For the first time, astronomers have discovered a star system with a strange supernova that brought a white dwarf star back to life after its explosion.
What makes this so unusual is that white dwarfs are stars at the end of their life cycles. They occur when a star doesn't have enough mass to become a neutron star, which is the type of star most of us are familiar with. In layman's terms, white dwarfs are almost dead. Once dead, their cores reignite or there is a gravitational collapse. Either way, the result is the same: the star explodes.
This supernova usually annihilates its white dwarf star. However, in this instance, scientists think that the supernova left the white dwarf behind, which in turn sucked energy from a companion star. This ignited a nuclear reaction in its core, brought it back to life and created a smaller second supernova.
This is associated with what is called a Type Iax supernova, and less common than the Type Ia, which is significantly brighter. As scientists have never seen the predecessor of the more common Type Ia, they may learn more about it by studying the Type Iax.
"Astronomers have been searching for decades for the progenitors of type Ia's," says Saurabh Jha of Rugers University. "Type Ia's are important because they're used to measure vast cosmic distances and the expansion of the universe."
After intensively studying enhanced Hubble images before the supernova, astronomers noticed a second object next to the white dwarf, apparently sharing some of its energy with it. After ruling out that this object was something not related to the supernova, they examined its colors and ran their findings through computer simulations.
They concluded that this second object was actually another star near the white dwarf. This star, though, lost its outer layer of hydrogen and was now mostly just helium.
"Back in 2009 when we were just starting to understand this class, we predicted these supernovae were produced by a white dwarf and helium star binary system," says research team member Ryan Foley of the University of Illinois at Urbana-Champaign. "There's still a little uncertainty in this study, but it is essentially validation of our claim."
Researchers suggest that the two stars shared energy with each other, creating a sort of "combined" star. The white dwarf eventually became unstable and exploded, but survived the event and came back to life for a second and smaller explosion. This is why astronomers refer to the star as a "zombie star."
