First But Not Last

A Supernova That Is the First of Its Kind

An extraordinarily bright, extraordinarily long-lasting supernova named SN 2007bi, snagged in a search by a robotic telescope, turns out to be the first example of the kind of stars that first populated the Universe. The superbright supernova occurred in a nearby dwarf galaxy, a kind of galaxy that’s common but has been little studied until now, and the unusual supernova could be the first of many such events soon to be discovered.

SN 2007bi was found early in 2007. The supernova’s spectrum was unusual, and astronomers subsequently obtained a more detailed spectrum. Over the next year and a half Berkeley scientists participated in a collaboration with the Weizmann Institute of Science to collect and analyze much more data as the supernova slowly faded away.

The analysis indicated that the supernova’s precursor star could only have been a giant weighing at least 200 times the mass of our Sun and initially containing few elements besides hydrogen and helium – a star like the very first stars in the early Universe.

“Because the core alone was some 100 solar masses, the long-hypothesized phenomenon called pair instability must have occurred,” says astrophysicist Peter Nugent. “In the extreme heat of the star’s interior, energetic gamma rays created pairs of electrons and positrons, which bled off the pressure that sustained the core against collapse.”

“SN 2007bi was the explosion of an exceedingly massive star,” says Alex Filippenko, an astronomy professor. “But instead of turning into a black hole like many other heavyweight stars, its core went through a nuclear runaway that blew it to shreds. This type of behavior was predicted several decades ago by theorists, but never convincingly observed until now.”

On the trail of a strange beast

The SNfactory has so far discovered nearly a thousand supernovae of all types and amassed thousands of spectra, but has focused on those designated Type Ia, the “standard candles” used to study the expansion history of the Universe. SN 2007bi, however, turned out not to be a Type Ia. For one thing, it was at least ten times as bright.

“The thermonuclear runaway experienced by the core of SN 2007bi is reminiscent of that seen in the explosions of white dwarfs as Type Ia supernovae,” says Filippenko, “but on a much larger scale and with a far greater amount of power.”

“The central part of the huge star had fused to oxygen near the end of its life, and was very hot,” Filippenko explains. “Then the most energetic photons of light turned into electron-positron pairs, robbing the core of pressure and causing it to collapse. This led to a nuclear runaway explosion that created a large amount of radioactive nickel, whose decay energized the ejected gas and kept the supernova visible for a long time.”

A fossil laboratory of the early Universe

“It’s significant that the first unambiguous example of a pair-instability supernova was found in a dwarf galaxy,” says Nugent. “These are incredibly small, very dim galaxies that contain few elements heavier than hydrogen and helium, so they are models of the early Universe.”

Dwarf galaxies are ubiquitous but so faint and dim – “they take only a few pixels on a camera,” says Nugent, “and until recently, with the development of wide-field projects like the SNfactory, astronomers had wanted to fill the chip with galaxies” – that they’ve rarely been studied.

Says Filippenko, “In the future, we might end up detecting the very first generation of stars, early in the history of the Universe, through explosions such as that of SN 2007bi – long before we have the capability of directly seeing the pre-explosion stars.”

The scientists expect that they will soon find many more ultrabright, ultramassive supernovae, revealing the role of these supernovae in creating the Universe as we know it today.

Source: Berkeley Lab