- Also called:
- M dwarf or M-type star
- Related Topics:
- Dwarf star
Red dwarf star
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What is a red dwarf star?
A red dwarf star is the most common and smallest type of star that burns hydrogen. They have a mass of about 0.08 to 0.6 times that of the Sun. Red dwarfs are cool main-sequence stars with a spectral type of M and a surface temperature of about 2,000–3,500 K. They are also the dimmest stars, with luminosities between about 0.0001 and 0.1 times that of the Sun.
Red dwarfs have very long lifetimes. The heaviest can last for tens of billions of years, while the smallest can last for trillions of years. The universe is only 13.8 billion years old, so even the oldest red dwarfs have not exhausted their internal supplies of hydrogen. They will eventually turn into blue dwarfs and then white dwarfs.
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How is a red dwarf star different from other stars?
Red dwarf stars are the most common and smallest type of hydrogen-burning star. They range from 0.08 to 0.6 times the mass of the Sun and are the coolest main-sequence stars, with surface temperatures between 2,000 and 3,500 K.
Red dwarfs are also the dimmest stars, with luminosities roughly 0.0001 to 0.1 times that of the Sun. Unlike stars such as the Sun, red dwarfs are completely convective, meaning they recirculate hydrogen from their outer regions to their core. Because of this, they burn their entire supply of hydrogen and will not evolve into red giants.
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Where are red dwarf stars found in the universe?
Red dwarf stars are the most abundant type of star in the universe. In the Milky Way Galaxy, they make up approximately three-fourths of the stars, and their proportion is even higher in elliptical galaxies.
Red dwarfs are small, with masses ranging from about 0.08 to 0.6 times that of the Sun. An example of a red dwarf is Barnard's Star, the second closest star to the Sun.
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Why do red dwarf stars appear red and dim?
Red dwarf stars appear red and dim due to their low surface temperatures and luminosities. With surface temperatures between 2,000 and 3,500 K, they are the coolest main-sequence stars. Because of these cool temperatures, spectral lines of molecules such as titanium oxide are prominent. Red dwarfs' luminosities range from 0.0001 to 0.1 times that of the Sun, making them the dimmest stars.
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How long do red dwarf stars live compared to other stars?
Red dwarf stars, the most common type in the universe, are much smaller than our Sun, ranging from 0.08 to 0.6 solar masses. This size difference has a significant impact on their lifespan. Smaller stars live longer than larger ones.
Stars like the Sun have a lifespan of roughly 10 billion years, but red dwarfs can live far longer. The heaviest red dwarfs have lifespans of tens of billions of years, while the smallest can exist for trillions of years. The universe is currently 13.8 billion years old, meaning that even the oldest red dwarf stars have yet to exhaust their hydrogen fuel. Unlike stars such as the Sun, red dwarfs will not evolve into red giants.
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Why are red dwarf stars important to astronomers studying planets beyond our solar system?
Red dwarf stars are important to astronomers studying exoplanets because of their low luminosity. This characteristic means that a planet within the habitable zone orbits the star closely and transits frequently. Because red dwarfs are small, the transits are noticeable, as the planet covers a significant portion of the star's surface.
For example, Proxima Centauri, the closest star to the Sun, is a red dwarf with three potential planets orbiting it: Proxima Centauri b, c, and d.
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Red dwarf star, the most numerous type of star in the universe and the smallest type of hydrogen-burning star.
Red dwarf stars have masses from about 0.08 to 0.6 times that of the Sun. (Objects smaller than red dwarf stars are called brown dwarfs and do not shine through the thermonuclear fusion of hydrogen.) Lighter stars are much more plentiful than heavier stars, and red dwarfs are thus the most numerous type of star. In the Milky Way Galaxy, about three-fourths of the stars are red dwarfs. The proportion is even higher in elliptical galaxies.
Stars that burn hydrogen through thermonuclear fusion are classified into seven spectral types on the basis of their surface temperature. These stars also lie in the Hertzsprung-Russell diagram, which plots stellar luminosities against temperature on a line called the main sequence. Red dwarfs are the coolest main-sequence stars, with a spectral type of M and a surface temperature of about 2,000–3,500 K. Because these stars are so cool, spectral lines of molecules such as titanium oxide, which would be disassociated in hotter stars, are quite prominent. Red dwarfs are also the dimmest stars, with luminosities between about 0.0001 and 0.1 times that of the Sun.
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Smaller stars have longer lifetimes than larger stars. While stars like the Sun have a lifetime of about 10 billion years, even the oldest red dwarf stars have not yet exhausted their internal supplies of hydrogen. The heaviest red dwarfs have lifetimes of tens of billions of years; the smallest have lifetimes of trillions of years. By comparison, the universe is only 13.8 billion years old. The dim red dwarfs will be the last stars shining in the universe.
Red dwarfs will not pass through a red giant phase in their evolution. Because convection occurs through the entire star, hydrogen is constantly recirculated from the outer regions to the core. Stars like the Sun are not completely convective and thus burn only the 10 percent of their hydrogen that is located in their cores. When that hydrogen is exhausted, such stars will expand enormously as they start to burn hydrogen on a shell surrounding their helium cores. Red dwarfs, however, because of convection, are completely efficient and will burn their entire supply of hydrogen. They will then become hotter and smaller, turning into blue dwarfs and finally ending their lives as white dwarfs.
Because of its low luminosity, a red dwarf’s habitable zone (the region near a star where liquid water could be found on a planet’s surface) is very close to the star. A planet in that region would orbit the red dwarf every few weeks and thus would often transit its star. Also, the transits would be quite noticeable, since a red dwarf’s small radius means that more of the star would be covered by the passing planet. Such a planet, Proxima Centauri b, was discovered in 2016 around the nearest star to the Sun, Proxima Centauri.