Credit: NASA/McGill/V. Kaspi et al

Supernova Bullseye

Exploding stars called supernovae are among the most violent galactic events. Stars which explode as supernovae are torn apart - their outer parts get blown outward into the galaxy (and form beautiful nebulae called supernova remnants), while their cores remain behind as incredibly dense objects called neutron stars or black holes. Though such explosions are extremely violent (one supernova can outshine the other billions of stars in a galaxy), they occur rather frequently, about once or twice per century in a given galaxy. Eight supernovae have been reported by astronomers over the last two millenia. By studying these "historical supernovae" astronomers can study the evolution of the expelled gas and the collapsed stellar core from the time of the explosion. In 1997 a group of Japanese astronomers led by Dr. Ken-íchi Torii, using the ASCA X-ray observatory, discovered X-ray pulsations from an object in a supernova remnant called G11.2-0.3, thought to have been produced by a stellar explosion reported by Chinese astronomers in AD 386. The X-ray pulses are thought to be produced by the rotating neutron star (a pulsar) which is actually the collapsed core of the exploded star. However this pulsar seemed to be rotating too slowly - from the observed slowing of the neutron star's spin, astronomers estimated that the neutron star was formed over 24000 years ago. But, if the neutron star is really so old, then it would have no physical connection to the supernova which occurred in AD 386. However, new evidence of a physical connection between the neutron star and the AD 386 supernova has just been provided by the Chandra X-ray observatory. The image of G11.2-0.3 above was obtained by Dr. Vicki Kaspi and colleagues using Chandra's ACIS camera. Chandra's exquisite spatial resolution was able to show that the pulsar (the bright white spot in the middle of the image) is at the exact center of the supernova remnant (which appears as the multicolored, circular shell around the pulsar). Since pulsars move through space, this shows that the pulsar has not moved appreciably from the place where it formed, consistent with an age of only 1600 years. Thus, taken together, the ASCA and Chandra results suggest that pulsars form with a much wider range of initial rotation periods than astronomers previously believed.

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Page Author: Dr. Michael F. Corcoran
Last modified January 22, 2001