The supernova 2007bi wasn’t your typical supernova: it was 10 times brighter than a Type Ia supernova, making it one of the most energetic supernova events ever recorded. Astronomers from the University of California Berkeley have analyzed the explosion, which was recorded by a robotic survey in 2007, and found that it is likely the first confirmed observation ever made of a pair-instability supernova, a type of extremely energetic supernova that has been theorized but never directly confirmed.
The confirmed observation of a pair-instability supernova has been long-awaited – the theory that they exist has been around since the 1960’s – but it appears as if the wait is over. The supernova 2007bi, seen by the Nearby Supernova Factory in April of 2007, is the first observed supernova that fits the bill for the unfathomably huge proportions of pair-instability supernovae explosions. A team of astronomers led by Alex Filippenko of the University of California Berkeley published their analysis in in the December 3rd issue of Nature. The discovery was initially made by the Nearby Supernova Factory, and emission spectra of the event was taken with the Keck Telescope and Very Large Telescope in Chile
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These type of supernovae occur only in stars above 100 solar masses, and are incredibly bright. Energetic gamma rays are created by the intense heat in the core of the star. These gamma rays, in turn, create antimatter pairs of electrons and positrons. Because of this antimatter production, the outward pressure exerted by the nuclear reactions in the core of the star is lessened, and gravity takes over, quickly collapsing the massive core of the star and creating a supernova.
There are theorized to be two kinds: those that explode with just enough force to allow for the mass around the leftover core of the star to recombine, and those that explode completely with not a smidgen left to form a black hole or neutron star. The supernova 2006gy, which had a luminosity 10 times that of a Type Ia supernova, is thought to be of the first variety. Here’s our story on that one, Could Antimatter be Powering Super-Luminous Supernovae? and Eta Carinae may also fit the profile.hese types of pair-instability supernovae will eject the outer shells of the star’s matter, settle down into an equilibrium, and repeat that process until the mass is low enough for a normal supernova to occur.
But 2007bi was much too massive to settle back down and explode multiple times. With a mass of 200 suns, the runaway thermonuclear explosion that happened in its core was energetic enough to effectively vaporize the entire star. Pair-instability supernovae in stars above 130 solar masses leave nothing behind in the way of black holes or neutron stars, but because they are so energetic and luminous, the increasing light from the explosion peaks over a very long time – 70 days in the case of 2007bi.
Though the team detected the supernova almost a week after the peak, they were able to calculate the duration of the light curve. They then studied the remnants of the explosion over the next 555 days as it faded away.
Filippenko said, “The central part of the huge star had fused to oxygen near the end of its life, and was very hot. 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.”
The star was unique in another way: it lies in a nearby dwarf galaxy, which contains little else but the elements hydrogen and helium. Because of this, 2007bi is much like the stars that existed near the beginning of the Universe, before the trillions of supernovae populated the Universe with heavier elements. Looking more closely at dwarf galaxies – the Universe has them in spades, but they are quite dim – may be the key to observing more supernovae of this kind. Being able to study its explosion and aftereffects will give scientists a look into what the earliest massive stars acted like.
Source: Berkeley Lab press release