Bad Astronomy

Crabby Hubble

When you go outside and look at the night sky, it seems changeless. But there are tremendous energies and motions out there, squashed by our lousy human perception of distance, perspective, and proportion.

Case in point: the Crab Nebula.

You may have already seen this image; it’s all over the blogosphere, as cool Hubble images tend to propagate quickly. Truly, it is amazing (it’s actually a composite of Hubble images with those from the Very Large Telescope). I’ve looked at the Crab Nebula my whole life and I’ve never seen it look so three-dimensional. But there’s more to this than just a pretty picture.

The Crab is an expanding supernova remnant; literally the gaseous remains of a star that blew up nearly a thousand years ago. The gas is really moving at a huge clip: 1400 kilometers per second. Given the amount of gas involved (a star blew up), the energy it took to get this stuff moving at that speed is mind-numbing– more energy than the Sun puts out in millions of years.

Yikes. All that energy was dumped into the gas all at once when the progenitor star exploded. And the Crab is considered to be relatively low-energy compared to other supernova remnants!

At a distance of 6500 light years or so, even that incredible expansion rate is apparently slowed to a crawl by perspective. Just like a distant car or airplane appears to hardly move at all, even the Crab Nebula’s ferocious expansion seems more reminiscent of a turtle than a crustacean. But it can still be seen.

If you take an old image, and compare it to a more recent one, the expansion is pretty easy to detect. Here’s a great example of such an animation. The two images, taken about 28 years apart, were carefully aligned, and you can see the movement of the gas as it screams out from the center of the nebula. Another good animation is here.

There’s more to it than a cool picture, too. By measuring the speed of the expansion, you can backtrack the gas to calculate how long it’s been expanding. That gives you the age of the nebula! When this is done, the number is about 1000 years– in fact, the date of the explosion is probably 1054 AD, in July. I’ve even heard an exact date of July 4, appropriately enough.

For my day job, developing educational activities for students, I rewrote and totally reworked an old lab exercise where you calculate this number (a preliminary draft version is online here; it’s the second activity listed). It was very cool to play with images of the Crab and see for myself how old it is. That exercise should be completed and ready to go in a couple of months. It can be performed online, so anyone who reads this blog will be able to do it for themselves. Imagine! With the tools literally at your fingertips right now, you can grasp some of the most incredible forces nature has to offer, and using some simple math determine when a star died, and when an iconic nebula was born.

I love this stuff.

Image credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)