Zombies are out there, NASA has decided, though happily of the star variety rather than the brain-eating undead. Digging through Hubble Space Telescope images revealed a surprising remnant of a star, left behind after a supernova that should, in theory at least, have blown it away.
The “unusually weak” explosion, SN 2012Z, took place 110 million light years away in the NGC 1309 galaxy. In the process, it’s believed to have stripped away the zombie star’s outer hydrogen envelope, though left behind its core of helium.
“The more massive star evolved more quickly to expand and dump its hydrogen and helium onto the smaller star. The rapidly evolving star became a white dwarf. The smaller star bulked up, grew larger and engulfed the white dwarf. The outer layers of this combined star were ejected, leaving behind the white dwarf and the helium core of the companion star. The white dwarf siphoned matter from the companion star until it became unstable and exploded as a mini-supernova, leaving behind a surviving zombie star” NASA
Actually spotting the zombie star was a matter of luck. The supernova itself was caught on camera by the Lick Observatory Supernova Search in January 2012, while the pre-explosion photos were tracked several years earlier by Hubble.
The hope is to use Hubble again come 2015, when it will observe the area once more. By that point, astronomers hope that the light from the supernova will have dimmed sufficiently that the zombie star and whatever helium companion it might have will be visible.
With that data, it’s possible that we’ll gain better understanding of how white dwarf explosions occur, which so far has proved something of a mystery. More than thirty mini-supernovas have been cataloged which may each have left a remaining zombie white dwarf.