The Sciences

The Dust of Magellan

Bad AstronomyBy Phil PlaitSep 6, 2006 4:22 PM


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When I saw the picture above, I was stopped in my tracks. I have looked at images of the Large Magellanic Cloud (a companion galaxy to the Milky Way) about a zillion times, and I am familiar with it. This one threw me. I just didn't recognize it. I realized it was an infrared image, since it came from the Spitzer Space Telescope, and that nebula look different in IR than they do in optical light. But why was this one so odd? Then it occurred to me to look up the dimensions of the image. It's seven degrees across. That's a huge chunk of real estate! Put it this way: take your hand. Make a fist. Hold it at arm's length away. That's roughly 7 degrees across. This is a big image. Normally, space telescopes don't see much of the sky at once. Even the big Hubble images are small compared to, say, the Moon, which is a half degree across. But this image is 14 times the width of the Moon! The astronomers made this image by taking images and then stitching them together into a mosaic. In this case, it was a lot of images. A whole lot: 300,000 of them. You read that right. So they took a third of a million small images and mosaicked them into one monster. The version I display here is heavily reduced in size and compressed to keep the file size small. But if you want this image in all its glory (and you do), then go grab the 10 Mb version. It's worth the wait.

Once you see the big version, things start to pop out at you. Near the top, the fuzziness appears to be cleared out in a circular shape (with a little uvula thingy hanging down). Why? I'm not sure. Maybe a bunch of stars blew up around the same time and cleared that region out. Maybe there were bright, hot stars whose winds did the job. Or maybe it's an illusion, and that region isn't a bubble at all. It's tricky trying to interpret images.

In another spot, there is a blue star right next to a red one. They make a lovely couple, don't you think? In reality, that star may not be blue at all; the color blue in the image corresponds to a wavelength three times what our eyes can see, and red is more than 20. So that star might actually be red to our eyes, and the red one might be invisible, glowing only in the IR! It might not even be a star at all; maybe it's a distant luminous galaxy, apparent size shrunken by its terrible distance, with its light reddened hugely by dust inside of itself. Again, I don't know, so I'm speculating a bit. The image was taken as part of a Legacy project -- 19 key projects for Spitzer. This one is to map dust in galaxies. This particular project has 50 scientists working on it (and no doubt a passel of grad students and postdocs, who also no doubt write the software to patch the 300,000 images together). A paper about the image won't be out until November, so my speculation will have to wait until then for an answer. There's obviously a lot of science in them thar dust, and a lot of beauty as well. That'll hold me for a while.

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