Glossary term: Red Giant

Also known as M giant

Description: A red giant is a star that has a comparatively cool surface, but a diameter typically tens, or sometimes hundreds, times larger than that of a main sequence star like our Sun. The comparatively low temperature gives the surface a reddish appearance, while the large diameter makes the star shine considerably brighter than our Sun. Red giants were originally main sequence stars, that is, Sun-like stars that burn hydrogen to helium in their cores via nuclear fusion. When such a star runs out of hydrogen fuel, it begins to burn helium into heavier elements. At that time, the star expands, its surface cooling down in the process, and with its increased size the star becomes more luminous. The Sun will enter the red giant phase billions of years from now, at which time life on Earth will likely not survive. Examples of red giants are Arcturus in the constellation Bo├Âtes and Mira in the constellation Cetus. Red giants are unstable to pulsation (an oscillation where the star repeatedly becomes smaller and larger in turn) and, as they pulsate, can vary in brightness. The star Mira is an extreme example that can vary in brightness by a factor of 1000. In the spectral classification used by astronomers, red giants are mostly of spectral type M, and some are of spectral type K. Red giants are similar to so-called red supergiants, but with a lower mass.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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