Supernova 2012Z may have left behind a "zombie star" that burst to life after a not-so-permanent death. This phenomenon is only recently recognized by astronomers.

Zombie stars are a mystery to the team that has now recognized their existence.

Supernovas are massive explosions that typically come in two classes. The first of these, core-collapse supernovas, mark the death of the most massive stars in the Universe. These catastrophic events usually spread much of the material of the star to space, leaving behind a small body that can collapse to form a neutron star or black hole.

Type 1A supernovas involve binary stars with one massive star and a white dwarf, the remains of dead stars originally having a mass similar to our own sun. Gaseous material from the larger star starts to fall onto the white dwarf. When this material reaches a critical mass, a massive thermonuclear reaction is triggered, destroying the smaller companion.

A new type of supernova, dubbed Type 1ax, may have been recorded by the Hubble Space Telescope in the 2012Z event. These events are much less energetic than typical type 1A explosions, and do not consume the white dwarf.

"A Type Iax supernova is essentially a mini supernova. It's the runt of the supernova litter," Ryan Foley, from the Harvard-Smithsonian Center for Astrophysics (CfA), said.

Foley and his team discovered 25 likely examples of Type 1ax supernovas while examining images recorded by the Hubble Space Telescope. None of these candidate systems were present in elliptical galaxies, which contain large numbers of older stars. This suggested to the team that Type 1ax supernovas may take place in younger binary systems.

Astronomers are uncertain as to the exact cause of the newly-discovered variety of supernova. Data collected so far suggests they may develop in stars where the larger member of the pair is rich in helium. A typical star like our Sun is roughly 75 percent hydrogen, and the vast majority of the rest of its mass is helium. Older, massive, stars can shed layers of gas, mostly hydrogen, leaving the remaining body with greater-than-normal concentrations of helium. The explosion could be triggered either by extra mass landing on the white dwarf, or when the incoming material becomes dense enough to trigger an explosion, sending shock waves down to the stellar remnant.

"Astronomers have been searching for decades for the star systems that produce Type Ia supernova explosions. Type Ia's are important because they're used to measure vast cosmic distances and the expansion of the universe... The similarities between Type 1ax's and normal Type 1a's make understanding [their formation] important," Saurabh Jha of Rutgers University, said.

Study of the Type 1ax supernova was detailed in The Astrophysical Journal.