Red dwarf
A red dwarf is a star with very low mass and low luminosity, whose surface temperature is less than about 4,000 K, giving it an orange-to-red hue. Classified as spectral type M (and sometimes late K), they are by far the most abundant stars in the Milky Way— accounting for about 70 to 75% of all stars—although they are too faint to be seen with the naked eye from Earth.
Very Efficient Stars
Red dwarfs burn their hydrogen so slowly that their lifespans far exceed the current age of the universe. A red dwarf with a mass of 0.1 solar masses could burn for more than ten thousand billion years—about 700 times the current age of the universe. Unlike the Sun, which burns only the hydrogen in its core, the least massive red dwarfs are entirely convective: they continuously mix all of their hydrogen, allowing them to use nearly all of it.
Red Dwarfs and Exoplanets
Red dwarfs are prime targets for the search for habitable exoplanets. Their habitable zone, which is closer to the star than that of the Sun, makes planetary transits more frequent and easier to detect. The TRAPPIST-1 system, a red dwarf 40 light-years away, has seven planets, three of which are in its habitable zone. However, the high magnetic activity of red dwarfs—frequent and intense eruptions—could compromise the habitability of nearby planets.
Importance in Astronomy
Understanding red dwarfs is fundamental to galactic astronomy: they account for the majority of the galaxy’s visible stellar mass. Their long lifespans make them archivists of the Milky Way’s chemical and dynamical history. Their abundance also makes them the most numerous candidates for potentially harboring extraterrestrial life.
Concrete example
Proxima Centauri, the red dwarf closest to the Sun at 4.24 light-years away, hosts at least one exoplanet in its habitable zone—Proxima b— discovered in 2016. Despite its proximity, Proxima is completely invisible to the naked eye, with an apparent magnitude of +11.1.
Frequently Asked Questions
Why are red dwarfs invisible to the naked eye?
Because they are not bright enough. Even Proxima Centauri, the closest star to Earth after the Sun, is thousands of times too faint to be visible to the naked eye. You need a telescope with a diameter of at least 50 mm to begin detecting them.
Can a red dwarf become a red giant?
The least massive red dwarfs (less than 0.35 solar masses) will probably never become red giants in the classical sense. Being entirely convective, they will gradually burn all their hydrogen and slowly become white dwarfs composed ofhelium over timescales that are incomprehensible on a human scale.
Are red dwarfs older than the Sun?
Some are, yes. Red dwarfs formed early in the galaxy’s history may be 13 billion years old or older. But their individual ages vary, just as with any type of star—some red dwarfs are young, while others are very old. Their long lifespans do not mean that they are all ancient.