Hey SciShow, Phil Plait from badastronomy.com here. Hank and I have a secret project in the works that will be coming out relatively soon. But in the meantime, I want to talk to you about the smallest star in the universe... Probably.

In 2013, astronomers at Georgia State University discovered what may well be the smallest star in the universe. About 8000 times dimmer and only 8.6% as wide as the sun, the star called 2MASS JO523-1403 can actually fit inside the planet Jupiter. It's not far away, either. Just about 42 light-years from us. But it's not like scientists declared this little starlet the smallest after measuring every other star in the universe, obviously. Instead, they think it is the smallest star because it's as small as a star can get and still be considered a star. See, if you look around the galaxy for smaller and smaller stars, you eventually run into a problem. After a while, you start finding brown dwarfs, so-called "failed stars" that weren't able to sustain nuclear fusion. These objects are so cold and so dim that they're considered "substellar objects" rather than actual stars. And ultimately, it's the mass of an object that determines whether it's a brown dwarf or an actual star. 

The problem is, the mass of these objects can only be measured directly through their gravitational interactions, like when two of them happen to be orbiting around their common center of mass. So instead, one clue scientists use to figure out the identity of an object is how hot it is. In the case of stars, for example, we know that the higher its surface temperature is, the more mass it has because more massive stars burn their fuel more rapidly and get hotter. And by extension, less massive stars have lower temperatures than more massive stars. But more importantly, the fusion reactions that happen inside stars keep them at a constant size, temperature, and brightness for a really long time, up to trillions of years for the smallest stars.

But that's not the case for brown dwarfs. They lack the sustained fusion to maintain such temperatures, so a brown dwarf that has more mass will be hotter for a while, but it will also cool off and dim down pretty quickly on a cosmic timescale. So when you're scouting out low mass objects, the trick is in telling the difference between a cool, old star and a warm, young brown dwarf, because both will be about the same size and have about the same temperature. And this brings us back to our smallest star.

Because both small stars and brown dwarfs are more than a thousand times fainter than the sun, we can only see them if they're really close to us. So the Georgia State team limited its search to our solar neighborhood; the tiny fraction of our galaxy closest to the sun. And they identified 63 very faint objects, determining the diameter and temperature of each. The object that stood out was the tiniest one around, with a surface that was a lukewarm, 2075 Kelvin. The temperature, size, and brightness were all right at the limit between a cool, low-mass star and a hot, young massive brown dwarf. It was almost too close to call.

But then there were other factors that the scientists considered. One was simple probability. Basically, the longer lived an object is, the more likely it is that we'll find it. Since stars keep burning long after brown dwarfs have cooled and faded away, that suggested that this warmish object was actually a star. So all things considered, the most likely answer is that this object is, in fact, a really old, cool star. And now astronomers are fascinated with their new little friend, because it might help them finally figure out how to best tell stars from brown dwarfs. We don't exactly know what the mass limit is that distinguishes a small star from a brown dwarf, but based on previous observations and theoretical calculations, we think that the threshold might be somewhere between six to eight percent the mass of the sun. So for now, this little star with a big name turns out to be the smallest that a star could be, earning it the title of smallest star in the universe... maybe.

Thanks for joining me for SciShow Space. If you want to learn how you can help us keep exploring the universe together, just go to Subbable.com/SciShow and don't forget to go to youtube.com/SciShowSpace and subscribe. Until then, I'm Phil Plait from badastronomy.com. Thanks