When Stars Explode

When astronomer J. Craig Wheeler first began observing supernovae, there was so little observational data available that scientists were forced to assume a great deal about the fundamental nature of the exploding stars.

One assumption made by the astronomical community, including Wheeler, was that the explosions were basically spherical in shape. That assumption, as Wheeler and a colleague document in a recent paper in the Annual Review of Astronomy and Astrophysics, appears to be wrong.

In the last decade or so, thanks to major advances in the ability of astronomers to both detect new supernovae and interpret the polarization of the light coming from them, enough data has been amassed to support a new consensus.

“When we got the data on 1987A, we noticed that there were some asymmetries,” says Wheeler, referring to the (relatively) nearby 1987 supernova that was a catalytic event in modern supernova observation. “We didn’t know, however, whether it was a theme or a peculiarity.”

As more supernovae were discovered and analyzed throughout the 1990s and 2000s, many of them by Wheeler and his collaborators, these asymmetries continued to pop up. In studying “core-collapse” supernovae, in particular, every single observation revealed light that was polarized in a way consistent with an elongated, rather than a spherical, explosion. The explosions seemed to move outwards faster along one axis than in other directions.

“There’s now a consensus that supernovae are polarized and hence intrinsically aspherical in some basic way,” says Wheeler.

Assuming that these asymmetries are intrinsic, says Wheeler, may also offer insight into one of the fundamental questions that continues to challenge supernova scientists: How do core-collapse supernovae explode in the first place?

The initial stages of the collapse, says Wheeler, are reasonably well understood.

Toward the end of the life cycles of certain types of massive stars, the cores of the stars become progressively denser and denser. At each stage of contraction, a process of fusion generates enough energy to hold the core stable against the gravitational pressure of the outer layers of the star. Eventually, however, an iron core is produced that can’t fuse, and the core implodes. The result is an environment in the interior of the star that forces electrons and protons to merge, producing what typically becomes a neutron star.

“Every time a proton and electron merge,” says Wheeler, “you make a neutrino, so you have a whole star’s worth of neutrinos produced all at once. You make a hundred times more energy than you need to blow the star up, but the star is basically transparent to the neutrinos, so most of that energy escapes.

"The question is what’s the mechanism whereby the star traps enough of that energy to blow itself up? Answering that question has been the project of decades.”

The evidence of asymmetry in the explosions, says Wheeler, may suggest that a greater role may be played by what happens to the star’s rotation and magnetic field when the core collapses than was once assumed. The rotation may channel the eventual explosion even if it occurs by some other mechanism.

“The magnetic fields would act like rubber bands,” he says, “constricting the flow inside the star. When it finally explodes, material squirts out up and down, resulting in a blast that’s asymmetric.”