A stellar explosion known as a type Ia supernova is more than 2 times brighter than accepted theory says it can be, a team of astronomers reports. The discovery sounds a note of caution for cosmologists, who have used the supernovae as "standard candles" to measure the expansion of the universe, although the finding does not contradict the observation that the expansion is bizarrely accelerating.
The basic physics of type Ia supernovae is simple. A white dwarf star--the corpse of a dead star that consists of tightly squeezed carbon and oxygen--consumes matter from a companion star. When the white dwarf grows massive enough, heat and pressure within it trigger a thermonuclear explosion. That should happen as soon as the mass of the white dwarf reaches roughly 1.4 times the mass of the sun, the so-called Chandrasekhar limit. As a result, all type Ia supernovae should shine equally brightly, which means that astronomers can judge how far away they are by measuring how bright they appear in the sky.
But now, astronomers have discovered a type Ia supernova that shines more than twice as bright as it ought to. Andrew Howell and Raymond Carlberg of the University of Toronto and colleagues with the Supernova Legacy Survey at the Canada-France-Hawaii Telescope on Mauna Kea spotted the explosion on 24 April 2003. They studied its spectrum on 6 May with the adjoining Keck Telescope when the explosion shown brightest. From the spectrum and the rate at which the supernova brightened and then dimmed, the researchers determined that the white dwarf must have reached a whopping 2.2 solar masses before it went off. "It didn't break the limit by a tiny bit," Howell says. "It smashed it."
The result, published in this week in Nature, doesn't necessarily undermine the measurements of the universe's expansion, Howell notes. The oddball is so unusual that it's easy to spot and discard from the data set, he says.
David Branch, an astrophysicist at the University of Oklahoma in Norman agrees. "If I were doing cosmology, I would note this and proceed with caution, but I wouldn't stop what I was doing," he says. Cosmology aside, the observation poses the puzzle of how the white dwarf got so massive without exploding. One possibility, Branch says, is that the white dwarf was spinning fast enough to throw matter slightly outward and reduce the pressure inside it.