Astronomers Discover Star No Hotter Than Your Sauna

This photo of the brown dwarf double star CFBDSIR 1458+10 was taken with the Keck II Telescope in Hawaii. It's the coolest pair of brown dwarfs found so far—the colder and dimmer of the two is a candidate for the brown dwarf with the lowest temperature ever found of 212 degrees. Credit: Michael Liu, University of Hawaii

I boil water for tea twice a day. And while boiling water burns like heck if it spills on your hand, it’s not usually what we think of when it comes to star temperatures. Those typically range from the low thousands to more than 70,000 degrees Fahrenheit. Many of the coolest stars, called brown dwarfs, can be under a thousand, but that’s still hotter than anything in my kitchen.

Today the European Southern Observatory reported that astronomers there, in coordination with Michael Liu of the University of Hawaii’s Institute for Astronomy,  were able to determine the temperature of the coldest star yet known. Called CFBDSIR 1458+10B, it’s a brown dwarf in a binary star system of two brown dwarfs 75 light years from Earth.

CFBDSIR 1458+10B is steaming away at the relatively pleasant temperature of 212 degrees Fahrenheit or the boiling point of water. That’s warm enough to support clouds of hot steam in its atmosphere. In comparison, the surface of the sun sizzles at 11,000 degrees.

The sun, a modest star, is large compared to the wee stars in the galaxy - the red and brown dwarfs

Although astronomers don’t yet know how large the dwarf is, typical brown dwarfs are similar to Jupiter in size but 15 to 75 times as massive. Those on the low end of the scale probably never burn hydrogen like real stars do, because they’re neither dense not hot enough in their cores to initiate the hydrogen fusion process responsible for a star’s light and heat.

Dwarfs on the higher end fuse a form of hydrogen at least early in their lives before cooling down and going dark like an old bulb. What’s remarkable about the new find is that it straddles the line between star and the hundreds of giant exoplanets that astronomers have discovered since 1995.

Used to be things were in neat categories like stars, planets and moons, but the more we dig around, the more connections and fine gradations we see between objects once thought to be separate and unique.

Take Pluto, which was recently re-designated a dwarf planet thanks to finding lots more objects like it in the outer solar system. As humans, we like to see things placed in nifty bins, but nature is more like a continuum, with room for just about anything, even oddball stars the temperature of your sauna.

The moon scoots up to Antares in Scorpius tomorrow morning. The two are best visible at the start of dawn in the southern sky. Created with Stellarium

Just a final observer’s note. If you’re up tomorrow at dawn, face south and look a few degrees to the lower left of the moon to find Antares, the brightest star in Scorpius the Scorpion. The moon pays regular visits to Antares, since the star lies so close to its monthly path through the sky.