When a star explodes, you'd expect the process to be messy. But astronomers have found that a star named Cassiopeia A blew itself up in a surprisingly ordered way. Cas A, as it's known, became a sort of cosmic onion, shedding its components into space one layer at a time. The event is helping astronomers piece the star back together and figure out how and why such stellar explosions occur.
Like all massive stars, Cas A once consisted of concentric shells of material. Heavy elements such as iron made up its core, overlain by oxygen, aluminum, and other elements in the middle, and finally hydrogen and helium near the surface. At least that's what computer simulations predicted. Now Cas A's aftermath has confirmed the models.
Located 11,000 light-years away, the star's explosion was close enough to be noticed 300 years ago, and its debris cloud has been expanding ever since. The cloud contains a lot of dust. By mapping the dust and measuring its temperature with the Spitzer Space Telescope's infrared sensors, a team of astronomers at the University of Minnesota in Minneapolis and elsewhere have been able to discern the supernova's composition in unprecedented detail.
As the dust streamed outward, it overtook and collided with the supernova's initial shock wave; this collision heated the dust. The lighter materials from the star's surface hit the shock zone first and therefore have been heating up longer, the team reports 20 November in Astrophysical Journal. The elements present in the star's middle and at its core collided with the shock zone more recently. They are relatively cooler and hence visible only in the infrared.
Principal investigator Lawrence Rudnick says the findings are giving astronomers their closest view yet of how a star explodes, or goes supernova. These data, he says, will provide researchers with better tools to study how stars burn out and what happens to them after they die.
"It's a neat result" and it tells us a lot about the nature of supernovae, says astrophysicist George Rieke of the Steward Observatory in Tucson, Arizona. He says current star models are highly reliable but studies of stellar explosions are still evolving. "Ultimately, understanding how they explode will show us in detail how supernovae produce all elements heavier than iron," he explains, which means the new observations further the understanding of where planets such as Earth come from.
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