Type Ia supernovae occur when white dwarf stars explode. Until recently, it was understood that the stars undergoing this type of explosion had an upper size limit, known as the Chandrasekhar limit after the Indian astrophysicist Subrahmanyan Chandrasekhar. It was thought that larger white dwarf stars would collapse under their own weight rather than becoming type Ia supernovae. This limit, of about 1.4 times the mass of our Sun, was important because it allowed cosmologists to measure distances to the galaxies containing those resulting surpernovae.
Brighter white dwarf supernovae are occasionally discovered, but scientists weren’t sure whether they were also larger than the Chandrasekhar limit. That changed when a French and American collaboration called the Nearby Supernova Factory (SNfactory) measured the mass of one such anomaly, dubbed SN 2007if (no, the ‘if’ doesn’t imply that anyone was wishing on a star). At about two times the mass of the sun, SN2007if is indeed above the limit.
It’s not clear how these “super-Chandrasekhar” supernovae form. Richard Scalzo of Yale, suggested that 2N2007if was actually two white dwarfs that had merged together. In any case, astronomers will have to rethink using type Ia supernovae as cosmic rulers.
As Scalzo said:
Supernovae are being used to make statements about the fate of the universe and our theory of gravity. If our understanding of supernovae changes, it could significantly impact our theories and predictions.