
 At first, like an  opening scene scene out a scifi classic, there didn't seem anything odd  or unusual about the tiny point of light blinking in the southern  Californian night sky in early April 2007. Only the robotic eyes of the  Nearby Supernova Factory, a project designed to spy out distant stellar  explosions, spotted it from the Palomar Observatory, high in the hills  between Los Angeles and San Diego.
The  project's computers automatically forwarded the images, following the  same routine that kicks in scores of times each year when a far-off star  in its death throes explodes into a supernova, before fading into the  vast cosmic backstory of the universe.
This extraordinarily bright 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. Despite  the prominence of large elliptical and spiral galaxies, most galaxies in  the universe appear to be dwarf galaxies. These tiny galaxies are about  one hundred times smaller than the Milky Way, containing only a few  billion stars
SN 2007bi was discovered by the Supernova Factory  (SNfactory) based at the U.S. Department of Energy's Lawrence Berkeley  National Laboratory.
Over the next year and a half the Berkeley  scientists participated in a collaboration led by Avishay Gal-Yam of  Israel's Weizmann Institute of Science to collect and analyze much more  of the unusual object's spectrum 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. A member of the SNfactory, Nugent is the  co-leader of the Computational Cosmology Center. "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, a professor in the Astronomy  Department at UC Berkeley whose team 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."
SN  2007bi was 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. 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."
Nugent contacted Gal-Yam,  then a Caltech postdoctoral fellow, the lead investigator for the  all-other category. "I asked, are you interested? He said, sure!" Nugent  then contacted Filippenko, who was about to conduct a night of  observation with the 10-meter Keck I telescope on the summit of Mauna  Kea in Hawaii. Filippenko immediately set out to obtain an optical  spectrum of the unusual supernova.
Caltech researchers  subsequently acquired additional spectra with the Keck telescope, as did  Paolo Mazzali's team from the Max Planck Institute for Astrophysics in  Garching, Germany, using the Very Large Telescope (VLT) in Chile.
Says  Mazzali, "The Keck and VLT spectra clearly indicated that an extremely  large amount of material was ejected by the explosion, including a  record amount of radioactive nickel, which caused the expanding gases to  glow very brightly."
"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."
Gal-Yam organized a  team of collaborators from many institutions to continue to observe SN  2007bi and obtain data as it slowly faded over a span of 555 days. Says  Gal-Yam, "As our follow-up observations started to roll in, I  immediately realized this must be something new. And indeed it turned  out to be a fantastic example of how we are finding new types of stellar  explosions."
Because it had no hydrogen or helium lines, the  usual classification scheme would have labeled the supernova a Type Ic.  But it was so much brighter than an ordinary Type Ic that it reminded  Nugent of only one prior event, a supernova designated SN 1999as, found  by the international Supernova Cosmology Project but unfortunately three  weeks after its peak brightness.
Understanding a supernova  requires a good record of its rise and fall in brightness, or light  curve. Although SN 2007bi was detected more than a week after its peak,  Nugent delved into years of data compiled by NERSC from the SNfactory  and other surveys. He found that the Catalina Sky Survey had recorded SN  2007bi before its peak brightness and could provide enough data to  calculate the duration of the rising curve, an extraordinarily long 70  days -- more evidence for the pair-instability identification.
"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."
These  tiny dwarf galaxies could be pristine remnants of the early universe,  preserving its composition and conditions in a cosmic museum. Their  degree of preservation could be the result of their sheer dwarfishness:  because gravity within them is weaker than within a normal galaxy, a  supernova exploding within it will eject the metal-rich products  outwards at such speed that they mostly escape altogether.
If  the original conditions of the universe were preserved in these dwarf  galaxies, there would be no reason why further waves of megastars should  not continually form and die within them throughout time. If it is the  absence of metals that determines stellar size, these monster stars are  not restricted to the furthest reaches of the universe: they could be  found in any dwarf galaxy with a low enough metal content, including  places well within reach of Earth's telescopes.
The  discovery of a nearby population of megastars in what amounts to  suspended animation would have huge implications for stellar science. We  do not understand the processes of star formation and death as well as  we would like to think.
"It is surprisingly difficult  to get the models to agree with the observations," says Gal-Yam. He  cites the example of gold, the abundance of which in the universe  essentially defies explanation, although most astronomers assume it must  somehow be made in supernovae. To find the answers, we might need to  look no further than dwarf galaxies orbiting the Milky Way.
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. SN 2007bi is expected to focus attention on what Gal-Yam and  his collaborators call "fossil laboratories to study the early  Universe."
"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," added  Filippenko .
With the advent of the multi-institutional  Palomar Transient Factory, a fully automated, wide-field survey to find  transients, the SNfactory, the Near Earth Asteroid Team, and other  surveys, the collaborators expect they will soon find many more  ultrabright, ultramassive supernovae, revealing the role of these  supernovae in creating the observable universe as we know it today.
Casey  Kazan via DOE/Lawrence Berkeley National Laboratory.
Other  Source:  
http://www.newscientist.com/article/mg20527470.900-primordial-giant-the-star-that-time-forgot.html?full=true