Red giant stars

A certain star of spectral type K or later that occupies the upper right portion of the H-R diagram. Red giants are evolved stars that have exhausted their hydrogen fuel in the core. They may have a luminosity up to 1000 times greater than main sequence stars of the same spectral type. Red giants belong to the luminosity class III or II (bright giants). They are luminous because of their great size, but have a relatively low surface temperature. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution. When a main sequence star has converted approximately 10% of its hydrogen to helium, nuclear reactions in the core stop (Schönberg-Chandrasekhar limit). The hydrostatic equilibrium is no longer maintained, and the core contracts while the outer layers expand and cool. This process produces the low surface temperature and large size (from 10 to 100 times that of the Sun) that characterize the red giant. In the core the temperature continues to rise. When it approaches 100,000,000 K helium will begin to fuse into carbon. Prominent bright red giants in the night sky include Aldebaran and Arcturus.