The Sciences

An octillion ton cannonball

Bad AstronomyBy Phil PlaitNov 30, 2007 1:58 PM


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Astronomers using the Chandra X-ray telescope made a pretty nifty observation of the supernova remnant Puppis A.

This pretty picture (click to embiggen) is a combination of X-ray and optical observations, and it shows just how impressively huge a supernova event can be. Attend: Puppis A formed more than three millennia ago when a massive star exploded at the end of its life. Over time the cloud of debris expanded, forming this intricately woven web of filaments of gas. Only the outer layers of the star exploded outwards, though. The core of the star collapsed to form a neutron star, an ultradense object with the mass of a star like the Sun, but squeezed into a ball only a few kilometers across. Neutron stars are funny beasts. Some of them are seen to be screaming across the sky at fantastic speeds, up to hundreds of kilometers per second (fast enough to circle the Earth in a few minutes). What could possibly toss around an object of that mass so quickly? There are two ways, in fact. The star could have been in a tight binary orbit around another massive star. When it exploded, it loses most of its mass. The sudden drop in mass lowers the gravity of the system, and its the gravity holding the two stars together. When the gravity lets go, the two stars slingshot away from each other at high speed. Another way is for the explosion itself to be off-center. When the core of the star collapses, triggering the explosion, there are several complicated processes that can make the explosion asymmetric. This acts like a rocket, kicking the new neutron star in one direction while the ejected matter goes off in the other. Usually, it's not easy to figure out which scenario took place in a given supernova. But in Puppis A, using Chandra, the astronomers found massive blobs of oxygen created in the explosion all moving in roughly the same direction. The neutron star that was once the core of the star that exploded was found to be moving in the opposite direction! When they added up the velocities and masses of the blobs, they matched those of the neutron star: but in the opposite direction. So the forces all balance out, indicating it was an off-kilter explosion that blasted the star off at high speeds. In the picture inset, you can actually see the motion of the neutron star over the six years between observations. From the images, the astronomers figured the neutron star is moving at at least 5 million kilometers per hour. That's fast. In the 3700 years since it formed, it's moved about 20 light years, or 200 trillion kilometers. I study this kind of thing all the time, and in fact I researched this very topic for my second book (to see how fast I could get a black hole to move, so that I could work out the math of what would happen if one approached the Earth... yikes!). And yet the sheer size and scale of events like these never ceases to give me a chill down my spine. The Universe is such an amazing place. I'm really glad we're here.

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