Glossary term: Giant Star
Description: Giant stars, giants for short, are stars that are unusually large and luminous, compared with other stars that have the same color. Stars do not start out as giants; instead, they intermittently become giants at various stages of their evolution.
For most stars that are not giants, there is a direct relation between their color and their luminosity (that is, the energy they emit over time). Those stars are called "main sequence stars", and their energy emissions are powered by the nuclear fusion of hydrogen into helium in their cores. When the hydrogen supply in the core is exhausted, hydrogen fusion continues in a shell around the core and the star expands to become much more luminous and redder. Eventually the temperature in the center may be high enough that the nuclear fusion of elements heavier than hydrogen becomes possible, providing an additional energy source for the star. In the course of this conversion, such stars expand to a much larger size, cool down to become more reddish, and overall become much more luminous – they become what is known as red giant stars, red giants for short. The Sun, for instance, will be hundreds of times larger and brighter as well as much cooler when it expands to become a red giant star. For stars of different masses, additional, often short-lived phases of evolution sees those stars become blue giants, or even more luminous red or blue supergiants.
Giants are classified as luminosity class III, brighter than subgiants (class IV) but fainter that bright giants (class II) and supergiants (class I).
Overall, giant stars are rare. This is due to the relatively short duration of the giant phase (for a star like the Sun a few hundred million years vs ten billion on the main sequence). But given their high luminosity, they are significantly over-represented among the stars that are visible to the naked eye in the night sky.
Examples of red giants include Arcturus, in the constellation Boötes, and Mira, in Cetus.
Related Terms:
See this term in other languages
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".
If you notice a factual error in this glossary definition then please get in touch.
In Other Languages
- Arabic: النجم العملاق
- German: Riesenstern
- Persian: ستاره غول
- French: Étoile géante
- Italian: Stella gigante
- Japanese: 巨星 (external link)
- Korean: 거성
- Brazilian Portuguese: Estrela gigante
- Simplified Chinese: 巨星
- Traditional Chinese: 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
Related Diagrams
Hertzsprung-Russell diagram
Caption: This diagram shows the temperature and luminosity of different stars. The size of each point represents the star’s radius and its colour is the colour the human eye would see. The stars range in colour from a washed-out blue to a washed-out reddish-orange. No star has a pure colour like red, green or blue as stars’ spectra include light from lots of different colours. However the reddest stars are commonly referred to as red and the bluest stars as blue. The sample of stars used to make this diagram was chosen to show a wide range of stars of different types so the relative number of each type of star is not representative of how commonly each type is found.
From the top left to bottom right there is a long line of stars burning hydrogen in their cores. This is called the main sequence. On this line, one sees the stars Mintaka, Achenar, Sirius A, the Sun and Proxima Centauri. The objects around Proxima Centauri at the lower right end of the main sequence are known as red dwarfs. To the lower right of the red dwarfs are Teide 1 and Kelu-1 A. These two objects are brown dwarfs, objects too low in mass to have cores hot enough to fuse hydrogen for a sustained period of time. As they do not burn hydrogen, brown dwarfs are not considered main sequence stars. The name brown dwarf is unrelated to their colour.
Above the main sequence, we find subgiants, giants and supergiants. These are stars that have finished burning hydrogen in their core and have evolved into larger objects. A star’s brightness depends on its temperature and size so giant stars are brighter than stars with a smaller radius but the same temperature. In time these objects will move towards the end of their lives and undergo either a planetary nebula phase or become supernovae. Stars which end their lives with a planetary nebula phase become a type of stellar remnant called a white dwarf. Such objects are much smaller than stars of the same temperature and thus are fainter and are found significantly below the main sequence. Stars which end their lives as supernovae become either black holes or neutron stars. These are not shown on this plot.
Credit: IAU OAE/Niall Deacon
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
