When a star becomes a white dwarf—an old, extremely dense star that would have once had a mass similar to our own sun’s—the eventful part of its life is over. It releases what heat and light it has left over billions of years, slowly cooling until it no longer shines. Some white dwarfs, however, are not content with this ending.

If a white dwarf exists in a two-star system with a companion, it can avert its fate and go out with a bang, not a whimper. It does so by causing a particular type of stellar explosion called a type Ia supernova. A type Ia supernova starts when the white dwarf drags material from its companion onto itself. It grows and grows until it cannot get any bigger. At this point, it implodes, then rebounds and explodes in a supernova bright enough to outshine whole galaxies.

The companion star from which the white dwarf steals matter is instrumental in this dramatic event. Its identity, however, has long been a mystery. Theoretical models say the companion star can be anything from a red giant to a main sequence star like the sun to another white dwarf.

Astronomers have been able to narrow the range of possible companions for a type Ia supernova spotted late last year. A telescope belonging to the Palomar Transient Factory (PTF) survey in Pasadena, Calif., spotted a bright spot at one minute before 9 P.M. on August 24. The new supernova, known as supernova 2011fe, won Palomar astronomers the rec­ord for the earliest ever detection of a type Ia supernova: just 11 hours after its initial explosion.

Last December researchers published two papers in Nature analyzing observations of supernova 2011fe. One paper, with lead author Peter Nugent of Lawrence Berkeley National Laboratory and PTF, found that the companion star was probably a main sequence star. The other work, spearheaded by Weidong Li of the University of California, Berkeley, rules out a red giant.

Li used observations from the Keck II telescope in Hawaii to pinpoint the location of the supernova, then analyzed Hubble Space Telescope images from before the supernova explosion to look for clues about the pair of stars from which it was born.

Supernova 2011fe is the nearest type Ia supernova to be discovered in many years and, because instrumentation has moved on considerably in that time, will be the most studied supernova in history. These two papers are just the beginning.

 This article was published in print as "Stars That Go Out with a Bang."