Scientists Map out Bubbly Interior of a Nearby Former Star

A team of astronomers, including a Harvard-affiliated scientist, detected bubbles in a supernova, the explosion of a massive star, that may hold clues to the asymmetry of stellar explosions.

Astronomers Dan Milisavljevic of the Harvard-Smithsonian Center for Astrophysics and Rob A. Fesen of Dartmouth College produced a three-dimensional map of the bubbly interior of the remnants of one such stellar explosion, Cassiopeia A (Cas A), that took place in the constellation Cassiopeia approximately 340 years ago.

Each star has a spherically layered structure at the time of explosion, with the heaviest elements such as iron closest to the center. Scientists had previously believed that explosions would not alter this arrangement of elements. In 1987, however, high-energy emissions originating from radioactive nickel were detected from a nearby supernova. Astronomers found that  nickel, a heavy element, had somehow sped past the lighter elements, overturning the theory of spherically symmetric explosions.

Milisavljevic and Fesen observed approximately half a dozen enormous cavities, or bubbles, inside the remains of Cas A using near-infrared wavelengths of light to penetrate the dust surrounding the former star.

They believe that the bubbles that they found uphold the hypothesis that supernovae are asymmetrical.

“The nickel radioactively decays to cobalt and emits high energy photons that produce a pressure that expands the blobs into the surrounding gas,” Milisavljevic wrote in an emailed statement. “It is this process – most important during the first couple weeks of the explosion – that carves out cavities or ‘bubbles.’”

To prove their claim, Milisavljevic and Fesen are now searching for iron deposits in the bubbles. The products of radioactive nickel decay, iron deposits would signal nickel’s activity in supernovae and the formation of bubbles.

“We have provided a map of what the end state of the explosion looks like,” Milisavljevic wrote. “Now using sophisticated simulations being run on some of the world’s most powerful computers, they can attempt to trace back how the explosion took place.”

This study may serve as a starting point in the search for the origin of asymmetry in supernovae, Milisavljevic and Fesen said.

“This is a fundamental process in the universe that produces the raw materials that make life possible. We should know a thing or two about how this happens!”

—Staff writer Jessica Kim can be reached at jessica.kim@thecrimson.com.