core helium fusing 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-fusing 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 fusing elements more massive than helium in their cores. In these cases, the star is then on the asymptotic giant branch.

Comparison between red giants and the Sun

Earth's Sun

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 current orbits of the solar system's inner planets, including Earth. However, the gravitational pull of the sun will have weakened by then due to its loss of mass, and it is possible (but unlikely) that Earth may escape to a wider orbit.