What is a Red Dwarf?
A red dwarf is a small, cool, faint star of spectral class M — the most common kind of star in the universe. Around three out of every four stars in the Milky Way are red dwarfs, yet not a single one is visible to the naked eye from Earth.
Red dwarfs sit at the bottom of the main sequence, between roughly 8% and 45% of the Sun's mass. Below that lower bound, the core never gets hot enough to fuse hydrogen at all and the object becomes a brown dwarf instead. Above it, fusion runs but slowly: a red dwarf can be ten thousand times fainter than the Sun, radiating most of its output in the infrared.
Their real superpower is patience. Small red dwarfs are fully convective — the whole star churns, so helium ash does not pile up in an inert core and essentially all of the star's hydrogen is available as fuel. Combined with a miserly burn rate, this gives lifespans measured in trillions of years. Every red dwarf ever formed is still on the main sequence; the universe is not old enough for one to have died.
They are not gentle, though. That same deep convection twists magnetic fields into knots that snap, producing flares which can briefly outshine the entire star in X-rays and strip the atmosphere from any nearby planet. Because the habitable zone of such a dim star sits very close in, this matters enormously: Proxima Centauri, the nearest star to the Sun, is a flaring red dwarf with a planet in its habitable zone.