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Red dwarfs are the most common type of star in the galaxy, but difficult to observe due to their dimness. They have a lower mass and slower nuclear reactions than larger stars, resulting in a lower surface temperature and longer lifespan. They lack metals, which is a mystery.
Red dwarfs are small, relatively cool stars that are the most numerous type of star in our galaxy, if not the universe. This is difficult to verify because red dwarfs are dim (0.01% to 10% of the Sun’s luminosity), making them difficult to observe from astronomical distances. Proxima Centuari, the closest star to the solar system, is a red dwarf.
Red dwarfs have a mass between 7.5% and 50% of that of the Sun. The more massive stars are called yellow dwarfs, while the less massive ones are called brown dwarfs. All dwarfs are part of the most common class of stars, known as the “main sequence.” Off the main sequence are white dwarfs, which have exhausted their nuclear fuel, and giant stars, which swell to form stellar nebulae or go supernova.
A red dwarf uses the same nuclear fusion reaction as the Sun to generate energy: fusion of hydrogen into helium through proton-proton chain interaction. But because red dwarfs are less massive, their cores are less compact and reactions proceed at a slower rate than in larger stars. Therefore, the surface temperature of a red dwarf is lower than 3.500 K, significantly compared to the surface temperature of the Sun, which is about 5.778 K.
Red dwarfs are small enough to be fully convective, meaning that material in their core and surface is constantly recirculating. Because of this, red dwarfs are able to burn a greater proportion of their nuclear fuel than more massive stars. Combined with the low rate of nuclear reactions from their low compression, this gives red dwarfs tremendous lifespans: tens of billions to trillions of years depending on mass.
One mysterious thing about red dwarfs is that they lack what astronomical jargon calls “metals,” which are elements other than hydrogen or helium. Since most stars in the universe are born from supernova debris, which contain many metals, all but the very first stars in the universe would be expected to contain heavier elements. Yet they are nowhere to be found in red dwarfs.
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