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The exploding stars known as type Ia supernovae are so consistently bright that astronomers refer to them as standard candles beacons that are used to measure vast cosmological distances. But these cosmic mileposts may not be so uniform. A new study finds evidence that the supernovae can arise by two different processes, adding to lingering suspicions that standard candles aren't so standard after all.

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The findings, which have been posted on the arXiv preprint server1 and accepted for publication in the Astrophysical Journal, could help astronomers to calibrate measurements of the Universe's expansion. Tracking type Ia supernovae showed that the Universe is expanding at an ever-increasing rate, and helped to prove the existence of dark energy advances that secured the 2011 Nobel Prize in Physics.

The fact that scientists don't fully understand these cosmological tools is embarrassing, says the latest study's lead author, Griffin Hosseinzadeh, an astronomer at the University of California, Santa Barbara. "One of the greatest discoveries of the century is based on these things and we don't even know what they are, really."

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"It's like having a tool that you know how to use, but you don't know how it works," Hosseinzadeh says. "Understanding the physics of the tool that you're using seems better than just using it blindly."

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