How cool is this? Literally, the coolest: NASA’s Wide-field Infrared Survey Explorer has found the lowest-temperature brown dwarfs ever seen, the tail end of the stellar class of brown dwarfs called Y dwarfs. How not hot are they? This one (called WISE 1828+2650 if you’re playing brown dwarf bingo), spotted by WISE, has a surface temperature of 25° Celsius — that’s 80° Fahrenheit!
As I sit here and write this, it’s warmer outside my window than it is on the surface of that object!
Not only that, another Y dwarf they found, called WISE 1541-2250, may be the seventh-closest object in the sky outside our solar system.
The distance found is not directly measured; it was calculated using the brightness of the brown dwarf. The distance was found using parallax. Even though it’s only about nine light years away, it’s incredibly faint. The only reason it was seen at all is that WISE is tuned to see in the far-infrared, where these things are far brighter than in visible light.
The most exciting part about this is it supports an idea I’ve had (and lots of others have had too) for a long time: Proxima Centauri may not be the closest object to the Sun. A Y-class brown dwarf could be even closer and still have evaded our detection. Even at four light years away — roughly how far Proxima Cen is — a Y star would be pretty hard to see. We may not know for a while yet, but it’s possible.
So what’s the deal with brown dwarfs? Read More
Astronomers recently discovered two brown dwarfs in our solar neighborhood, and they’re actually pretty close by: 15 and 18 light years away!
[Click to hugely unendwarfenate.]
The two objects were spotted in observations made by WISE, the Wide-field Infrared Survey Explorer, which recently finished its mission to observe the entire sky in infrared light. In the false color images from WISE, brown dwarfs tend to appear very green (fun details are in that link), making them somewhat easy to spot against full-blown stars which tend to appear blue — remember, this is false color!
Anyway, the astronomers were looking for nearby brown dwarfs (PDF), so they searched for green objects that had no obvious counterparts in older infrared surveys. All stars orbit the center of the Milky Way galaxy at different velocities, and over time that means they move across the sky. Nearby stars appear to move fastest (just like the nearby trees fly past you while driving, but distant mountains appear to move more slowly), so nearby brown dwarfs would have moved in the time separating the older surveys from that of WISE.
The astronomers actually found quite a few objects, most of which were known. But these two, called WISE J0254+0223 and WISE J1741+2553, were not previously known — in the picture above, their positions in the older survey from 2000 are labeled compared to their positions in the 2010 WISE images. Remarkably, only 39 star systems (I include multiple stars as one system here) are known to be closer to us than J1741 (which is 15 light years away), and only a handful of them are brown dwarfs (what are called T class objects).
Which, as always when we find new nearby stars, make me wonder: are there faint, cool brown dwarfs even closer to us? Is it possible that Proxima Centauri, a red dwarf 4.2 light years away, is not the closest star to the Sun?
Maybe. The WISE data used to find these two neighbors is not the full set taken by the spacecraft. There’s still quite a bit of data to sift through. Who knows, we may yet find out there’s a star or stars passing by still waiting to make our acquaintance.
- WISE finds the coolest stars. Literally.
- The galaxy may swarm with billions of wandering planets
- Are we in danger from a rogue planet?
- The case of the brown star that’s really red or possibly blue
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:
[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.
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.
Image credits: JAC/UKIRT, Spitzer Space Telescope, University of Hertfordshire, and Subaru Telescope (NAOJ), University of Hertfordshire.