Class L star

A **Class L star** is a stellar class that includes early brown dwarfs and ultracool low mass stars. They are generally 65 - 90 times as massive as Jupiter and their temperature is generally 1,300 - 2,400 K. More massive Class L stars are at the limits of smallest stars capable of fusing hydrogen, whereas less massive ones fuse deuterium and lithium instead. They appear to be a dim orange to orange-red with their spectrum peaking in the infrared.

Class L stars 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.

Class L stars have a habitable zone in between 0.007 and 0.04 AU, or between 1,050,000 to 6,700,000 km, which is 10 - 54 times as close as Mercury's orbit to the Sun. This close distance would leave the planet tidally locked to the star. This combined with the low temperatures, leave Class L stars extremely poor candidates for life.

While most Class L stars are either very massive brown dwarfs or less massive main sequence stars, it may be possible for atypically cool supergiant stars to be examples of Class L stars. V838 Monocerotis is a known example of an L-type supergiant candidate. Such supergiants likely cannot form from typical stellar evolution and have to be created via collisions of stars.

## Properties of L-type main sequence stars

Class L main sequence stars and less massive Class M stars that will evolve into Class L stars are the least massive and coolest possible stars capable of hydrogen fusion. One of the least energetic known main sequence stars is a star of spectral type L2.5 V called **2MASS J0523−1403**—a star with a mass comparable to the minimum possible mass for a star.

The spectra of Class L stars are characterized by weak or nonexistent metallic oxide bands (e.g. TiO and VO)—a defining feature of Class M stars, and the presence of strong metallic hydride bands (e.g. CrH and FeH) and alkali metal bands (e.g. Na I, K I, Rb I, and Cs I). A test that can be used to distinguish between L-type stars and L-type brown dwarfs is the lithium test. Stars tend to burn away all of its lithium very quickly while brown dwarfs do not so if an L-type fusor has lithium in its spectrum, then it is likely that the object is a brown dwarf. If it does not have lithium, then the object is likely a true star. Based on the lithium test, it is likely that roughly two-thirds of all L dwarfs are true stars. Despite being a fair indicator between low mass L stars and brown dwarfs, the lithium test is flawed as a means of differentiation as it is possible for very massive brown dwarfs to completely burn away their lithium supply and for lithium to be detected in extremely young stars in the process of burning away their lithium supply.

## Evolution of L-type main sequence stars

Class L main sequence stars between 0.075 and 0.09 solar masses are formed from T Tauri pre-main sequence stars and can stay in the main sequence for up to twelve trillion years. At the beginning of the star's main sequence stage, the star is as hot as a Class M main sequence star due to residual heat from the star's formation, though the star will lose said heat relatively quickly and become an L-type star kept warm by its own heat from hydrogen fusion.

Class L main-sequence stars are fully convective and as such, helium will not accumulate at the star's core and the star is able to burn a much larger proportion of its hydrogen compared to more massive main-sequence stars. After the star has exhausted its hydrogen, it will go through a transitional stage between red dwarf and white dwarf called a **blue dwarf**. During its time as a blue dwarf, the star will increases in temperature and luminosity, though compared to blue dwarfs formed from more massive Class M red dwarfs, the blue dwarfs that evolved from Class L stars are much cooler. A blue dwarf formed from a 0.08 solar mass red dwarf will reach a maximum temperature of about 4250 K—more comparable in temperature to a Class K "orange" star than a Class O "blue" star. The blue dwarf will eventually form a very cool helium white dwarf.

## Examples of L-type brown dwarfs

* 2MASP J0345432+254023, an L0 dwarf
* 2MASS J13595510-4034582, an L1 dwarf
* Kelu-1A, an L2 dwarf
* DENIS-P J1058.7−1548, an L3 dwarf
* GD 165B, an L4 dwarf
* 2MASS 1507−1627, an L5 dwarf
* 2MASSW J2244316+204343, an L6 dwarf
* WISE 1147-2040, an L7 dwarf
* Luhman 16A, an L8 dwarf
* WISE 0049+0441, an L9 dwarf

## References

1. ↑ Various; Brown\_dwarf - Wikipedia, the free encyclopedia
2. ↑ Various; Stellar classification - Wikipedia, the free encyclopedia
3. ↑ A. Evans, T. R. Geballe, M. T. Rushton, et. al; *V838 Mon: an L supergiant?*. 2003
4. ↑ S. B. Dieterich, T. J. Henry, W. Jao, et al; *The Solar Neighborhood XXXII: The Hydrogen Burning Limit*. 2013
5. ↑ J. D. Kirkpatrick, I. N. Reid, J. Liebert, et. al; *Dwarfs Cooler than "M": The Definition of Spectral Type "L" Using Discoveries from the 2-Micron All-Sky Survey (2MASS)*. 1998
6. ↑ F.C. Adams, P. Bodenheimer, G. Laughlin; *M dwarfs: planet formation and long term evolution*. 2005

## See Also

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