Space

An enormous explosion observed in 2007 was the death of one of the most massive stars known in the universe, new calculations suggest. Similar blasts may have polluted the early universe with heavy elements, altering its evolution.

A team of astronomers led by Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, detected the explosion in a dwarf galaxy on 6 April 2007.

Measurements of the supernova's light spectrum and fading brightness over the next 18 months suggest the explosion, called SN 2007bi, ripped apart a star with 100 times the mass of the sun. The star must have been even heavier at birth, because massive stars shed a lot of material as they age. Based on its mass at death, the star was likely born with about 200 times the sun's mass, the team says.

No star is known to contain so much mass today. Among the heaviest stars that have been precisely weighed is one near our galaxy's centre with 114 times the sun's mass.

SN 2007bi also distinguishes itself as the best case yet for an exotic type of stellar death called a pair-instability supernova – a type of explosion that only affects stars born with more than about 140 times the mass of the sun.

Best case

Stars slightly below this threshold collapse to form black holes or neutron stars after burning up their nuclear fuel.

But heavier stars never get a chance to finish burning their fuel. As they age, high pressures and temperatures in their cores lead energetic photons to turn into pairs of electrons and their antimatter counterparts, positrons. This conversion reduces the pressure from radiation that supports the star's weight, triggering an explosion that rips the star apart.

The huge amount of radioactive nickel observed in SN 2007bi's light spectrum, amounting to more than seven times the mass of the sun, is characteristic of pair-instability supernovae, the team says. A 2006 explosion suspected of being this kind of event lacked such a clear signature.

Volker Bromm of the University of Texas in Austin, who is not a member of Gal-Yam's team, agrees. "2007bi is the first really convincing case," he told New Scientist.

Stunted growth

Though rare in the modern universe, pair-instability supernovae may have been common in the early universe, when the lack of elements heavier than hydrogen and helium would have favoured the formation of high-mass stars.

More than 22 solar masses of silicon and other heavy elements were blasted into space during this supernova, several times the amount from a normal supernova.

The heavy elements spewed into space in the deaths of similar early, massive stars may have stunted the growth of later stellar generations. That's because gas clouds containing iron and other heavy elements tend to fragment into smaller knots that give birth to relatively lightweight stars like the sun.

Though some less-massive stars also spew metals when they die, pair-instability supernovae are especially prolific polluters. "One such explosion can pollute an entire small ancient galaxy," Gal-Yam told New Scientist.

The dwarf galaxy where SN 2007bi occurred seems to have low metal content, the team says, which might explain how the star that went supernova was able to form.

Journal reference: Nature (DOI: 10.1038/nature08579)

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