Astronomers Discover New, Third Type of Supernova: Exploding Stars May Not be Deadly After All

Supernovae, the massive, exploding stars that hurl cosmic material into space, were once thought to occur only in two main types. Now, astronomers have discovered a new, third type of supernova called Type Iax.

In the past, researchers divided these exploding stars into two main categories: core collapse and Type Ia. A core-collapse supernova occurs when a star about 10 to 100 times the size of our own sun explodes. A Type Ia supernova, in contrast, is the complete disruption of a tiny white dwarf.

This new type, though, has characteristics that are different from both. Type Iax is both fainter and less energetic than Type Ia, even though it also comes from an exploding white dwarf. It's also rarer--only about a third as common as a Type Ia supernova. This possibly explains why researchers haven't seen the phenomenon before. The faintest of this category are only one-hundredth as bright as a Type Ia.  In addition, this new type may not completely destroy the white dwarf--it's essentially a mini supernova.

"It's the runt of the supernova litter," said lead author, Ryan Foley, in a press release.

In order to categorize the tiny supernova, researchers examined 25 examples of the new type. None of these supernovae appeared in elliptical galaxies, which are filled with old stars. This suggested that the supernovae are unique to young star systems.

The Type Iax supernovae may shun older star systems, but they seem to thrive in binary star systems. Researchers saw the new category in systems containing a white dwarf and a companion star that had lost its outer hydrogen, leaving it helium dominant. The white dwarf collects helium from the normal star.

Despite their observations, though, the researchers still aren't sure exactly what triggers a Type Iax. It's possible that the outer layer of helium ignites first, sending a shock wave into the white dwarf. Another possibility is that the white dwarf may ignite first due to the influence of the overlying helium shell. Whatever the case, it seems that the supernova isn't completely deadly for the white dwarf, which often survives the explosion. It's a sharp contrast to a Type Ia supernova, which completely destroys the star.

"The closer we look, the more ways we find for stars to explode," said Mark Phillips, one of the researchers, in a press release.

The findings have been accepted for publication in The Astrophysical Journal.