TRAPPIST-1 is classified as what kind of small, cool star?

TRAPPIST-1 is classified as an ultra-cool red dwarf.

Red dwarfs are small, relatively cool main-sequence stars whose light is strongest in red and infrared wavelengths. TRAPPIST-1 is an especially cool example, with spectral type M8V and a mass of roughly 9% of the Sun’s. Despite its small size, it is massive enough to sustain hydrogen fusion.

The star lies about 41 light-years away in Aquarius and became famous after observations revealed seven roughly Earth-sized planets. The name TRAPPIST-1 comes from the TRAPPIST telescope project, which discovered the system’s first planets in 2016; additional planets were announced in 2017.

A common mix-up is calling TRAPPIST-1 a brown dwarf. Brown dwarfs are substellar objects that never sustain ordinary hydrogen fusion, whereas TRAPPIST-1 is a true star. Its planets are also not red dwarfs: they orbit the red-dwarf star.