Brown dwarfs form like stars but never gather enough mass to sustain stable hydrogen fusion. They radiate away formation heat and cool for billions of years, drifting down through spectral classes.
- Below ~13 M♃ — no deuterium fusion; a planetary-mass object.
- 13–80 M♃ — brief deuterium burn only, then cools through L, T and Y classes.
- Above ~80 M♃ (~0.075 ☉) — stable hydrogen fusion; a true M-dwarf star holding a steady temperature.
- Radius barely changes across the whole brown-dwarf mass range — electron degeneracy pressure keeps them all close to Jupiter's size even as mass climbs sixfold.