Supernovas Slosh Before Exploding
Feb. 19, 2014: A longstanding mystery of astronomy, how supernovas explode, might finally have been solved with the help of NASA's Nuclear Spectroscopic Telescope Array (NuSTAR). The high-energy X-ray observatory has mapped radioactive material in the supernova remnant Cassiopeia A (Cas A). The map reveals how shock waves likely rip massive dying stars apart--by sloshing.
"Stars are spherical balls of gas, and so you might think that when they end their lives and explode, that explosion would look like a uniform ball expanding out with great power," said Fiona Harrison, the principal investigator of NuSTAR at Caltech. "Our new results show how the explosion's heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating."
Harrison is a co-author of a paper about the results appearing in the Feb. 20 issue of Nature.
How supernovas explode has been a mystery for a long time: video. When researchers simulate supernova blasts using computers, as a massive star dies and collapses, the main shock wave often stalls out and the star fails to shatter. The latest findings strongly suggest the exploding star literally sloshed around, re-energizing the stalled shock wave and allowing the star to finally blast off its outer layers.
NuSTAR's target, Cas A, was created when a massive star blew up as a supernova leaving a dense stellar corpse and its ejected remains. The light from the explosion reached Earth a few hundred years ago, so we are seeing the stellar remnant when it was fresh and young.
"With NuSTAR we have a new forensic tool to investigate the explosion," said the paper's lead author, Brian Grefenstette of Caltech. "Previously, it was hard to interpret what was going on in Cas A because the material that we could see only glows in X-rays when it's heated up. Now that we can see the radioactive material, which glows in X-rays no matter what, we are getting a more complete picture of what was going on at core of the explosion."
NuSTAR is the first telescope capable of producing maps of radioactive elements in supernova remnants. In this case, the element is titanium-44, which has an unstable nucleus produced at the heart of the exploding star. The NuSTAR map of Cas A shows titanium concentrated in clumps at the remnant's center, which suggests a sloshing action.
The NuSTAR map also casts doubt on other models of supernova explosions, in which the star is rapidly rotating just before it dies and launches narrow streams of gas that drive the stellar blast. Though imprints of jets have been seen before around Cas A, it was not known if they were triggering the explosion. NuSTAR did not see the titanium, essentially the radioactive ash from the explosion, in narrow regions matching the jets, so the jets were not the explosive trigger.
"This is why we built NuSTAR," said Paul Hertz, director of NASA's astrophysics division in Washington. "To discover things we never knew -- and did not expect -- about the high-energy universe."
Why Won't the Supernova Explode? -- this ScienceCast video explores the longstanding mystery of supernova explosions
Supernovas seed the universe with many elements, including the gold in jewelry, the calcium in bones and the iron in blood. While small stars like our sun die less violent deaths, stars at least eight times as massive as our sun blow up in supernova explosions. The high temperatures and particles created in the blast fuse light elements together to create heavier elements. Learn more