GoKawiilOur basic understanding of core-collapse supernovae hasn’t changed in decades. Large stars burn through all the fuel at their cores and start creating heavier elements in reactions that consume energy. The lack of energy from these reactions allows gravity to pull the interior of the star in on itself, collapsing it into a neutron star or black hole. The energy released by this process then blows the remainder of the star apart.
And, generally, that’s right. But there’s an entire busload of devils in the details. The statistics of supernovae that we’ve observed indicate that the model may be seriously incomplete. And on the theoretical side, there are still plenty of uncertainties, including over some of the basics, such as whether all core collapses actually result in a supernova.
A paper being released by Physical Review D provides what might be a potential explanation for the discrepancy: flavor-changing neutrinos. Neutrinos play a key role in our current models of supernovae, and right now, those models don’t take into account one of neutrinos’ most striking features: their ability to change identity.
Supernovae and their discontents
We’ve observed plenty of supernovae, so it would seem like there’d be little mystery left. But a number of observations suggest there are some subtleties that we might be missing. For example, if we compare the rate of star formation in the Universe to the frequency of supernovae, there’s a discrepancy; it appears we’re forming enough stars to fuel a much higher frequency of supernovae than we actually observe. Also, in cases where we can identify the progenitor star that exploded, we find too few red supergiants, indicating that they may be contributing to this discrepancy.
Other problems come from observations of gravitational waves generated by mergers of the black holes left behind after a supernova. These mergers suggest there’s a “mass gap” in black hole formation—a range of masses where there are fewer black holes than you’d expect from an even distribution. But that data is complicated by the fact that theorists haven’t definitively identified the conditions that determine when a neutron star tips over into a black hole instead.