Red Dwarf

A red dwarf is a class of small star with a mass lower than that of the Sun. Its light is dim, and its surface temperature is low, which makes its color appear red or reddish. Because of its small mass, its density and core temperature are low, and therefore the nuclear fusion of hydrogen proceeding in its core does so at a very slow rate.

Red dwarfs are the most numerous stars by count in the Milky Way galaxy. However, because their light is faint, none of them can be seen with the naked eye in the night sky. Proxima Centauri, the star closest to the Sun, is a red dwarf.

Physical characteristics

Their mass lies between a lower limit of about 8 percent and an upper limit of about 50 percent of the Sun's mass. Those below that threshold cannot initiate hydrogen fusion and become what is called a brown dwarf. Their surface temperature is mostly below 4,000 kelvin. Their total luminosity (brightness) may be as little as a fraction of one percent of the Sun's.

Internal convection and long lifespan

Small red dwarfs are composed entirely of convective gas throughout the star. This means that hydrogen from the surface is brought to the core, so all the fuel is used, not only the hydrogen in the core. The combination of efficient fusion and the use of all available fuel gives them a very long lifespan—many trillions of years, longer than the current age of the universe.

Magnetic activity and stellar flares

Many red dwarfs exhibit strong magnetic activity. They emit flares that rapidly increase ultraviolet and X-ray radiation. The most magnetically active varieties are called UV Ceti stars. This activity is considered important when assessing the possibility of life on planets orbiting them, because intense radiation can affect a planet's atmosphere.

Orbiting planets

Many studies have shown that rocky planets with sizes comparable to Earth are far more common around red dwarfs. Because the star is faint, the habitable zone (the region where liquid water could exist) is much closer to the star. This often results in the planet being tidally locked, with one side always facing the star. The Proxima Centauri and TRAPPIST-1 systems are studied examples.