WISE finds the coolest stars. Literally.

The entire universe in blog form
Nov. 12 2010 7:00 AM

WISE finds the coolest stars. Literally.

I have a few pet objects in astronomy that fascinate me endlessly. One of these is brown dwarfs, objects that are bigger than planets, but too small to be bona fide stars. They are much cooler and fainter even than dinky red dwarfs, making them very difficult to find... unless you are WISE:


Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


[Click to embiggen.]

See that pale green dot in the middle? That's a brown dwarf! I know, it's not brown, it's green, but that's kosher since brown dwarfs are really magenta.

OK, hang on a sec. I'll explain that in a minute.

The important thing is that this image shows a very nearby brown dwarf, maybe 18 - 30 light years away (the distance is hard to determine, but observations taken over the next year or so should pin it down). That's really close! The nearest known star, Proxima Centauri (a faint red dwarf) is 4.2 light years away, and only a few hundred stars are within 30 light years. That makes this brown dwarf, named WISEPC J045853.90+643451.9 (after its location in the sky), one of the closest stars known.

You'd think think we'd have a pretty good idea of all the stars near us, since they'd be among the brightest in the sky. But in fact brown dwarfs are so faint that to optical telescopes they can escape detection even if they're our cosmic neighbors. WISE, however, looks in the infrared, where brown dwarfs glow considerably brighter.

And that brings me to the weird colorful adjectives we use for these objects. The color of a star depends mostly on its temperature (and the way our eyes see the mixed colors from stars, which is complicated, and I will ignore here). Very hot stars are blue, middlin' hot are white (like the Sun), cooler ones are orange, and very cool ones are red. Brown dwarfs are even milder than red dwarfs, and their spectrum actually peaks in the infrared.

But it's a bit more complicated than that. Brown dwarfs are so cool that actual molecules can exist in their atmosphere, like water (well, steam), titanium oxide, ammonia, even hydrogen sulfide (which makes eggs smell rotten). These molecules absorb light at certain colors, and that absorption can really mess with a brown dwarf's color. Sometimes, a cooler brown dwarf can actually be more blue than a warmer one, because those molecules absorb the redder light the object emits. It's weird. So a lot of these brown dwarfs actually would appear to be magenta to the eye.

So why are they called brown dwarfs? Blame SETI's Jill Tarter. She just wanted a name for the critters, and compromised on a color between red and black. Stars can't be brown, really, but the name stuck.

Anyway, if they're magenta and not brown, why does that one look green in the WISE picture? It's because WISE look in the infrared, which our eyes can't perceive. So when they make pictures form the data, astronomers use false colors; they let blue represent the shortest IR wavelength WISE sees, green the intermediate, and red the longest wavelength -- this corresponds to visible light, with wavelength getting longer from blue to green to red. It's a shorthand that astronomers use that makes images like this easier to interpret.

As it happens, brown dwarfs at a certain temperature emit very strongly in the infrared wavelength astronomers code as green in the WISE images. So finding brown dwarfs is actually not too hard: just look for green stars! Those are really the magenta brown dwarfs.


Anyway, I for one welcome WISEPC J045853.90+643451.9 to our little clan of nearby stars, and hope we find more. For quite a long time I've wondered if Proxima Cen really is the closest object to the Sun, or if there might be a faint brown dwarf even closer. Since brown dwarfs aren't considered stars, Proxima may yet hold on to the title of "nearest star". But if we ever do find something even closer, it would literally be very cool.

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