Bad Astronomy | 04 Oct 2010
A thousand years ago, and 6500 light years away from Earth, a high mass star exploded. An octillion tons of gas blasted outwards at speeds of thousands of kilometers per second, forming tendrils and wisps as it raced away. At the center of the conflagration, the core of the star had collapsed into an ultradense object called a neutron star. It has the mass of the Sun crammed into a ball only 20 – 30 km (12 – 18 miles) across, and is spinning at a rate of 30 times per second.
This image is a brand spanking new shot of the heart of the Crab Nebula taken by the Hubble Space Telescope. And by new, I mean it was taken on Saturday, October 2! It’s a bit hard to see what’s going on, so I created an annotated version:
The pulsar is labeled. It’s sitting right at the center of the gas cloud, which extends way beyond the edges of this picture. As the pulsar spins, it emits a fast stream of particles that act like a wind, compressing the gas in the nebula and creating those circles of light. They look elliptical because the whole system is tilted, and you’re seeing it like a DVD held at an angle. From what I can tell, the bottom left is the side toward us, and the upper right is farther away, as if we’re looking down on it.
In mid-September, just a couple of weeks ago, several orbiting observatories noted that there was an increased amount of gamma rays coming from this part of the sky. Gamma rays are the highest energy form of light, and there aren’t many sources in the sky that can create them at all, let alone in quantities that can be seen. The Crab is the brightest continuous gamma-ray source we know, and so it was immediately put on the Most Wanted list.
Hubble was quickly pointed at the pulsar, and that image above was taken. Now note the rectangular bit to the left of the pulsar. Some of that stuff was seen in earlier images, but not this bright! You can compare it to the image on the right, taken in December 1993 by Hubble, which I’ve rotated and scaled to be close to what we see in the new image. The overall structure is very roughly the same, but clearly that gas to the left is brighter in the new image. I suspect that rectangular structure in the new image is actually a ring, the shape distorted by density variations in the gas.
What must have happened [note: I'm conjecturing here and may be way off, but the basic stuff is probably correct] is that something occurred on the pulsar: maybe it had a starquake, or a bit of material fell on it (the crushing gravity of a neutron star guarantees that anything hitting it will be doing so at a large fraction of the speed of light, generating a lot of explosive energy; even a marshmallow hitting at that speed explodes like an atomic bomb!). But something violent happened, and a blast of gamma-rays ensued.
Image Credit: NASA/ESA/Hubble