If you live in the northern hemisphere and go outside in the winter, hanging not too far from Orion's left shoulder is a small, tight, configuration of stars. A lot of people mistake them for the Little Dipper -- I get asked about it all the time -- but really it's the Pleiades (pronounced PLEE-uh-dees), an actual cluster of stars about 400 light years away. To the eye you can usually spot six of the stars (the seventh, seen in ancient times, may have faded a bit since then), and in binoculars you can see dozens.
But when NASA's Wide Field Infrared Survey Explorer (WISE) looked at it in February, this is what it saw:
Coooool. Literally! WISE looks in the infrared, and can see cool objects that are invisible to our eyes. The Pleiades stars are bound together in a cluster by their own gravity, and are currently plowing through a dense cloud of dust and gas in the galaxy. The material has been warmed up by the hot stars, and glows in the infrared. Deep images in visible light also show the material, but it looks blue as it reflects the optical light from the stars. In the WISE images, we're seeing the matter actually glowing on its own, emitting infrared light. When I was younger it was thought that this material was the leftover stuff from which the stars formed. But it was later found that the stars are older than first thought; about 100 million years old. While still quite young -- the Sun is 4.5 billion years old! -- that's long enough for the original cocoon of material that made up these stars' nursery to have dispersed. So it's a cosmic coincidence that we happen to see the cluster as it's ramming through this material. On the other hand, the Milky Way galaxy is loaded with lots of junk floating out there, and the Pleiades are in an area of high traffic. It's not too surprising we'd see something like this happening, and it's nice that it's going on close enough that we get a good view of it.
WISE doesn't just get pointed wherever astronomers see something interesting: it's an all-sky survey, spinning on its axis and taking snapshots continuously. These are stored, and astronomers on the ground can then put them together in a mosaic. This image is actually pretty big, covering 2x3° of the sky. That's about the size of a postage stamp held at arm's length, and is a fair bit bigger than the full Moon on the sky. This image was released to celebrate the fact that as of July 17, WISE has now scanned the entire sky, and its primary mission has been fulfilled. Yay!
Funny, too: I've observed the Pleiades a lot, and seen lots of pictures too, yet it's difficult to identify the stars in the WISE image -- I had to rotate the visible image to match the one from WISE, but even then it's not entirely obvious how they line up. In the IR, stars are bright that might be dim in optical, and vice-versa! But I'd recognize the sheets and filaments of the disturbed dust anywhere. One of my favorite things in astronomy is seeing a familiar object in an unfamiliar way. It reminds me that there's still plenty to learn about the Universe.
Image credits: NASA/JPL-Caltech/UCLA and NASA, ESA and AURA/Caltech
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