Brown dwarfs are poorly named: they're not really brown. They're objects that are too small to really be called stars; they lack the oomph needed to fuse hydrogen into helium in their cores, which is the the mark of a true star. Because of this, they are far cooler than actual stars. Since cool stars are red, you'd think brown dwarfs would actually be really red.
And they are. Unless they're blue.
Yeah, let me explain this one. First, here are two images of a newly discovered brown dwarf, perhaps the coolest ever seen, and certainly one of the closest to the Earth:
[Click to redgiantize.]
The star SDSS1416+13A is the brighter one in the image, and is a regular ol' brown dwarf. The other star is its lower mass and cooler companion, called SDSS1416+13B. How cool is it? Scientists estimate that it's at about 200 Celsius (400° F). I ate chicken last night hotter than that! So as stars go, 1416+13B is pretty cool.
Observations taken some time apart show that the two stars are in fact binary, orbiting around each other. Since we don't know exactly how far away these two are, we can't say exactly just what their masses are, but the way they give off light is a dead giveaway they are both brown dwarfs. It's possible to estimate their distance, and scientists think they are between 15 and 50 light years away. That makes them very close to us as stars go! The Milky Way is 100,000 light years across, so these guys are basically sitting in our front yard.
Now, let me take a sec to explain some jargon. Blue light has a shorter wavelength than red light. Because of this, astronomers sometimes use the words "blue" and "red" as adjectives, meaning shorter and longer wavelengths, respectively. So blue is bluer than red, and red is redder than blue. Duh. But they can also say with a straight face that red is bluer than infrared, and infrared is redder than red! That's because red has a shorter wavelength than IR, and is therefore "bluer", while the IR is longer wavelength than red, and is therefore "redder". Got it? It actually makes sense, and you eventually get used to it. I'll be using this jargon below, so be ye fairly warned.
The pictures above are false color; both are in infrared light (the left is from the ground-based UKIRT telescope, while the one on the right is from the space-based Spitzer telescope). You might expect that since 1416+13B is cooler than its companion, it should be giving off more long-wavelength (redder) IR light. But in the case of the left image, the blue color still means 1416+13B is giving off more light at the shorter (bluer) end of the IR part of the spectrum. What gives?
Brown dwarfs are weird, that's what gives. They have atmospheres almost like planets do, and that air is filled with methane, water vapor (steam!), and sometimes even vaporized iron for hotter ones -- in cooler brown dwarfs, that iron precipitates out... in other words, it rains molten iron droplets!
In the case of 1416+13B, the atmosphere is cool enough that methane and steam absorb the light coming from below. Those two molecules are picky about what light they absorb, and they soak up quite a bit of IR at different wavelengths, allowing other wavelengths through. So what's happening here is that some of the redder IR light gets sucked up, while bluer IR passes right through. What we see from outside is the star emitting bluer IR light, so images taken in IR make the star look blue.
This spectrum, taken with the Subaru telescope, might help:
Think of the vertical axis telling you how much light the gas in the star's atmosphere lets through, and the horizontal is the color. Bluer IR is on the left, redder on the right. You can see that a handful of blue colors blast right through, but the star emits very little in the red. So when we look at it with our infrared telescopes, we see it looking blue.
Mind you, to our eyes, this guy would look very, very red. But that's in visible light, off to the left (blue) of this graph.
So, given all this, why does the star look red in the Spitzer image? Aiiiiieeee!
OK, don't panic. That's because Spitzer looks at a different part of the IR spectrum. It sees light at 3.6 and 4.5 microns, well off to the right (red) of the spectrum shown above. In those wavelengths, 1416+13B looks redder.
So here we have a brown dwarf that looks red, or maybe blue. It all depends on how you look at it.
But that's the whole point! By looking at stars at different wavelengths, we can find out a lot about them. In this case, we can estimate the distance to the star, its temperature, and even what's in its atmosphere... all from hundreds of trillions of kilometers away!
Things like this never cease to amaze me. Science! I love this stuff.
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Image credits: JAC/UKIRT, Spitzer Space Telescope, University of Hertfordshire, and Subaru Telescope (NAOJ), University of Hertfordshire.