The Glint in the Cat's Eye

Yesterday's Astronomy Picture of the Day has a special place in my heart. The object is the Cat's Eye nebula, a planetary nebula, a little over three thousand light-years away.

Why was I glad to see this one in particular? Because, years ago, I was a part of the team that studied the X-rays coming from the Cat's Eye Nebula. (Also known by the less romantic name of NGC 6543.) Our original press release is still online at the Chandra Observatory webpage. I helped to make the X-ray images that became part of the multicolor composite picture in the original release, and to study the spectrum of that hot gas. Now, image people at NASA, Chandra, and the Space Telescope Institute have gotten together to combine images from Hubble's Advanced Camera for Surveys (ACS) with a new Chandra X-ray image to create the lovely false-color picture you now see. The neat thing about such astronomical images is that they're not just beautiful, they're also amazingly informative.

Follow up:

Take this Cat's Eye Nebula picture, for instance. The bright inner parts of the nebula are about a thousand years old, and expanding at a rate of over sixteen kilometers per second. (You can actually see the expansion in some of the Hubble images.) The outer circles are older, from gas blown off the star late in its evolution, before it entered its final death throes. Everything you see in blue in the new image is coming from X-rays, which in turn come from gas heated to millions of degrees. The other colors are showing areas giving off optical light - gas that is heated to "only" tens of thousands of degrees.

For a beginning, then, this image shows us that in the center of this planetary nebula is a tiny point, which is what's now left of a once-larger star. It's now about the same mass as our Sun, but was once five times more massive than it is now. (In the long run, this star will become a tiny stellar cinder called a white dwarf.) Where has all that mass gone? You can see it all around that central point - all that beautiful structure is the shells of gas that have been "shed" by the star over its dying centuries. The light of the central star energizes those shells of gas, causing them to glow in visible light.

Where, then, do the X-rays come from? Some of them come from the central star itself, but the blue-lit inner parts of the nebula in this picture show that the million-degree gas extends far beyond the star. That central star is still losing mass, as it turns out, blowing off more layers of gas in what's called a "fast stellar wind". We're talking about trillions of tons of gas leaving the star every second, and being hurled out at four million miles per hour. The X-rays could be generated by shock waves caused by the speeds with which material is hurled off of the star's surface. (Studying these X-rays was the subject of our paper, which showed that they were coming from the stellar wind itself, not from outer regions of the nebula.)

They say that a picture is worth a thousand words, but in truth, it's taking scientists many thousands of words to describe everything that's going on in this one image!