The heat and the light of a dusty galaxy

By Phil Plait | May 19, 2010 11:23 am

The European Southern Observatory just released a new image of the spiral galaxy M83, and it’s a pretty cool shot:


[Click to embiggen, or go here to get access to monstrously bigger images, including wallpapers.]

M83 is pretty close to us, if you happen to think 15 million light years is close. I’m an astronomer, so yeah, I consider 150 quintillion kilometers — that’s 150,000,000,000,000,000,000 kilometers, or 90 quintillion miles — practically next door in galactic terms.

This image is fascinating. It was taken with the ridiculously huge 8 meter Very Large Telescope in Chile, and is in the infrared at a wavelength of 2.2 microns, about three times the wavelength of the reddest light your eye can see. That turns out to be important! Galaxies like M83, and our Milky Way for that matter, are littered with huge clouds of dust. These aren’t like the clumpy bunnies you find under your sofa when you accidentally drop some M&Ms under there while gaming and realize you haven’t vacuumed in a couple of months. Say.

No, this dust is actually composed of complex organic molecules, and they are opaque to visible light. A cloud of dust a few light years across might as well be a concrete wall if you’re trying to look past it with a telescope. Whatever’s behind it is hidden.

But infrared light has a longer wavelength than visible light, and it passes through that dust. So an image of a galaxy in IR can look a lot different than one taken in visible light. When I first saw the picture above, I didn’t even recognize the galaxy! Mind you, I am a vastly huge astronomy dork, and can recognize dozens of galaxies at a glance (that would sound like bragging if the topic were any different). So to be stumped, even for a moment, when seeing such a picture is disturbing. Here’s a side-by-side shot to show you the difference:


[Again, click to enspiralate, and to get much huger pix.]

See why I was confused? They look pretty different, and not just due to the colors. In the visible light image on the right, the spiral arms are lit up by really hot stars (some many times hotter than the Sun), and the glowy pink and red parts are clouds of gas where stars are being born. The hydrogen in the clouds gets heated up (astronomers call that being "excited", a gentle reminder that we’re dorks) and emits a lot of light, making them very showy and obvious. Those regions are punctuated by long streamers of darkness, the dust clouds scattered along the arms. They are essentially gone from the IR image on the left! The star forming regions are hard to see as well; they don’t emit nearly as much IR as visible light, so they fade into the background.

I’ll note that just because these images are in the infrared doesn’t mean they’re tracking heat; at least, not like what you normally think of when you think "infrared". A lot of folks equate IR with heat, but that’s not really the case. When you see images from a thermal camera (like in those awful ghost hunting shows) those are way way farther out than what your eye sees, like at 10 – 20 microns. Objects at human body temperature glow at those wavelengths. The picture of M83 was at 2.2 microns, which is where objects at about 1000°C (1800° F) glow. So in a sense we’re still seeing heat, but from objects much hotter than even an oven set to broil — this IR light is from red stars, like giants and supergiants near the ends of their lives.

Looking in the IR tells us a lot about galaxies, more than we’d know otherwise just looking in the visible part of the spectrum. And while the image from the VLT is pretty cool, I have to admit I like the one in visible light better aesthetically. It’s prettier. And it just goes to show you: we need our dark side. It provides contrast against which to see our warmer side.

Image credits: ESO/M. Gieles. Acknowledgment: Mischa Schirmer

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (15)

  1. These aren’t like the clumpy bunnies you find under your sofa when you accidentally drop some M&Ms under there while gaming and realize you haven’t vacuumed in a couple of months.

    You sound like you speak from experience here.

  2. Awesome. I used to live in Munich and rock climbed with astronomers from ESO. Great shots.

  3. Fritriac

    1st link produces a “Document not found” error. You should remove the “target=” in the URL (and set the target to “_blank” AFTER the href).

    Oh, and: Nice pics indeed! :)

  4. Stupid learner here.
    So…if a starship, or lets be specific and say the starship Enterprise, were to go through this cloud it would become toast like. Or at least warmish.

    No one ever discusses the possibility of boiling inside the spaceship except when going through atmosphere. This what happens when you let your science knowledge come from watching science fiction….

  5. Russell

    BA always makes me go look further for more information about the contents of the post.
    (which really means I am geeking out at lunchtime !)

    I found the definition of “organic molecule” on many web sites, wiki etc.

    “The four major classes of organic molecules include carbohydrates, proteins, lipids and nucleic acids.”

    “carbs & proteins”…sound like lunch to me !

    Now see, I have always been confused by the term “dust in space” as meaning those dust bunnies and M&M’s and old rubber bands… To find that it is the building blocks of LIFE !

    wow…. When you know about the universe and how it is put together, you really don’t need to construct your own beliefs about all this….the reality is powerful enough!

    Thanks for the chills up my spine !

  6. Very nice post, Phil.
    But I would like to elaborate a bit on the name “The Very Large Telescope”; Although 8.2 diameter primary mirrors make for a “Pretty Big Telescope”, it is the fact that there are FOUR of these guys together that makes it (the four of them) the VLT. The light-paths of these + four 1.8m auxiliary telescopes (ATs) can be combined (only three at a time, for the time being) interferometrically to result in the Very Large Telescope Interferometer (VLTI), having a resolving power corresponding to that of a 200m telescope! Check “my” (ESO’s) web-site for details.
    Non-Believer @ 4: “Stupid learner here.”
    You are so wrong about that! – one who keeps learning is the opposite of stupid :-)
    The density of the dust and gas in those nebulae you see in astro-pix is so low – lower than the best vacuum we can produce in a lab – that despite high temperatures, the energy density is still so low that any energy transferred to a ship wouldn’t be enough to burn a flea.
    Russel @ 5: “organic molecule” are pretty much anything that involve carbon atoms. The most common ones in interstellar dust, are poly-aromatic hydro-carbons (PAH) which, here on Earth, are major components of soot… But vinegar, sugar and amino-acids have also been found out there.
    Cheers, Regner

  7. jcm

    ” 1000°C (1800° F)”

    Make that 1273 K.

  8. Russell

    Regner@6: Thank you for the info !

    So by what you say about the sugar & vinegar I suppose the universe is somewhat like sweet & sour sauce? :)

    Back in the 70’s I designed a little star ship just for fun. I always imagined that it would need some sort of beam or field in front of it to peel away all the atoms it is about to hit at light speed.

    This from the mind of a kid! Star trek will do that to ya !

  9. TMB

    Okay, by a weird coincidence I am right now at a telescope in Chile observing M83. Or I would be if it weren’t cloudy. . In the optical, though… those are very very nice near-IR images!


  10. Messier Tidy Upper

    When I first saw the picture [M83 in IR] above, I didn’t even recognize the galaxy!

    Ah what a difference a wavelength makes eh? 😉

    I agree the visual view is more aesthetically appealing however there is certainly something special about the infra-red image too. It makes the barred spiral structure stand out clearer methinks.

    … this IR light is from red stars, like giants and supergiants near the ends of their lives.

    You forgot to include the bright giants (luminosity class II*) there too!

    Seriously though, what about the main-sequence red dwarfs – don’t they show up in this wavelength as well? After all, while they’re exceedingly dim, there’s also a huge lot of them.

    Finally, if folks here want more info. on Messier 83 then here’s a link to its wiki-page :


    * For those who don’t already know, luminosity classes are the division of stars by their basic nature, divided up based on size, diameter, metallicity, stage of stellar evolution and type or even absence of fusion :

    I = Supergiant, evolved high-mass, fusing helium (He) + other elements at core
    II = Bright giant, evolved high~ish mass but not as much as supergiants. He-fusing
    III =Giant, evolved He-fusing, much larger diameter & thus much brighter than dwarfs.
    IV = Sub-giant, star evolving from dwarf into giant, no longer Hydrogen (H) fusing.
    V =Main sequence /dwarf, ordinary core Hydrogen fusing stars (90% of all stars)
    VI = Sub-dwarf, metal (elements beyond H & He) poor thus smaller, hotter & fainter stars.
    VII = White dwarfs, evolved non-fusing stellar remnants – the dead cores of ex- stars.

  11. Messier Tidy Upper

    @4. Non-Believer Says:

    Stupid learner here.

    Not stupid at all – there’s nothing stupid about asking questions or trying to learn more. :-)
    As (#6.) Regner Trampedach has noted.

    So…if a starship, or lets be specific and say the starship Enterprise, were to go through this cloud it would become toast like. Or at least warmish.

    A good question & I’m not 100% sure of the answer but I don’t think it would because the gas and dust that makes up these clouds (nebulae – dark or bright) is very thinly spread and mostly vacuum. So I agree with Regner Trampedach again there.


    PS. Listening to local radio program discussing the Hayabusa asteroid sample return mission ( ) as I type this! :-)

  12. Thank you Regner Trampedach and Messier Tidy Upper for the info. I like to get to the bottom of things.
    I am glad that Russell mentioned the organic material. I was going to look it up and got distracted by the heat.

  13. Ken B (#1) – Ask Phil to show you the article he wrote in college about toenail clippings.

  14. mike burkhart

    This is an amazing photo .I take it this galaxy is part of the local group.To #4 As a Star Trek fan since the age of 6 I do have some answers.Even thro the Enterprises warp engines can cause the starship travel at speeds faster then light there are parts of the galaxy that would take millons of years for the Enterprise to reach .And to reach a nearby galaxy ,even at top warp speed it would take millons of years.The Enterprises deflector sheilds protect it form space hazzards in addtion to Kilngon Disrupter wepons.However the ship dose have problems when going thro a nebula it sheilds,sensors are useless due to the gas.

  15. Anchor

    Considering that M83 is 15 million light-years away and has produced 6 supernovae in the last century, and if that rate is reliable as an average for the next 15 million years (which seems likely) then the light of about 900 THOUSAND separate supernovae is already on its way here, strung out over that distance at the (assumed) average interval of one about every 17 years.

    Man, that thing has got to look like a frenzied flurry of flashbulbs going off in a jam-packed stadium to beings that live sufficiently long and slow lives…


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