The biggest stars in the Universe are the monster red hypergiants, measuring up to 1,500 times the size of the Sun. But what are the smallest stars in the Universe? (from universetoday.com)

The smallest stars around are the tiny red dwarfs. These are stars with 50% the mass of the Sun and smaller. In fact, the least massive red dwarf has 7.5% the mass of the Sun. Even at this smallest size, a star has the temperature and pressures in its core so that nuclear fusion reactions can take place.
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One example of red dwarf star is the closest star to Earth, Proxima Centauri, located just 4.2 light-years away. Proxima Centauri has 12% the mass of the Sun, and it’s estimated to be just 14.5% the size of the Sun. The diameter of Proxima Centauri is about 200,000 km. Just for comparison, the diameter of Jupiter is 143,000 km, so Proxima Centauri is only a little larger than Jupiter.

But that’s not the smallest star ever discovered.

The smallest known star right now is OGLE-TR-122b, a red dwarf star that’s part of a binary stellar system. This red dwarf the smallest star to ever have its radius accurately measured; 0.12 solar radii. This works out to be 167,000 km. That’s only 20% larger than Jupiter. You might be surprised to know that OGLE-TR-122b has 100 times the mass of Jupiter, but it’s only a little larger.

And that is the smallest known star. But there are certainly smaller stars out there. The smallest theoretical mass for a star to support nuclear fusion is 0.07 or 0.08 solar masses, so smaller stars are out there.