An international team of astronomers using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) has revealed how stars blow up in supernova explosions.
This image shows Cassiopeia A, a remnant that blew up as a supernova more than 11,000 years ago, leaving a dense stellar corpse and its ejected remains. Image credit: Lawrence Livermore National Laboratory.
A supernova is the cataclysmic death of a star, which is extremely luminous and causes a burst of radiation that often briefly outshines an entire galaxy before fading from view.
The explosion expels much or all of a star’s material at a velocity of 10 percent of the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
Dr Fiona Harrison from the California Institute of Technology and her colleagues created the first map of titanium-44 thrown out from the core of a star that exploded in 1671. That explosion produced the beautiful supernova remnant known as Cassiopeia A.
“Our new results show how the explosion’s heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating,” said Dr Harrison, the second author of a paper published in the journal Nature.
Cassiopeia A is about 11,000 light-years from Earth and the most studied nearby supernova remnant.
In the 343 years since the star exploded, the debris from the explosion has expanded to about 10 light years across, essentially magnifying the pattern of the explosion so that it can be seen from Earth.
Earlier observations of the shock-heated iron in the debris cloud led some astronomers to think that the explosion was symmetric, that is, equally powerful in all directions. However, the origins of the iron are so unclear that its distribution may not reflect the explosion pattern from the core.
“We don’t know whether the iron was produced in the supernova explosion, whether it was part of the star when it originally formed, if it is just in the surrounding material, or even if the iron we see represents the actual distribution of iron itself, because we wouldn’t see it if it were not heated in the shock,” said study co-author Prof Steven Boggs from the University of California Berkeley.
The new map of Cassiopeia A, which shows the titanium concentrated in clumps at the remnant’s center, points to a possible solution to the mystery of how the star met its demise.
“Cassiopeia A was a mystery for so long but now with the map of radioactive material, we’re getting a more complete picture of the core of the explosion,” said co-author Dr Bill Craig, also from the University of California Berkeley.
“The surprising thing, which we suspected all along, is that the iron does not match titanium at all, so the iron we see is not mapping the distribution of elements produced in the core of the explosion,” Prof Boggs said.
The scientists also launch balloon-borne high-energy X-ray and gamma-ray detectors to record the radioactive decay of other elements, including iron, in supernovae to learn more about the nuclear reactions that take place during these brief, catastrophic explosions.
“The radioactive nuclei act as a probe of supernova explosions and allow us to see directly into densities and temperatures where nuclear processes are going that we don’t have access to in terrestrial laboratories,” Prof Boggs said.
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B.W. Grefenstette et al. 2014. Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A. Nature 506, 339–342; doi: 10.1038/nature12997
