🪐 Brown Dwarf Classification Explorer

About this simulation

Brown dwarfs occupy the twilight zone between giant planets and true stars. They form the same way stars do — from a collapsing cloud of gas — but they never accumulate enough mass to ignite stable hydrogen fusion in their core. Instead they radiate away the heat of their own formation and slowly cool for billions of years, drifting down through spectral classes as they fade.

How it works

• Below ~13 Jupiter masses — not even deuterium fusion ignites; this is a planetary-mass object.
• ~13–80 Jupiter masses — deuterium burns briefly early on, but ordinary hydrogen (protium) never ignites. This is the brown dwarf regime: L, T and Y spectral classes, from dusty and warm down to cold and nearly planet-like.
• Above ~80 Jupiter masses (~0.075 ☉) — core pressure and temperature are high enough to sustain stable hydrogen fusion. The object becomes a true star (a red or M-type dwarf) and holds a steady temperature for billions of years instead of cooling away.
• Because brown dwarfs have no steady energy source, their spectral class is not fixed by mass alone — the same object drifts from a hot, dusty L dwarf to a cold, methane-rich T dwarf and eventually a frigid Y dwarf as it ages.

Controls

• Mass — sets the object's mass in Jupiter masses (M♃). Crossing ~80 M♃ crosses the hydrogen-fusion boundary.
• Age — how long the object has been cooling since it formed, from a few million years to over ten billion years.
• Jump to preset — snaps mass and age to well-known real-world examples.
• Cutaway core view — slices the object open to show whether its core is fusing hydrogen, briefly fusing deuterium, or simply cooling.

Did you know?

Radius barely changes across the entire brown dwarf mass range — from 13 to 80 Jupiter masses, degeneracy pressure keeps every one of them close to Jupiter's own size. The scale bar on the right shows Jupiter and the Sun for comparison, so you can see just how little a substellar object grows even as its mass increases sixfold.