Andromeda’s Warm Glow

By Phil Plait | October 16, 2005 9:06 pm

First, let me start with a gorgeous picture:

What are you looking at there? Ah, that’s a cool story… literally.

For a long time, astronomers figured visible light was all there is. In astronomy, what you saw was literally what you got.

That changed in the 20th century. Astronomers quickly found the utility of observing the sky in different parts of the electromagnetic spectrum. What we call visible light is really only a tiny portion of that wide, wide spectrum of light, which includes radio, infrared, ultraviolet, X-rays, and gamma rays.

I could go on and on (and on and on, believe me) about this. But in general, the type of light emitted by an object tell you a lot about its temperature. Cold objects (like clouds of dust in space, or the ice balls way out past Pluto) give off radio or infrared light, warmer objects like the Sun give off visible light, and very hot objects emit UV, X-ray, or gamma rays. I am way oversimplifying here, but you get the point.

In 2003, NASA launched the last of its “Great Observatories”, a telescope called Spitzer which observes objects in infrared light. Int he past couple of years it’s done some amazing science.

The latest release by the Spitzer folks is a dramatic image of our little sister in the sky, the Andromeda Galaxy. It’s a nearly edge-on spiral, about 3 million light years away (yet still visible as a smudge in the sky… actually, as a I write this, it’s up right now). Like our Milky Way, it’s got lots of stars, as well as lots of dust.

Interstellar dust is dark in visible light. There’s enough of it in Andromeda to absorb the light behind it, so when you look at a visible picture, you see long streamers of dark dust.

But the dust is warm– well, warm meaning about 100 Kelvin (about -170 C). That means it glows in infrared! That makes a pretty different picture of Andromeda taken by Spitzer. Those two images I posted at the top of this entry are of Andromeda. The top one is in visible light, and the bottom is by Spitzer in IR. Take a minute to compare them…

They look really different, don’t they! The dust lanes are dark in the visible picture, but bright in the IR. Below, I enlarged the right side of the images, and you can actually see how the stuff that’s dark in the top picture is bright on the bottom, like a negative!

Images like these will give astronomers all manner of insight into dust in Andromeda, which in turn can tell them the mass of the galaxy, how stars formed (and continue to form) there, and quite a lot else. That also helps us understand our own Milky Way, which, ironically, is hard to study because of all the dust blocking our view of the really interesting parts, like the Galactic center, and dense star-forming regions. There’s a lot to learn, even in our own neighborhood, and telescopes like Spitzer (and, eventually, the James Webb Space Telescope) open a new window on sky, letting us see the Universe in a whole new light.


Comments (12)

  1. scotia47

    Very nice, although something has always confused me about images taken in the IR spectrum, UV spectrum etc. This is probably a stupid question, but given that these forms of radiation are invisible to the naked eye, what are we actually looking at in these images?

  2. Thanks for posting these awesome images!
    I found your site completely by accident, and I must say I’m glad I found it.

    I have been an armchair Astronomy buff for years, I only own a modest telescope, but am able to make my way around the galaxy fairly well.
    I recognized Andromeda immediately in this post. It is one of my favorite objects to point out to friends in the night sky, First visually, using a laser pointer, then with binoculars.

    Whats cool with the Bino’s is that they can follow the green laser beam while looking through them and thus can easily find the target. Then I let them see it through the scope. Their amazement at what they are seeing never ceases to amaze me! Most never really have a clue to whats up there. They are too busy watching where they are going in life.

  3. Evolving Squid


    What happens is that the astronomer takes a photo with film/CCD/whatever that is sensitive to the particular kind of light desired. As you are aware, a human cannot see infrared or ultraviolet or x-rays… but film and other devices and materials can. Images made with devices seeing into those wavelengths would be greyscale because infrared and so on have no “colour” – you’d have an image of bright and dark spots and everything in between… just like a black and white photograph.

    When the photograph is processed, false colour is added to bring out details. In the case above, I would conjecture that it was made red-scale, with white being the brightest spot in infrared, and so on down through yellow, red and brown to black which represents very dark spots in infrared. That would be an easy transformation to make.

    Often images are also combined: the infrared image might be made red-scale , combined with a UV image that has been made bluescale and an X-ray image that has been made green-scale to give a false “true-colour” type of image that shows details that no human could see with the naked eye.

  4. Evolving Squid


    One way you can see this yourself, is if you have a digital camera. Most digital camera CCDs are sensitive to infrared light.

    Turn your camera on, and while looking at the screen at the back, point your TV remote at the camera and push some buttons. On most cameras you will see bright lights where the LEDs are when looking at the screen, but to your eye, the LEDs on the remote will be dark.

    If you translate that to a photograph in infrared ONLY, you’d have two bright spots in a black field. That is like what you’re looking at in the photo above.

  5. scotia47

    Thanks for the explanation Squid, makes perfect sense now. :-)

  6. TheBlackCat

    Very cool. And very beautiful to. It looks kind of like one of those pictures of a populated area of Earth from space at night, where all the city lights are visible.

  7. Leon

    Bah, that’s nothin’. Astrologers predicted all this stuff long ago…um, er…didn’t they?

  8. Nigel Depledge

    Yes, indeed. Thank, BA, very cool pics.

  9. JusANuttaBackYahdah

    Welcome to the club Wizard; it is fun showing folks various DSO’s, I enjoy doing the same
    Nice explanation Squid but but I was almost shocked at how sirius (couldn’t help myself) your entries were.
    BlackCat, I guess we live in a small universe eh? ūüėČ
    Clear Skies all!

  10. Eric

    These pictures are even more impressive as an animation


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar