What is the difference between a red and brown dwarf?

Proximal Centauri, the nearest star outside our Sun at 4.2 light years away, is a red dwarf. Red dwarfs typically have between 7.5% and 50% of the mass of the Sun. Occasionally, slightly more massive stars are included, but our Sun is definitely too massive. None of these has more than a tenth the luminosity of the Sun, and some smaller red dwarfs can shine on for trillions of years (our Sun will last a total of only about 10 billion years).

Brown dwarfs are far less luminous than even red dwarfs; they are referred to as "substellar objects" while red dwarfs are full stars. Because of their low luminosity and relatively low temperatures, brown dwarfs are more easily detected by infrared emissions rather than being seen visually, even with powerful telescopes. The nearest (known) brown dwarf is 6.5 light years away. Brown dwarfs are distinguished by their inability to fuse ordinary hydrogen, but they can fuse some less abundant isotopes like deuterium and lithium.

Red dwarfs are low-mass stars that can sustain hydrogen fusion in their cores; brown dwarfs are substellar objects that lack the mass to sustain hydrogen fusion and instead produce heat and light through gravitational contraction. These two types of dwarfs are primarily distinguished by their mass and the fusion processes taking place in their cores.