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Supernova Legacy Survey Finds a Superbright, Supermassive Type Ia Supernova in a Young Galaxy

A group of scientists affiliated with the SuperNova Legacy Survey (SNLS) have found startling evidence that there is more than one kind of Type Ia supernova, a class of exploding stars which until now has been regarded as essentially uniform in all important respects. Supernova SNLS-03D3bb is more than twice as bright as most Type Ia supernovae but has much less kinetic energy, and appears to be half again as massive as a typical Type Ia.

The supernova SNLS-03D3bb was discovered on April 24, 2003 in a small, young, star-forming galaxy, a satellite of the larger galaxy in this picture. Image on the left is before maximum brightness; at maximum brightness (right), the supernova was much brighter than its host. Image: www.lbl.gov

Because almost all Type Ia supernovae found so far are not only remarkably bright but remarkably uniform in their brightness, they are regarded as the best astronomical "standard candles" for measurement across cosmological distances. In 1998, after observations of many distant Type Ia supernovae, the Supernova Cosmology Project and the rival High-Z Supernova Search Team announced their discovery that the expansion of the universe is accelerating — a finding that would soon be attributed to the unknown something called dark energy, which fills the universe and opposes the mutual gravitational attraction of matter.

"Type Ia supernovae are thought to be reliable distance indicators because they have a standard amount of fuel — the carbon and oxygen in a white dwarf star — and they have a uniform trigger," says Nugent. "They are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass, which is about 1.4 times the mass of our sun. The fact that SNLS-03D3bb is well over that mass kind of opens up a Pandora's box."