Glossary term: M-type Star

Also known as M star or M-star

Description: A star with spectral type "M". Astronomers identify M-type stars by the presence of molecular absorption bands, primarily from titanium oxide, in their spectra. They have typical (effective) temperatures between around 2500 kelvins (K) and 3700 K. Compared to other stars, they appear red to human eyes. Main sequence stars with spectral type M are known as red dwarfs. M-type stars can also be red giants or red supergiants; these classes are mostly M stars but include some K-type stars as well as some more exotic types of stars with strong spectral features from carbon. Betelgeuse in Orion is an M-type supergiant.

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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 choppy line increasing at longer wavelengths with large wide dips and a few sharper dips.

Spectrum of an M-type star

Caption: The spectrum of the M-type star 2MASS J15581272+8457104. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms, ions and molecules of different elements in the star’s atmosphere. These atoms, ions and molecules absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. The atmospheres of M-type stars are cool enough for some chemical compounds to form. These are often referred to as molecules in astronomy, even if they are not strictly molecules in chemistry. These molecules produce so many lines in an M-type star’s spectrum that the lines appear to merge together in huge bands that remove large chunks from the spectrum. In M-type stars, titanium oxide has a large number of these bands in visible light, dominating huge regions of the spectrum.
Credit: IAU OAE/SDSS/Niall Deacon

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