Washington, Nov 2 : A new finding by astronomers indicates that the actual cause of the supernova discovered last year was the collision of two white dwarf stars.

The white dwarfs, which were siblings orbiting each other, slowly spiraled inward until they merged, triggering off a titanic explosion. CfA (Centre for Astrophysics) observations show the strongest evidence yet of what was, until now, a purely theoretical mechanism for creating a supernova.

"This finding shows that nature may be richer than we suspected, with more than one way to make a white dwarf explode," said first author Malcolm Hicken, who is a Harvard graduate student.

Astronomers characterize an observed supernova based on whether its spectrum shows evidence of hydrogen (Type II) or not (Type I). In Type II, a massive, short-lived star undergoes core collapse and explodes. In the conventional picture for Type Ia, the most common supernovae lacking hydrogen, a white dwarf star collects gas from a companion star until it undergoes catastrophic nuclear fusion and blasts itself apart.

The new find, supernova 2006gz, was classified as a Type Ia due to the lack of hydrogen and other characteristics. However, an analysis combining CfA data with measurements from The Ohio State University suggested that SN 2006gz (the supernova) was unusual and deserved a closer look.

SN 2006gz showed the strongest spectral signature of unburned carbon ever seen. The spectrum of this supernova also showed evidence for compressed layers of silicon.  Silicon was created during the explosion and then compressed by a shock wave that rebounded from the surrounding layers of carbon and oxygen. Additionally, SN 2006gz was brighter than expected, indicating that its progenitor exceeded the 1.4 solar mass Chandrasekhar limit - the upper bound for a single white dwarf.

"Although 2006gz is also extra bright, the chemistry we see, particularly unburned carbon, is well observed and very unusual," said Harvard astronomer Robert Kirshner, a member of the discovery team.

In addition to providing the first example of a new way to make supernovae, SN 2006gz holds implications for the field of cosmology. Type Ia supernovae typically have a narrow spread in brightness, which makes them useful as "standard candles" for calculating cosmic distances.

Also, it was the study of Type Ia supernovae that led to the discovery of dark energy, the mysterious force causing the expansion of the universe to accelerate. (ANI)