Typically speaking, when a supernova explodes, it doesn’t leave anything in its wake, as the white draw fueling the explosion is usually destroyed. That’s apparently not always the case, as NASA believes they’ve found something resembling a “zombie” star, left in the aftermath of a particularly weak supernova event.

Astronomers at Rutgers were pouring over Hubble images taken years before the stellar explosion when they noticed something odd – a blue companion star feeding energy to a white dwarf. This eventually led to a Type Iax supernova dubbed SN 2012Z, significantly weaker than its more common cousin, Type Ia.

“I was very surprised to see anything at the location of the supernova. We expected  the progenitor system would be too faint to see, like in previous searches for normal Type Ia supernova progenitors. It is exciting when nature surprises us,” said Curtis McCully, a graduate student at Rutgers and lead author of the paper.

What they saw following the explosion was the helium core of a star that had lost its hydrogen shell. Though they’ll need to use Hubble again in 2015 to confirm, the find lends credence to their hypothesis that supernovas are the result of white dwarf/helium star binary systems.

How these things evolve is uncertain, but one theory involves a game of “seesaw” between the two stars. The larger star grows faster, dumping its helium and hydrogen onto the smaller star and eventually becomes a white dwarf. The smaller star then catches up, engulfing the white dwarf and rejecting the outer layers of the new combined star. The white dwarf then siphons off energy from its helium companion until it eventually explodes, leaving the “zombie” star we see today.

“SN 2012Z is one of the more powerful Type Iax supernovae and SN 2008ha is one of the weakest of the class, showing that Type Iax systems are very diverse,” explained team member Ryan Foley of the University of Illinois at Urbana-Champaign, lead author of the paper on SN 2008ha. “And perhaps that diversity is related to how each of these stars explodes. Because these supernovae don’t destroy the white dwarf completely, we surmise that some of these explosions eject a little bit and some eject a whole lot.”