When most white dwarf stars supernova, they're basically obliterated. These explosions expel most or all of the star's matter into space. But in rare cases (only 30 have been spotted by astronomers) another type of supernova occurs, and it's kind of a miniaturized version.
A smaller percentage of the star's mass is expelled, and the star can find a second life after the supernova.
“Astronomers have been searching for decades for the star systems that produce Type Ia supernova explosions,” Rutgers astronomer and study co-author Saurabh Jha said in a statement. “Type Ia’s are important because they’re used to measure vast cosmic distances and the expansion of the universe. But we have very few constraints on how any white dwarf explodes. The similarities between Type Iax’s and normal Type Ia’s make understanding Type Iax progenitors important, especially because no Type Ia progenitor has been conclusively identified. This discovery shows us one way that you can get a white dwarf explosion.”
The astronomers believe that the weak supernova may have been caused by the white dwarf's interaction with a nearby companion star. While the white dwarf started as a bigger star, as it evolved it dumped hydrogen and helium onto the smaller companion. Eventually, the larger star became a white dwarf, while the smaller was bulking up on its leftovers.
Once larger than the white dwarf, the companion star engulfed it. This massive combined star ejected its outer layers, leaving behind the white dwarf and the helium core of the companion star, which astronomers believe they've detected. The white dwarf then pulled matter from the companion star until it was unstable, leading to the mini-supernova. Because of the (relatively) small scale of the explosion, a white dwarf that should have been on its last legs was instead hoisted from its cosmic grave.
The astronomers are waiting for the light of the explosion to fade, which will allow them to take a closer look at the white dwarf - and maybe, if they're lucky, the companion star as well. "Usually during a supernova, a white dwarf would just be totally gone," study co-author and astrophysicist Curtis McCully said. "In this case, the white dwarf is still just a white dwarf. How the companion star changed, if it survived the explosion, is another question."
No word on whether either stars came back with a taste for brains, but it's still pretty cool. Read more from NASA here.