Scientists know that stars explode resulting in supernovae. They know at about what age stars explode but not very much about the internal process of that explosion. Now researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) and Dartmouth College believe they’ve uncovered important new information.

The researchers, whose work appears in the January 30 issue of the journal Science, generated a 3-D map using the astronomical equivalent of a CAT scan. Their images of the interior of Cassiopeia A (Cas A), a start 11,000 light-years from Earth that exploded 340 years ago, shows a collection of half a dozen massive “bubbles”. By studying relatively young supernovae, located close enough to get good imaging the researchers hope to gain insight into the processes that drive stellar explosions.

“We’re like the bomb squad. A bomb’s gone off and I want to understand how that bomb exploded. So when I go in the room, the first thing I’m going to say is: Where did the debris go? Did it go in all directions equally or did it go in some directions preferentially, like a pipe bomb or something? That’s step one. And that’s what we’ve done here,” Dan Milisavljevic of the CfA told Space.com.

To create their map, Milisavljevic and co-author Rob Fesen of Dartmouth College used the Mayall 4-meter telescope at Kitt Peak National Observatory, southwest of Tucson, AZ to examine Cas A in near-infrared wavelengths which can penetrate dust.
The infrared wavelengths the researchers used is particularly common in Cas A and the sulfur they observed showed the outline of bubbles.

When Cas A exploded hot, radioactive material spread outward from the stars core. This research, combined with other studies indicates that radioactive metal, specifically nickel, would have moved into the expanding cloud of material from the exploding star. The decay of the nickel would produce photons which would push outward on the material forming caverns or bubbles.

Although the bubbles would have collapsed, or “popped” with nothing to push inward they would remain permanent features of the cloud. This finding explains the rings of debris which can be seen in the outer shell of Cas A.

The most well-defined of these cavities are 3 and 6 light years in diameter, giving a Swiss cheese appearance to the remains of the star. After the radioactive decay of the nickel to iron the researchers believe the interior bubbles should be enriched with as much as a tenth of a solar mass of iron.

The nickel bubbles may also show that material inside a star begins to move prior to the supernova. Millsavijevic notes that prior models of supernovae assume an interior with elements separated neatly into layers, with the lightest on top. Bubble theory could indicate much greater mixing of materials prior to the explosion.

“It may be that the star is quite turbulent inside. So it’s not this onion skin interior. There may be some kind of mixing that’s going on in the chemical layers of the star immediately prior to the explosion. And simulations have shown that if that’s the case, then the explosion proceeds much more easily and that may be aiding in the explosion all together,” said Milisavljevic.

An interactive 3-D map which demonstrates the teams findings can be found on the Harvard-Smithsonian website.