Glossary term: Red 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 Boötes 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
The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".
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In Other Languages
- Arabic: العملاق الأحمر
- German: Roter Riese
- French: Géante rouge
- Italian: Gigante rossa
- Japanese: 赤色巨星 (external link)
- Simplified Chinese: 红巨星
- Traditional Chinese: 紅巨星
Related Media
The red supergiant Betelgeuse
Caption: The image shows Betelgeuse, a red supergiant in the constellation Orion, observed by the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA consists of many antennae spread across a plain in Northern Chile. The observations from all of these receivers is synthesised together by a central computer to form an image. The wide distances between the antennae mean that is can resolve very fine details.
Most stars we observe are just seen as points of light, but Betelgeuse is so large (with a radius about 1,400 times larger than the Sun) and is sufficiently nearby that it is one of the few stars to have been resolved to show it as an extended object.
Betelgeuse is a massive star, more than 14 times the mass of the Sun and is relatively young for a star (less than 14 million years old). However, its high mass led to it having a very hot core which burned through its hydrogen fuel quickly. It has since evolved through many stages and now appears as a red supergiant, it's final stage before exploding as a supernova. When such an explosion will happen is not known for certain, but it could be in around 100,000 years. Such an explosion would be visible from Earth, even during the day.
Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella
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License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
