Red Dwarf
Red Dwarfs are low mass stars that are just massive enough to make it to the Main Sequence and burn
hydrogen to helium by nuclear fusion. They have between about 0.075 and 0.50 times the mass of the
Sun. Smaller stars have lower core temperatures and so burn their fuel much more slowly than more
massive stars and so live much longer. Red Dwarfs are extremely long lived, with the smallest red dwarfs
living for around one thousand billion years (10^12 years). Burning fuel so slowly, red dwarfs are cool and
dim, with atmospheric temperatures of less than 3500 degrees kelvin and about 0.001 to 0.0001 times the
Sun's luminosity.

The smaller red dwarfs are of spectral class M, but the largest are the cooler of the K class stars (hotter K
class stars are orange dwarfs). Red dwarfs are only about 1% of the Sun's diameter.

Red dwarfs with planets orbiting them are known, but these planets have to be much closer to their parent
star to have warm surface temperatures than planets around hotter and more luminous stars.

Stars that have insufficient mass to obtain the core temperatures to burn hydrogen never make it to the
Main Sequence and become
brown dwarfs, which may briefly burn deuterium or lithium fuel, for around 10
000 years before slowly cooling. The coolest brown dwarfs may have methane or even superheated steam
in their atmospheres.

The atmospheres of red dwarfs are turbulent, or at least
fully convective, with hot plasma rising to the
surface, cooling and then sinking again. In other stars heat may be transported mainly by radiation. In the
Sun (a yellow dwarf) the heat travels most of the way from the core by radiation (photons are scattered off
the plasma, especially from the electrons, and also absorbed during photoionisation) and then enters a
thin convective zone near the surface, but in red dwarfs the whole atmosphere is in convective motion.
This motion, combined with the rotation of the star, generates a strong magnetic field in red dwarfs,
especially when the star is young and rotating rapidly. Such stars may become
flare stars - a flare star is a
magnetically very active (and probably young) red dwarf.

It is thought that red dwarfs are the most numerous stars in the galaxy, though they are so dim and faint
that their numbers are hard to determine as only those that are nearby are visible. Certainly in the Sun's
local neighbourhood, red dwarfs are the most numerous star type.
Proxima Centauri, the nearest star to
the Sun, at 4.22 light years and is a red dwarf flare star with about 0.1 solar masses. With its very active
flares it is a source of intense low energy X-rays and high-energy UV light, despite being very faint.