A brown dwarf: not enough mass to shine like a star NASA/JPL-Caltech

They may be small, but they’re still stars. New observations indicate that objects born with a mass just 6.7 per cent that of the sun can shine for trillions of years rather than fizzle out as failed stars known as brown dwarfs.

Stars like the sun shine as a result of nuclear reactions that convert hydrogen into helium at their hot centres. The hotter the core, the more intense the reaction and the brighter the star’s surface. Those born less massive have cooler cores and therefore slower reactions, making them dimmer.

Brown dwarfs, meanwhile, have so little mass that their centres stay cool and they cannot sustain nuclear reactions – even though they are still far more massive than our solar system’s largest planet, Jupiter. Despite their name, brown dwarfs glow red when they form, then slowly cool and fade to black. If our sun was a brown dwarf, noon would look darker than a moonlit night.


Calculations of how stars evolve previously suggested that the boundary between brown dwarfs and red dwarfs – dim stars just massive enough to sustain nuclear fusion – was at a mass 7 to 8 per cent that of the sun. But until now, this had never been directly measured.

Watching them revolve

Trent Dupuy at the University of Texas at Austin and Michael Liu at the University of Hawaii in Honolulu measured the masses of 37 red and brown dwarfs by watching them slowly revolve around each other. Their orbits, which are dependent on their gravity, revealed their masses.

Only six such measurements of these types of object had been performed before – not enough to see the boundary between red and brown dwarfs.

“I know the kind of pain it takes to do this right, so they showed great fortitude,” says Todd Henry at Georgia State University in Atlanta.

For red dwarfs, the less massive the star, the cooler and dimmer it is. But this relationship does not hold for brown dwarfs. They cool and fade over time, so an old brown dwarf is cooler and dimmer than a young one, even if they have the same mass

“There should be a break where there’s no longer any correspondence between mass and surface temperature,” says Dupuy. Ranking the objects by mass revealed this break.

The boundary between successful and failed stars turns out to be at a mass of about 6.7 per cent that of the sun, or 70 times that of Jupiter. This is slightly smaller than expected.

“It’s superb work,” says Henry. “This is definitely a breakthrough.”

Reference: arxiv.org/abs/1703.05775