WISE finds the coolest stars. Literally.

By Phil Plait | November 12, 2010 7:00 am

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:

wise_browndwarf

[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.

See?

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.


Related posts:

Two nearby galaxies peek out through the dust
A WISE flower blooms in space
The seven WISE sisters
Three views to a comet


CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (46)

  1. 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.

    Does this mean that you can eat green brown dwarfs that are really magenta?

  2. GB

    Is it possible that a brown dwarf’s gravity could sweep up enough mass (perhaps by passing though clouds of interstellar dust) to ignite the fusion process?

  3. Aaron

    Al Feersum-Does this mean that you can eat green brown dwarfs that are really magenta?

    Only if they are in season.

  4. @Aaron – I don’t think I’d want to try one if it wasn’t ripe.

  5. Messier Tidy Upper

    Well done WISE guys! :-)

    Superb discovery, I love this mission.

    So finding brown dwarfs is actually not too hard: just look for green stars!

    Using heat and false colour – and they’re really purple but called brown.

    Plus technically speaking are the brown dwarfs really “stars” at all or just super-successful Jupiters instead?

    Yep, that clears things up very nicely I must say! ;-)

    (Not knocking it srsly – Well written up here tho’ BA.)

    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.

    Well it makes a kind of sense on the scale of what’s cooler than red but not black.

    Violet-y purple oddly enough seems to fit for the very hottest and bluest stars or am I thinking more ultra-violet there so having a similar hue also for the magneta brown dwarfs is paradoxially apt. :-)

    Three suggested alternative names : infra-red dwarfs or heat dwarfs or mauve dwarfs?

  6. Messier Tidy Upper

    Green stars – there’s always Zubeneschamali : (Beta Librae – a few variations on the spelling exist.)

    http://stars.astro.illinois.edu/sow/zubenes.html

    which optically does appear green although opinions vary. I’ve seen it as having a greenish colour on occassion. Some of that may be imagination but still. :-)

    There’s also Antares B, the red supergiants companion star, and a handful of others that are probably due to contrast effects although maybe there’s more than just that.

    Both Antares B and Zubeneschamali are B8 V spectral class~wise.

  7. Is there actual fusion going on in the core of these brown dwarves (Tolkienesque pluralization FTW!)? Or are they just hot from pressure?

  8. JMW

    If Pluto is not a planet, is a brown dwarf not a star?

    Seems we need a definition from the IAU.

  9. Alareth

    I will not eat green dwarfs and ham
    I will not eat them Sam I Am

  10. Jamey

    @Carey – was just reading the Wikipedia articles on these puppies recently, and the larger ones do fuse deuterium for a short period early in their life. The funniest thing though is that no matter their mass, their size is like within 1% or so of Jupiter’s.

  11. On a more serious note, there has long been speculation (even amongst astronomers) that there is ‘something large’, possibly a brown dwarf, just outside the Oort cloud – no, I’m not talking about Niburu.

    I know Space is Deep – but surely WISE (or it’s predecessor WIRE) would have seen it by now (being wide field observers and all that). Or is it long sighted? Or does the Oort cloud hide it? Or, horror amongst horrors, does this ‘something large’ not actually exist, which is why we can’t find it?

  12. Messier Tidy Upper

    For more info green stars wise – though not WISE~wise but still a wise place to look for more ;-) – See :

    http://blogs.discovermagazine.com/badastronomy/2008/08/28/followup-green-objects-in-space/

    & my comment (back when I was posting as StevoR) there here :

    http://blogs.discovermagazine.com/badastronomy/2008/08/28/followup-green-objects-in-space/#comment-114922

    I also quoted an idea on on the possible cause for Zubeneschamali’s greenness at comment # 71 there.

    Plus there’s alist of green stars of spectral type B further down there :

    http://blogs.discovermagazine.com/badastronomy/2008/08/28/followup-green-objects-in-space/#comment-115532

    Notably Almach (Gamma Andromedae) as well as Zubeneschamali & Antares B are all B8 “blue / green” dwarfs. More than just co-incidence perhaps?

    But then, Rigel is a B8 blue supergiant that’s non-green & many other non-green B98 stars are also known eg. Sirrah a.k.a. Apheratz (Alpha Andromedae) and Gomeisa (Beta Canis Minoris) so, yeah, really not sure if there’s really anything to this B8 = emerald hued star possible pattern or not and likely not. All the same – does anyone have any ideas on what might be the possible explanation here please?

  13. named WISEPC J045853.90+643451.9

    See, personally, I’m not sure I’d call that a name.. ;-)

    A catalogue designation sure, but a name – no. The simple descriptive False Green Star or the WISE Green Star are my temporary “working title” suggestions.

    @8. JMW Says:

    If Pluto is not a planet, is a brown dwarf not a star?
    Seems we need a definition from the IAU.

    Agreed entirely there.

  14. Come on people, that’s obviously not a BROWN dwarf. It’s the Starbug from RED Dwarf. :D

    http://en.wikipedia.org/wiki/File:Starbug800x600.jpg

  15. Messier Tidy Upper

    @14. ^ Lewis : Nice one. :-)

    @8.JMW. Part II continuing from # 13 : [out of edit time, sorry.]

    I think we need to revise the IAU’s “planet” definition mistake & get it corrected sooner rather than later.

    I’m for something like their initial definition before they undemocratically rushed through the additional Pluto-bashing superfluous and unnecessarily complicating “orbital clearance ” criterion. A criterion that means, btw., that Jupiter orbiting at Pluto’s distance would NOT still be a planet but become a dwarf variety of gas giant instead whilst Pluto orbiting at Mars distance *would* be a planet – just because of the relative amounts of space they have to cover! Consistency and common sense FAIL there.

    Let’s broaden the membership of the “Planets club” & let the ice dwarfs in! :-)

    Of course, this is a whole other story but worth a quick mention methinks – and who knows if a small brown dwarf or superjovian world is located by WISE inside our Oort Cloud then perhaps it too will have to be labelled a “dwarf planet” under the ridiculous IAU definition! :-o ;-)

  16. Messier Tidy Upper

    PS. Off topic sorry but still interesting here Pluto-wise see :

    http://blogs.discovermagazine.com/80beats/2010/11/08/new-bragging-rights-for-pluto-it-may-be-the-biggest-dwarf-planet/#comments

    for more news – turns out it may be larger than Eris after all although still less massive. Plus my preferred definition of planet is given at comment #5 there.

  17. That makes three brown dwarfs so far, and the WISE team has said that they expect to discover 1000 of them so that leaves just a few more…like you, I’m most looking forward to the discovery of something closer than Proxima Centauri. A discovery like that would be phenomenal in the way it would change our view of our corner of the universe over the past decade or two. Instead of nine planets followed by a lot of nothing and then finally another star, we would end up with eight big planets, dozens and dozens of smaller ones, a belt of far away objects, then a cloud of other stuff, then a brown dwarf or two or three or more, and THEN the next star. Much, much better.

  18. Gary Ansorge

    2. GB

    Yes, it IS possible. Though it might take a few million years.

    Hydrogen sulfide. Not only stinky but also about ten times more toxic than cyanide. That was the last compound to be flared off(burned) at Berri NGL plant in Saudi Arabia. We captured nearly all the rest of the interesting gasses(propane, methane, pentane, butane) and solid sulphur.

    Gary 7

  19. If these are stars that ‘didn’t quite make it’, is it possible that they are very common, perhaps even more so than ‘real’ stars? Thus adding their non-luminous bulk to that of galaxies and representing some of the mass that they attribute to dark matter?

    Or is that just silly.

  20. Duane

    “So when they make pictures form the data, astronomers use false colors”
    I think you meant to say “make pictures FROM the data”?
    *
    I’m writing an SF story about explorers stranded on an Earth sized planet closely orbiting a brown dwarf, so I’m avidly educating myself as much as possible about these ultra cool objects (in more ways than one!). I don’t have all the details nailed down yet, but I DO know the explorers won’t mistake their accidental surroundings for Miami Beach….

  21. Can brown dwarfs have planets around them? If they are not stars per se, would the planets then be moons? Grrr….semantics!

  22. Utakata

    I think they should rename them pink dwarves, because it sounds a lot nicer than brown…

  23. Misora

    how warm.. er, cold would this be in comparison to our sun?

  24. Gonçalo Aguiar

    I think “brown” dwarfs are not considered stars because they aren’t massive enough to create the conditions to have an auto-sustained nuclear fusion in their cores.
    See it like a wanna-be-star that failed because it ain’t big enough.

  25. Utakata

    …like homeopathy compaired to real medicine, Gonçalo Aguiar @ 24?

  26. amphiox

    The funniest thing though is that no matter their mass, their size is like within 1% or so of Jupiter’s.

    I think this relationship holds all the way up to the very smallest red dwarfs, at around 80 Jupiter masses before the radiant pressure from stellar fusion starts to make the stars get bigger again.

    This interestingly makes these high-mass brown dwarfs and low-mass red dwarfs just about the densest normal-matter objects in the universe. Jupiter’s density is about 1.3g/cm3, so a 70-80 J mass brown/red dwarf is going to have a density of around 90 – 105 g/cm3. By comparison this is almost 10X the density of lead (11 g/cm3), and 5X the density of depleted uranium (20 g/cm3).

  27. Duane

    The Universe doesn’t really care about our efforts to categorize things that exist in it. I think some people still refer to the Sun as a “yellow dwarf” despite the fact it’s bigger than 70% of the stars in the galaxy.

  28. Brian Too

    @19. J. Major,

    Yes, I remember quite a bit of talk that brown dwarf stars might be the “missing” matter. It never seemed to be the leading theory but for a while it was respectable to talk about it.

    Now the theorists seem to have settled on the idea that Dark Matter and Dark Energy are the only reasonable candidates to explain galactic scale (and larger) dynamics.

    Personally I’m not convinced. Seems to me there is insufficient evidence yet to say definitively that DM and DE are known and provable.

  29. @ Brian Too:

    “Brown dwarfs vs. Dark Matter” is a false dichotomy.

    “Dark Matter” is a catch-all term for ANY matter we can’t see. It is, in fact, a synonym for “missing mass.” It can include exotic things like neutrinos and supersymmetry particles, but it can also include mundane stuff like un-illuminated dust clouds and brown dwarfs.

    I believe the term you’re looking for is “Exotic Dark Matter”, “Non-baryonic Dark Matter”, or “WIMPs”.

  30. Regarding the changing definition of what a planet is – I think we ought to go back to the original Greek for planet (πλανήτης): The seven objects that wander against the background stars; the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. None of these new fangled and non traditional modern world changes to something people were so used to for so many thousands of years. Of course if we’re going to use the Greek word πλανήτης, we should probably use the original Greek names too…. Naw.

    Sadly, even by this traditional and ages old definition, Pluto is still not a planet.

    jbs

  31. amphiox

    I think that the “brown dwarf/dark matter” issue comes down to the fact that when current projections on how many brown dwarfs there are are taken into account (as well as planets, ejected planets, debris, black holes, black dwarfs, and every other “normal” matter non-radiant object that would technically qualify for being a MACHO) the total mass still isn’t enough to account for the observed effects of dark matter, by a long shot. It doesn’t mean these things might not be part of the effect, though.

  32. amphiox

    Sadly, even by this traditional and ages old definition, Pluto is still not a planet.

    Nor is earth!

  33. jcm

    Neil deGrasse Tyson has a nice write-up on color and how it relates to astronomy.

  34. Ronnie Harris

    Click to enCOOLenate

  35. Nikhil

    Dear Dr Plait,

    What are the red stars in the frame ? What can be cooler than a Brown Dwarf ? Since there is no motion, they cannot be comets/asteroids, could they ?

  36. So, false color is like transposing music into another key. “they let blue represent the shortest IR wavelength WISE sees, green the intermediate, and red the longest wavelength…”

  37. Grand Lunar

    “WISE finds the coolest stars. Literally.”

    I thought we already found David Tennant. :D

    In all seriousness, this is very cool. And good point you bring up too, Phil; could there be a brown dwarf closer to us than Proxima?

  38. DigitalAxis

    @Messier Tidy Upper:
    I don’t think your planet analogy quite works, because if we stuck Jupiter out where Pluto is, it would very quickly eat or throw the rest of the Kuiper Belt objects out of the Kuiper Belt, or turn them into new Jupiter Trojan asteroids.

    Meanwhile, if we replaced Mars with Pluto, I’m betting the asteroid belt would start to migrate inwards, because they would not be deflected by Mars’ considerable gravity any longer. The asteroid belt has, collectively, 4% of the mass of the Moon, while Pluto has 20% of the mass of the Moon, and Mars has 850% the mass of the Moon. Pluto would probably become a planet at that point, but still a really really small one with far less influence on the asteroid belt. I think a better analogy would be Mercury (440% the mass of the moon)

    On the other hand, I quite like your planet definition, because it doesn’t rely on other nearby objects. We could extend it to ‘major’ and ‘minor’ moons; there are nice large moons (Ganymede, Titan, The Moon, Oberon, Enceladus) that should probably be distinguished from the teeny tiny peanut moons.

    There IS the problem of genuine effects related to other nearby objects (Ganymede, Europa and Io have a nice resonance going, not to mention tidal heating from Jupiter; Enceladus and Dione have another resonance; GJ0876b and c are in a resonance)

    @21 J. Major:
    Brown dwarfs can and do have planets: See 2MASS 12073346-3932539 (also named for its location in space). This brown dwarf has a temperature of a very cold star, but it’s also very young. It has a planet-mass companion with the temperature of a brown dwarf.

    They are considered brown dwarf and planet, although I’ve heard J Davy Kirkpatrick of WISE say he prefers to call these “objects”, as we’re not sure where the line between star and brown dwarf, or brown dwarf and planet, is. If the object in the article is the one he was talking about that day, it is a potentially 5 Jupiter mass Brown dwarf, which some would obviously call a planet, if it weren’t free-floating. But its luminosity is due to emitted light, not reflected light, so it’s a T9+ Brown Dwarf…

  39. @ 32 amphiox Says on November 12th, 2010 at 10:08 pm

    Sadly, even by this traditional and ages old definition, Pluto is still not a planet.

    Nor is earth!

    Yeah, I did notice that. ;-)

    JBS

  40. Somebody

    I remember a post a while back you put up that said there were no green stars.. I dont know what to believe anymore.

  41. Messier Tidy Upper

    @23. Misora Says:

    how warm.. er, cold would this be in comparison to our sun?

    Our Sun’s temperature is about 5,800 K at its surface.

    http://en.wikipedia.org/wiki/Sun

    Whilst a brown dwarf has a temperature below that of the coolest “proper stars” whihc are about 3,000 degrees celcius :

    http://en.wikipedia.org/wiki/Brown_dwarf

    Not excatly sure how cool this particular one is though I’m afraid.

    @ 21. J. Major Says:

    Can brown dwarfs have planets around them? If they are not stars per se, would the planets then be moons? Grrr….semantics!

    Indeed! ;-)

    As has been pointed out several uh .. sub-stellar objects are known to have even more (less?) sub-stellar objects orbiting around them.

    See :

    http://en.wikipedia.org/wiki/2M1207b

    for example.

    @38. DigitalAxis Says:

    @Messier Tidy Upper : I don’t think your planet analogy quite works, because if we stuck Jupiter out where Pluto is, it would very quickly eat or throw the rest of the Kuiper Belt objects out of the Kuiper Belt, or turn them into new Jupiter Trojan asteroids.

    Really? I don’t think that’s actually the case. In fact I’m pretty sure its not. Would you care to back that asseryon there up with a source?

    Meanwhile, if we replaced Mars with Pluto, I’m betting the asteroid belt would start to migrate inwards, because they would not be deflected by Mars’ considerable gravity any longer.

    The gravity of Mars is hardly “considerable” and the red planet doesn’t keep the asteroid belt from spiralling inwards. Not as far as I’m aware anyhow. Once again, do you have a source or anything to back up what your saying please?

    Sorry, but I think you’re wrong on both counts there.

  42. Messier Tidy Upper

    @40. Somebody :

    I remember a post a while back you put up that said there were no green stars.. I dont know what to believe anymore.

    Check out this :

    http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/

    Which I think is the one from the BA (back in July 2008) that you were most likely thinking of – & my earlier comment #12 which linked the follow up to that and more.

    Or try reading the post more closely where the BA noted that :

    Anyway, if they’re magenta and not brown, why does that one look green in the WISE picture? It’s because WISE looks 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.

    (Emphasis added.)

    As to what to beleive, well that’s entirely up to you. ;-)

    Personally, I believe I’ll have another beer! ;-)

    [Drowning my sorrows after Aussie Mark Webber just failed to win this year’s F1 Grand Prix championship in the final race. :-( ]

  43. amphiox

    What would happen if a brown dwarf got captured into orbit around a larger star? Would it become a planet then? Would it’s planets then become moons? What if it was a very small brown dwarf, perhaps only 15 Jupiter masses or less, making it potentially smaller than the largest super-jovian planets? If we discovered such a system a long time after the capture event, how would we tell the difference?

  44. AJ in CA

    @#36 HTwoWhoah: So, false color is like transposing music into another key. “they let blue represent the shortest IR wavelength WISE sees, green the intermediate, and red the longest wavelength…”

    I like that analogy :D But then, as long as I’m not the one who has to do the transposing, I’m happy :P

  45. Daniel J. Andrews

    See it like a wanna-be-star that failed because it ain’t big enough.

    Instead of calling them failed stars I prefer to think of them as overachieving planets. A much more positive spin, so to speak. ;)

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