Astronomers have discovered the coldest-ever failed star with a surface temperature of 100o C. To give you some perspective, consider: 1) the hardiest Finns can easily withstand 120o C saunas; and 2) our Sun, a relatively mediocre star, has a surface temperature of almost 5,500o C.
This lukewarm object, called CFBDSIR J1458+1013B, appears to be a brown dwarf, a class of objects somewhere between star and super-planet. The distinction between star and brown dwarf is clear — a star is only a star if it undergoes nuclear fusion in its core — but the distinction between brown dwarf and super-planet isn’t as clear. Brown dwarfs are all about the same size as Jupiter, but they pack a lot of mass into that space — at the high end of the range, they can be 90 times the mass of Jupiter — and unlike planets, they sometimes emit X-rays. CFBDSIR J1458+1013B is at the lower end of the range with a mass of about 6-15 Jupiter-masses. With its super-cool surface temperature, astronomers speculate that it could even have water clouds in its atmosphere.