Type Ia supernovae are explosions of white dwarfs, the carbon-oxygen remnant cores of medium-sized stars that have exhausted their hydrogen fuel. They’re distinguished from other supernovae by the absence of hydrogen and the presence of silicon in their optical spectra. And they appear remarkably uniform—although not identical—which makes them useful to cosmologists as standard candles for gauging distances across the universe.

That uniformity has been attributed to a common formation mechanism: A white dwarf in a binary system accretes material from its companion star until it nears the Chandrasekhar limit of 1.4 solar masses (M). At that point, electron degeneracy pressure no longer supports the white dwarf against its own weight. When the center of the dwarf is sufficiently hot and dense, carbon fusion begins, which releases enough energy to unbind the star.

Several years ago researchers with the Supernova Legacy Survey spotted a supernova, SN 2003fg,...

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