spaceSpace and PhysicsspaceAstronomy

We Finally Know How Big A "Failed Star" Can Be


Jonathan O'Callaghan

Senior Staff Writer

Artist's impression of a brown dwarf. NASA/JPL-Caltech

It may not quite be the answer to life, the universe, and everything. But the number “70” is about to get propelled to stardom, as it’s just solved one of the greatest mysteries in astronomy.

Scientists have long wondered how much mass a star needs to become a star in the first place. They are born from clouds of dust and gas, which form a collapsing ball. If there is enough mass at the start, fusion is ignited at the core of the resulting star. If not, a failed star – or a brown dwarf – is formed.


Now we know how much mass there needs to be. It’s 70 times the mass of Jupiter. This discovery was made by Trent Dupuy from the University of Texas at Austin and Michael Liu from the University of Hawaii. They’re presenting their findings today at the 230th meeting of the American Astronomical Society in Austin, Texas. A paper will also be published in the Astrophysical Journal (also available on arXiv).

“When we look up and see the stars shining at night, we are seeing only part of the story,” said Dupuy in a statement. “Not everything that could be a star ‘makes it,’ and figuring out why this process sometimes fails is just as important as understanding when it succeeds.”

It wasn’t just the limiting mass they discovered, though, but also the temperature. They found that if an object is cooler than 1,300°C (2,400°F), then it will be a brown dwarf and not a star.

To make their findings, the duo spent a decade observing 31 ultracool binaries using the Keck Observatory and the Canada-France-Hawaii Telescope, and the Hubble Space Telescope. These are pairs of either brown dwarfs, or the lowest mass stars.


Binaries are useful because by observing the size and speed of each object’s orbit around a binary's center of mass, the mass of each object can be measured. Ultracool binaries are a bit tricky, though, as brown dwarfs are so much dimmer than true stars, which is why such powerful telescopes are needed to study them.

The final figure of 70 Jupiter masses is slightly lower than previous predictions, which had suggested about 75 Jupiter masses. Above this, objects cannot be cold enough to be brown dwarfs, suggesting they have nuclear fusion taking place at their cores.

While we have an upper mass limit for brown dwarfs now, the lower mass limit is still not clear. The difference between a large gas giant planet and a brown dwarf remains a gray area, but maybe someone can figure that out in the future.


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