The whole point of white dwarfs is that they're the remains of stars not big enough to go supernova. Or so we thought. Many of these stars are held together by their super-fast spins... and slowing down could mean an explosion.
Yes, it's pretty much the plot of the movie Speed, except with thousands of stars scattered throughout the Milky Way. The idea that white dwarfs can go supernova isn't new - indeed, there's an entire category for this phenomenon, known as Type Ia supernova. It can happen whenever a white dwarf becomes unstable enough to reignite runaway nuclear fusion.
All previous instances of Type Ia supernovae occurred because white dwarfs exceeded their critical mass due to sapping mass from a stellar companion. (The collision of two white dwarfs is also a potential explanation, but the current consensus is leaning against that as a serious possibility.) But there are some issues with this theory, not the least of which is it's hard to find evidence of the donor stars that gave these exploding white dwarfs their extra mass.
Researchers at the Harvard-Smithsonian Center for Astrophysics say they have an explanation: the reason we're not seeing the so-called "donor star" is that the white dwarf isn't exploding straight away. Instead, as it gains more mass, it also gains angular momentum, and that builds in a huge time delay before the now-inevitable supernova explosion. The white dwarf could remain a ticking time bomb for as long as another billion years, and by that time any direct evidence of the white dwarf leeching mass would be long since erased.
This should mean that there are many - perhaps several thousand - white dwarf stars in the Milky Way that are currently well above the so-called Chandrasekhar mass limit, meaning they could go supernova at any time as soon as their spin slows down. We haven't detected any of these stars yet, but we've only just learned that we should be looking for them, and the hope is that upcoming telescope surveys could find them.
Lead researcher Rosanne Di Stefano explains:
"We haven't found one of these 'time bomb' stars yet in the Milky Way, but this research suggests that we've been looking for the wrong signs. Our work points to a new way of searching for supernova precursors. Our work is new because we show that spin-up and spin-down of the white dwarf have important consequences. Astronomers therefore must take angular momentum of accreting white dwarfs seriously, even though it's very difficult science."
Via arXiv. Image by David A. Aguilar.