A red dwarf is a small and relatively cool star on the main sequence. Red dwarfs range in mass from about 7 to roughly 50 percent the mass of the Sun and have surface temperatures of less than 4,000 K. They are by far the most common type of star in the Milky Way, but because they are dim in the visible light spectrum, individual red dwarfs cannot easily be observed. Indeed, not a single one can be seen by the naked eye from Earth. According to some estimates, three-fourths of the stars in the Milky Way are red dwarfs—as is our nearest neighbor.
Proxima Centauri is a red dwarf about 4.24 light-years from the Sun and is the nearest known star to the Sun. Its distance to the second- and third-nearest stars, which form the bright binary Alpha Centauri, is sufficiently close (about .24 light-year) that it is very likely part of a triple star system with Alpha Centauri A and B, but its orbital period in that system may be greater than 500,000 years.
n 2016, a planet designated Proxima Centauri b was found orbiting the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of about 11.2 Earth days. Its estimated mass is at least 1.3 times that of the Earth, and the planet’s average temperature is estimated to be within the range of where liquid water could exist. While the planet is within the “habitable zone,” Proxima Centauri is a red dwarf and a flare star, so it’s not considered likely to support life.
Image Credit: NASA