M51 is one of my favorite galaxies. It's a face-on spiral, a mere 30 million light years away - on a cosmic scale, that makes us neighbors - and pretty easy to spot even in small 'scopes. And you can see by the image above why it's nicknamed the Whirlpool Galaxy. But what the heck is going on in that image?
This false-color Hubble picture is taken in the near-infrared, just outside the range of what our eyes can see. This is where warm dust is very bright, and the swirling pattern you see is from all that dust. Normally, a picture like this would show lots of stars as well as dust lit up by those stars, but astronomers used an image taken in visible light to subtract off the stars in the IR image, leaving just the dust behind.
This picture shows just the inner region of the galaxy, an area about 18,000 light years across. The dust is following the spiral arms of the galaxy, and is clumpy... but not as clumpy as was expected. M51 is interacting with a nearby galaxy (off screen here) and that gravitational barn dance may be what's smoothing out the dust here. Or it might be something entirely different. We're not exactly sure. Images like this one will help astronomers understand how dust behaves inside galaxies, which in turn will help us understand galaxies as a whole.
And, since we live inside a galaxy ourselves, this strikes me as a good idea. It's always nice to explore your own neighborhood.
Image credit: NICMOS Image: NASA, ESA, M. Regan and B. Whitmore (STScI), and R. Chandar (University of Toledo)