Until recently, it was thought that white dwarfs could not exceed what is known as the Chandrasekhar limit, a critical mass equaling about 1.4 times that of the Sun, before exploding in a supernova.

Since 2003, four supernovae have been discovered that were so bright, cosmologists wondered whether their white dwarfs had surpassed the Chandrasekhar limit, dubbed the "super-Chandrasekhar" supernovae.

Now, a team of astronomers has measured the mass of the white dwarf star that resulted in one of these rare supernovae, called SN 2007if, and confirmed that it has exceeded the Chandrasekhar limit. They also discovered that the unusually bright supernova had not only a central mass, but a shell of material that was ejected during the explosion as well as a surrounding envelope of pre-existing material. The team hopes this discovery will provide a structural model with which to understand the other supermassive supernovae.

Using observations from telescopes in Chile, Hawaii and California, astronomers were able to measure the mass of the central star, the shell and the envelope individually, providing the first conclusive evidence that the star system itself did indeed surpass the Chandrasekhar limit. They found that the star itself appears to have had a mass of 2.1 times the mass of the Sun (plus or minus 10 percent), putting it well above the limit.

Being able to measure masses for all parts of the star system tells the physicists about how the system may have evolved—a process that is currently poorly understood. "We don't really know much about the stars that lead to these supernovae," said Richard Scalzo, an astronomer at Yale. "We want to know more about what kind of stars they were, and how they formed and evolved over time."

There's a good chance, the team says, that SN 2007if resulted from the merging of two white dwarfs, rather than the explosion of a single white dwarf.

Theorists continue to explore how stars with masses above the Chandrasekhar limit, which is based on a simplified star model, could exist without collapsing under their own weight. Either way, a subclass of supernovae governed by different physics could have a dramatic effect on the way cosmologists use them to measure the expansion of the universe.

"Supernovae are being used to make statements about the fate of the universe and our theory of gravity," Scalzo said. "If our understanding of supernovae changes, it could significantly impact of our theories and predictions."

Citation: Scalzo et al., 'Nearby Supernova Factory Observations of SN 2007if: First Total Mass Measurement of a Super-Chandrasekhar-Mass Progenitor', Astrophysical Journal, 2010; doi: http://arxiv.org/abs/1003.2217