When a star becomes a white dwarf — an old, extremely dense star that would have once been similar to our own Sun — 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.

Usually. 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 this by causing a particular type of stellar explosion called a type 1a supernova. A type 1a 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.

Despite their origins being somewhat muddy, type 1a supernovae have given us a lot over the past year. Saul Perlmutter, Brian Schmidt and Adam Riess, along with their respective research groups, used them to discover the accelerating expansion of the universe — caused by the mysterious force we call dark energy — that won them the 2011 Nobel prize in physics.

Now, a type 1a supernova spotted in August this year has allowed astronomers to narrow down the range of possible companions that give white dwarfs the mass boost they need to explode.

At one minute to four in the morning on 24th August 2011, an alert was sent out to astronomers working on the Palomar Transient Factory (PTF). Their telescope had spotted an extremely bright object in the Pinwheel galaxy that hadn't been there before — a new supernova, now known as supernova 2011fe. At that time it was, relatively speaking, quite faint, but over time it brightened. You may have seen it yourself: ten days after it was first seen it became bright enough to see through a pair of good binoculars.

The Palomar Transient Factory had noticed the star just 11 hours after it exploded, winning them the record for the earliest ever detection of a type 1a supernova, and were quick enough to get a glimpse of the light coming from it just 16 hours after explosion using an instrument on the robotic Liverpool Telescope in the Canary Islands. Since then, other telescopes have looked over their observations of that night to see if they saw it too.

Watching a supernova as soon as possible after it starts is key to discovering what happened to make it explode in the first place. Theoretical models say the companion star to an exploding white dwarf can be only one of three types: a red giant, a main sequence star like the Sun, or another white dwarf. Astronomers are keen to narrow this down further.

In Nature earlier this month a team from California published two papers analysing observations of supernova 2011fe in the hope of finding clues that will enable them to do this. One paper, led by Peter Nugent from the Lawrence Berkeley National Laboratory found that the companion star was probably a main sequence star. Nugent's paper also confirms that the star that exploded was a white dwarf.

The other paper, led by Weidong Li of the University of California, Berkeley, rules out a red giant companion.

Li used observations from the Keck telescope on the summit of Mauna Kea in Hawaii to pinpoint the precise location of the supernova, then analysed images taken by the Hubble Space Telescope from before the supernova explosion to look for clues about the pair of stars from which it was born. In the space where supernova 2011fe was later detected, they saw nothing. This did not mean they had made a mistake — just that the system preceding the supernova was not bright enough to be detected. This information was enough for them to rule out a red giant as the companion because, at one hundred times as luminous as the Sun, it would have been bright enough to show up. They could not, however, rule out other types of stars.

Supernova 2011fe is the first type 1a supernova to be discovered for many years and, because instrumentation has moved on considerably in that time, will become the most studied supernovae in history. These two papers are just the beginning.


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Li W, Bloom JS, Podsiadlowski P, Miller AA, Cenko SB, Jha SW, Sullivan M, Howell DA, Nugent PE, Butler NR, Ofek EO, Kasliwal MM, Richards JW, Stockton A, Shih HY, Bildsten L, Shara MM, Bibby J, Filippenko AV, Ganeshalingam M, Silverman JM, Kulkarni SR, Law NM, Poznanski D, Quimby RM, McCully C, Patel B, Maguire K, & Shen KJ (2011). Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe. Nature, 480 (7377), 348-50 PMID: 22170681

Nugent PE, Sullivan M, Cenko SB, Thomas RC, Kasen D, Howell DA, Bersier D, Bloom JS, Kulkarni SR, Kandrashoff MT, Filippenko AV, Silverman JM, Marcy GW, Howard AW, Isaacson HT, Maguire K, Suzuki N, Tarlton JE, Pan YC, Bildsten L, Fulton BJ, Parrent JT, Sand D, Podsiadlowski P, Bianco FB, Dilday B, Graham ML, Lyman J, James P, Kasliwal MM, Law NM, Quimby RM, Hook IM, Walker ES, Mazzali P, Pian E, Ofek EO, Gal-Yam A, & Poznanski D (2011). Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star. Nature, 480 (7377), 344-7 PMID: 22170680