Another cosmic mystery, solved

Scientists have used "standard candles," Type 1a supernovae, for years to measure the universe's rate of expansion. Thousands of Type 1a supernovae have been studied for their light, but until now, scientists have relied wholly on models - not direct evidence - to explain why these supernovae explode and what their star systems might have looked like ahead of the explosion.

On August 24, Peter Nugent of the U.S. Department of Energy's Lawrence Berkeley Laboratory (the Berkeley Lab), going through data from an automated telescope on California's Palomar Mountain, saw something notable: a confirmed, very young Type 1a supernova, officially named SN2011fe. This supernova is in the Pinwheel Galaxy, relatively close at 21 million light-years away.

SN2011fe was seen 11 hours after its explosion, allowing Nugent and other members of the multi-institutional Palomar Transient Factory (PTF) to calculate the moment of SN2011fe's explosion to within 20 minutes.

Direct observations of SN2011fe, particularly how the light it emitted evolved (the "early-time light curve"), allowed the PTF team to confirm some of the standard assumptions about Type 1a supernovae while ruling out some models of the physics behind these exploding stars.

SN2011fe started out as a carbon-oxygen white dwarf star, confirming long-held models that assume these supernovae form in binary systems where one dense, small, carbon-oxygen white dwarf orbits another star, from which it cannibalizes matter.

The early-time light curve also provided information that ruled out certain models of the supernova-producing binary system. The companion star, for example, couldn't have been a red giant, which would have made the supernova much brighter from the beginning. It also probably wasn't another white dwarf, which would have produced a hard-to-miss flash of light in the curve.

The observations point instead to a main-sequence companion star.

While overall, the observations of SN2011fe point to a "normal" Type 1a supernova, the PTF team expects that this particular supernova will become the best-studied stellar explosion in our history, forming the foundation for further knowledge of Type 1a supernovae.