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11,000 light-years away but only 340 years ago as seen on Earth, a massive star exploded in a supernova. This incredible explosion has left a set of rings of gas and dust, together forming a continuously expanding sphere centered on the original object, the remnant of which has collapsed into a neutron star. The shell of debris is now about twelve light-years across. But lest you imagine a hollow ball, note that there’s gas and debris throughout that entire spherical region.
This supernova remnant is known as Cassiopeia A, or Cas A for short, and is one of the most well-studied examples of its kind. Nonetheless, it still has some mysteries. The specifics of these star-shattering explosions are still poorly understood and difficult to model, even with state-of-the art simulations run on the most powerful supercomputers. But, like a doctor taking a CAT scan of a patient, a team of researchers has now constructed a 3-D map of Cas A’s interior structure, using new observations from the Mayall 4-meter telescope at Kitt Peak National Observatory. The map they’ve created should provide insight into the star’s explosion.
“We’re sort of like bomb squad investigators. We examine the debris to learn what blew up and how it blew up,” says Dan Milisavljevic, an astronomer with the Harvard-Smithsonian Center for Astrophysics and the paper’s lead author. “Our study represents a major step forward in our understanding of how stars actually explode.”
The observations were taken in the near-infrared wavelengths of light—a part of the spectrum just outside our eyes’ ability to see.
The interior structure of the remnant turns out to be filled with large cavities, forming a swiss cheese-like structure. The researchers’ map shows a link between the large cavities and the individual ring-like structures that comprise the spherical shell.
“We interpret Cas A’s main-shell rings of ejecta to be the cross sections of reverse-shock–heated cavities in the remnant’s internal ejecta now made visible by our survey,” the authors write in their paper. And since the cavities and the rings are related, it’s likely they share a common explanation as well. The researchers conclude that the cavities probably formed as a result of plumes of radioactive nickel that appeared during the initial explosion. And the plumes themselves could have been the result of turbulent mixing processes within the star.
Massive stars, toward the end of their lives, are no longer as symmetrical as they once were, and their irregularities could have also contributed to the formation of the cavities. Mixing processes were likely churning up the star’s material both before and after the explosion, leading to the cavities and rings seen today.
An additional clue that supports the role of mixing is that the neutron star at the center of Cas A is inferred to be moving roughly in the opposite direction from the largest of the cavities, the northwest cavity. That cavity is coincident with a large concentration of (mostly iron) debris, suggesting it might have been formed by a massive plume. The force involved in generating this plume was so large that it sent the neutron star flying in the opposite direction due to conservation of momentum.
The plumes, being composed of nickel, should have decayed to iron, so the researchers predict that the cavities should be enriched with iron—about a tenth of a solar mass of it. However, this iron hasn’t been detected in previous observations. That could just be because those weren’t sensitive enough. The next generation of infra-red and x-ray space telescopes should be able to detect it, if it’s there, and thus potentially confirm the researchers’ hypothesis.
Despite the decades of scrutiny, there are still a number of unresolved issues with Cas A—and this study serves as a reminder that there’s plenty of work to be done. Nonetheless, this study is a clear step forward, not just in the endeavor to understand Cas A but also for the study of supernovae in general.
Cas A shares a lot of similarities with other supernova remnants of various ages, and it’s likely they have a similar bubble-like structure, with cavities throughout. As such, understanding Cas A can help astrophysicists extrapolate to the other supernovae, some of which can’t be observed as clearly as Cas A. And supernovae play an important role in the evolution of stars and planets, and even life, by creating elements that can’t be produced in stellar cores.
Even if the researchers’ hypothesis is correct, many questions remain, they acknowledge. They’d expect dozens of bubbles to form in Cas A, when only half a dozen are observed. It’s uncertain why this is, but when the next generation space telescopes are complete, we may learn more.