Class M star

A **Class M star** is the type that consists of some of the dimmest of the main sequence stars. Such main sequence Class M stars are known as **red dwarfs**, though the category can also include very late Class K stars that may be nearly convective and early Class L stars able to fuse hydrogen. They are typically described as red or red-orange. They are by far the most common type of luminous star, making up roughly 76% of all main sequence stars.

A typical Class M star has a mass of 0.20 (0.08 to 0.45) solar masses (M☉), a radius of 0.3 (≤0.7) solar radii, a luminosity of 0.01 (≤0.08) solar luminosities, a surface temperature of 3,000 K (2,400 K to 3,790 K), and a lifespan of 1 to 10 trillion years.

Although they have very long lifespans, red dwarfs' low mass and luminosity (and their habitable zones of about 0.1 AU) means that many planets will end up frozen or tidally locked to their parent star; these are generally not good conditions for intelligent life. However, it may be possible or even common for simple life to develop on these planets.

Even though almost all of the class M stars are red dwarfs, most giants, supergiants and hypergiants, including leading candidates for largest known stars, are also class M. Examples of Class M supergiants and hypergiants include Antares A, Betelgeuse, Mu Cephei, Westerlund 1-26, VV Cephei A, VY Canis Majoris, and UY Scuti.

Brown dwarfs often begin their life with emissions in the Class M spectrum with a temperature of over 3,000 K but cool off and become dimmer over time.

## Properties of M-type main sequence stars

Compared to the Sun, Class M main sequence stars are significantly smaller, significantly less massive, and significantly less luminous. The largest red dwarfs are about two thirds the mass and radius of the Sun and less than a tenth of the Sun's luminosity. The smallest Class M main sequence stars are some of the smallest possible stars capable of hydrogen fusion.

The spectra of Class M stars are characterized by strong lines from diatomic molecules such as TiO, VO, and MgH and extremely weak to completely nonexistent hydrogen absorption lines. Strong lines from sodium and neutral metals such as Ca I and Fe I are also present in the spectra of Class M stars.

## Evolution of M-type main-sequence stars

Class M red dwarfs are formed from T Tauri pre-main sequence stars and will stay in the main sequence for up to trillions of years. The less massive the star, the longer the star will stay in the main sequence. For example, a star about 0.16 solar masses (comparable to Barnard's star) will last about 2.5 trillion years in the main sequence while a star 0.10 solar masses will last about 6 trillion years in the main sequence. The least massive Class M stars are ones partially heated from residual heat from the star's formation. Said stars will eventually radiate away its residual heat and become Class L stars heated by hydrogen fusion and stay that way for the rest of their time as main sequence stars.

Class M red dwarfs are fully convective and because of this, helium does not accumulate in their cores and compared to larger Class K or Class G stars, they can slowly burn a larger proportion of their own fuel before exiting the main sequence. Currently, in our universe, no M-type main sequence star has evolved out of the main sequence as the universe is not old enough for that to happen.

When M-type stars larger than about 0.25 solar masses lose enough hydrogen, they will drastically swell up, become more luminous, and enter the subgiant branch and red giant branch before becoming a helium white dwarf. M-type stars less massive than 0.25 solar masses will go through a transitional stage between a red dwarf and a white dwarf known as a **blue dwarf** after they exhaust their hydrogen. Blue dwarfs formed from more massive red dwarfs can reach temperatures up to 9000 K—comparable to that of Class A stars—before becoming white dwarfs while blue dwarfs formed from the least massive Class L red dwarfs can only reach temperatures of 4250 K—comparable to the temperatures of Class K stars—before becoming cool white dwarfs.

## Examples of M-type brown dwarfs

* 2M1101A, an M7 dwarf
* Teide 1, an M8 dwarf
* 2M1207, an M8 dwarf
* 2MASS J04414489+2301513, an M8.5 dwarf
* TWA 28, an M8.5 dwarf
* DEN 1048-3956, an M9 dwarf
* 2MASS J20004841-7523070, an M9 dwarf
* 2MASS J00274197+0503417, an M9.5 dwarf

## References

* Mamajek, E., *A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence*. 2019
* L. Kaltenegger, W.A. Traub; *Transits of Earth-Like Planets*. 2009
* F.C. Adams, P. Bodenheimer, G. Laughlin; *M dwarfs: planet formation and long term evolution*. 2005
* Various; Stellar Classification - Wikipedia, the free encyclopedia
* Cram, Lawrence E., and Leonard V. Kuhi. *FGK Stars and T Tauri Stars*. NASA, 1989.

## See Also

**Brown dwarf classes**: **Y** · **T** · **L** · ****M****
**Wolf-Rayet and carbon star classes**: **S** · **C** · **W**
**Stellar remnant classes**: **D** · **N** · **Ω**