Andromeda’s Warm Glow

Bad Astronomy
The entire universe in blog form
Oct. 16 2005 9:06 PM

Andromeda’s Warm Glow

First, let me start with a gorgeous picture:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


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.


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