Red dwarf is a small, relatively cool star that glows with a dim, reddish light. Red dwarfs range in mass (amount of matter) from about 1/12 to 1/2 the mass of the sun . They generally give off from about 1/10 to less than 1/10,000 as much light as the sun does. Their reddish color results from relatively low surface temperatures, typically less than 4500 K. One kelvin (K) equals one Celsius degree above absolute zero (–273.15 °C). Red dwarfs shine so faintly that they are difficult to detect at great distances. Surveys of nearby stars, however, have revealed that red dwarfs are the most common type of star in our galaxy. Red dwarfs include Proxima Centauri , the star closest to the sun, and most of the other nearby stars.
Astronomers classify red dwarfs as main-sequence stars . Main-sequence stars, including the sun, produce nearly all of their energy by combining hydrogen nuclei to form helium nuclei in their cores. This reaction, a type of nuclear fusion gives off a tremendous amount of energy. Red dwarfs rank as the smallest and least massive main-sequence stars. Astronomers sometimes have difficulty distinguishing the coolest red dwarfs from brown dwarfs . Brown dwarfs are objects more massive than planets but not massive enough to sustain fusion in their cores.
The more mass a main-sequence star has, the more quickly it fuses hydrogen. Because red dwarfs have such low masses, they use up hydrogen extremely slowly. Their cores can continue to fuse hydrogen for tens of billions or even several trillions of years, a period much longer than the current age of the universe . For this reason, no red dwarf has yet used up its hydrogen fuel. Nevertheless, astronomers have determined that, unlike larger stars, red dwarfs will never be able to fuse elements heavier than hydrogen. As a result, a red dwarf that used up the hydrogen in its core would leave the main sequence to become a type of burned-out star known as a white dwarf .
A planet orbiting a distant star is called an extrasolar planet or exoplanet. In 2016, astronomers discovered an exoplanet orbiting Proxima Centauri. The exoplanet, designated Proxima b, is slightly larger than Earth and probably has a rocky surface. Its orbit lies extremely close to Proxima Centauri. However, red dwarfs are smaller and cooler than other stars. For this reason, scientists think that the surface of Proxima b might be an appropriate temperature for liquid water. Many scientists consider liquid water to be an essential ingredient for life . For this reason, the region around a star where water can exist on a planet’s surface is known as the star’s habitable zone. Later in 2016, astronomers discovered another planetary system around the red dwarf TRAPPIST-1 . After further study, they announced in 2017 that the system included seven Earth-sized exoplanets, three of them within the star’s habitable zone. 
