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Glossary term: Luminosity Class

Description: The intrinsic brightness of a star depends on its temperature and physical size. Hotter stars are brighter, as are larger stars. Spectral type categorizes stars based on the appearance of their spectral lines. This roughly follows changes in temperature. The luminosity class categorizes stars based on how bright they are compared to other stars of their spectral type. Roman numerals are used to denote luminosity class: Stars on the main sequence burning hydrogen are classified as dwarfs (V). As an example, the class of the Sun is G2 V. Above these lie (in order of increasing size and brightness) subgiants (IV), giants (III), bright giants (II), and supergiants (Ib for less luminous and Ia for the most luminous). All of these are types of evolved stars that have finished burning hydrogen in their core. Metal poor subdwarfs are hydrogen burning stars with a low metal content that lie below the main sequence. Finally white dwarfs (stellar remnants) lie further still below the main sequence.

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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".

Related Diagrams


A line of stars goes from cool faint stars to hot bright stars. Some stars lie above or below this line

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

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