December 2, 2009
Berkeley, Calif. – A discovery of an extraordinarily bright, extraordinarily long-lasting supernova named SN 2007bi turns out to be the first known example of the earliest types of stars that populated the Universe. The unusually luminous supernova could provide astronomers with clues about the earliest stars in the cosmos and could be the first of many similar events soon to be discovered.
SN 2007bi was found in 2007 by the Nearby Supernova Factory (SNfactory) based at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. Over the next 18 months, observations of the exploding star were made by an international team of astronomers using the 10-meter Keck I telescope on the summit of Mauna Kea in Hawai’i and the Very Large Telescope in Chile.
Based on the data, the team determined that SN 2007bi was the explosion of an exceedingly massive star, said astronomer Alex Filippenko of the University of California Berkeley whose group helped obtain, analyze and interpret the data. “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.”
According to the data, which was collected in a collaboration led by Avishay Gal-Yam of Israel’s Weizmann Institute of Science, the supernova’s precursor star could only have been a giant having at least 200 times the mass of the Sun and initially containing few elements besides hydrogen and helium – a star similar to the first stars in the early Universe.
SN 2007bi is also the first confirmed observation of a pair-instability supernova. The long-hypothesized phenomenon suggests that “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,” said astrophysicist Peter Nugent, co-leader of Berkeley Lab’s Computational Cosmology Center (C3), a collaboration between the Lab’s Physics Division and Computational Research Division, or CRD.
The researchers describe the data to support the pair instability supernova finding in the Dec. 3 issue of Nature.
On the trail of a strange beast
SN 2007bi was first recorded on images taken as part of the Palomar-QUEST Survey, an automated search with the wide-field Oschin Telescope at the California Institute of Technology’s Palomar Observatory, and was quickly detected and categorized as an unusual supernova by the SNfactory. 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.
“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, but on a much larger scale and with a far greater amount of power,” Filippenko said. SN 2007bi was at least ten times as bright as the standard Type Ia supernovae.
Rollin Thomas of CRD, a member of C3 and the SNfactory, used the Franklin supercomputer at the National Energy Research Scientific Computing Center to match synthetic supernovae spectra with the real SN 2007bi spectrum. The model fit was unambiguous: SN 2007bi was a pair-instability supernova.
“The central part of the huge star had fused to oxygen near the end of its life, and was very hot,” Filippenko explained. “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
Finding the first unambiguous example of a pair-instability supernova in a dwarf galaxy is significant, Nugent said. Dwarf galaxies are incredibly small and dim and contain few elements heavier than hydrogen and helium, so they are models or fossil laboratories of the early Universe. Dwarf galaxies are also ubiquitous, but, they are so faint and dim that they’ve rarely been studied. SN 2007bi is expected to focus attention on these fainter galaxies.
Studying the dwarf galaxies and their remnant supernovae might, in the future, allow astronomers to— through explosions such as that of SN 2007bi— “detect the very first generation of stars, early in the history of the Universe, long before we have the capability of directly seeing the pre-explosion stars,” Filippenko explained. So while SN 2007bi is the first of its kind to be detected, it is likely not the last.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for the Department of Energy’s Office of Science and is managed by the University of California. For the full release, visit http://newscenter.lbl.gov/.
The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i. The twin telescopes feature a suite of advanced instrumentation including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a scientific partnership of the California Institute of Technology, the University of California and NASA. For more information please call 808.881.3827 or visit http://www.keckobservatory.org.