As Carl Sagan always said, “We’re made of star-stuff.” That’s because dying stars explode, expelling stardust — which scientists now know contains water in addition to carbon and other organic, life-promoting compounds — throughout the galaxy. In fact, some scientists believe that the universe may have been created when a massive, four-dimensional star went supernova, shedding its outer layers while its inner layers collapsed into a black hole. But supernovae remain somewhat elusive, especially when it comes to the details of the explosion. Until, that is, they are seen with a special telescope. A study published today in Nature by an international team of scientists provides new information about what happens inside a dying star.
Computer simulations have shown that stars won’t explode if they retain their perfectly round shape, so astronomers knew that something else had to be happening. They had some ideas about what that might be, but until now they haven’t been able to determine which, if any, were accurate. NASA’s NuSTAR (nuclear spectroscopic telescope array) telescope, housed at Caltec, enabled scientists to map radioactive material in the remnants of supernova Cassiopeia A. The telescope provided the first ever glimpse at the high-energy X-rays generated by a dying star.
Instead of exploding uniformly or retaining their spherical shape, stars that turn into supernovae get warped and distorted, “possibly because the inner regions literally slosh around before detonating.” That sloshing gives more energy to the shock wave that causes a star to shed its layers, but without that sloshing movement, as shown by the simulations, the shock wave would stall out, rather than causing an explosion. The information allowed scientists to eliminate previous theories, including that the center of a dying star emits jets that might power the explosion.
NuSTAR can also identify the X-ray emissions coming from radioactive elements such as titanium. They were able to construct a map indicating the radioactivity of Cassiopeia A, shown above. The red, yellow, and green indicate non-radioactive elements previously detected using low-energy X-rays, and the blue shows the radioactive elements identified by NuSTAR’s high-energy X-rays. A dying star’s titanium decays into calcium, which allows the telescope to track it. Those pockets of titanium confirm an asymmetrical explosion.
Cassiopeia A is a familiar supernova from a star located far enough away that the light from the explosion only reached us a few hundred years ago. Relatively speaking, it’s a young stellar remnant, which makes it particularly useful for study. I think we can expect NuSTAR to uncover more new data about Cassiopeia A’s cosmic death throes, and as long as those images continue to dazzle (with or without the false-color composites), it will continue to be the most picturesque death in the universe.