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In the News this month: first observational evidence of a new type of supernova

A schematic of the material ejected from SN 2007bi, the radioactive nickel core (white) decays to cobalt, emitting gamma rays and positrons that excite surrounding layers (textured yellow) rich in heavy elements like iron. The outer layers (dark shadow) are lighter elements such as oxygen and carbon, where any helium must reside, which remain unilluminated and do not contribute to the visible spectrum.
A schematic od SN 2007bi CREDIT: Lawrence Berkeley National Laboratory
Stars the size and mass of our Sun end their lives by first expanding as red giants, then shrinking to white dwarfs. Stars heavier than this however, come to a much more violent end.  For stars with masses between about 10 and 100 times that of the Sun, they continue fusing hydrogen to form helium in the core, until they run out of hydrogen. They then begin to fuse the helium nuclei together to form heavier elements such as carbon, nitrogen and oxygen. This carries on through the elements until iron, at which point the core collapses to form either a neutron star or a black hole, and the outer layers are expelled in a giant explosion known as a supernova. For stars heavier than 140 times the mass of the Sun, theory suggests that there may be another mechanism causing the explosion.

Counterbalancing gravity inside a star is radiation pressure, the force of the photons themselves which help stop a star collapsing under its own gravitational pull. In very massive stars, when the temperature rises above 1,000,000,000 kelvin, these photons can undergo a process known as pair production where they create an electron and its anti-particle, a positron. This reduces the number of photons in the star, reducing the radiation pressure and, if it happens on a large enough scale, allows the star to begin collapsing. The result is the ignition of oxygen in the core, and the end of the star in what is known as a pair-instability supernova.

Models of this type of supernova are fairly robust, but have never been confirmed observationally. But now a team led by Avishay Gal-yam at the Weizmann Institute of Science in Israel, have discovered the first evidence of such an explosion taking place. They observed the supernova known as SN 2007bi and compared their observations with predictions from models of pair-instability supernovae. Their data fit the models very well, providing the first clear evidence of this type of explosion. Stars this size and much larger are thought to have been common in the early evolution of the universe, contributing significantly to the chemical evolution of the early galaxies, so confirmation of the models is an important step in understanding how the universe came to be the way we see it today.

Gal-Yam, A., Mazzali, P., Ofek, E., Nugent, P., Kulkarni, S., Kasliwal, M., Quimby, R., Filippenko, A., Cenko, S., Chornock, R., Waldman, R., Kasen, D., Sullivan, M., Beshore, E., Drake, A., Thomas, R., Bloom, J., Poznanski, D., Miller, A., Foley, R., Silverman, J., Arcavi, I., Ellis, R., & Deng, J. (2009). Supernova 2007bi as a pair-instability explosion Nature, 462 (7273), 624-627 DOI: 10.1038/nature08579

Posted by Megan on Monday 04th Jan 2010 (01:48 UTC) | 4 Comments | Permalink

Comments: In the News this month: first observational evidence of a new type of supernova

Wow. So the rate of photon pairing to produce electron-positron pairs exceeds the electron-positron combination which produces photons? Stellar cores are such strange places; as a chemist I'm not accustomed to thinking of light being converted into other particles. In my laboratory electron + positron is an irreversible reaction. :)

What temperatures and pressures are required before we have to stop talking about hydrogen and other elements and instead deal with a "particle soup"? What particles are distinguishable in the soup?

Posted by MadScientist on Monday 04th Jan 2010 (04:17 UTC)

MadScientist: The real question is whether a star which approaches "particle soup", or any condition other than the usual fusion sequence, will still retain enough expansive pressure to prevent collapse. Once collapse begins, any unusual pressure-caused condition may only become worse.

For that matter, I expect that stellar physicists are now examining other star-collapse situations to see if pair-instability conditions may fleetingly exist and contribute to nova events.

Posted by KathyT on Monday 04th Jan 2010 (05:19 UTC)

Thanks - I never stopped to think about how close nuclei would be packed in the cores of stars and it never occurred to me that fusion would indeed halt in a situation where it was no longer possible to distinguish nuclei of different elements.

Posted by MadScientist on Tuesday 05th Jan 2010 (11:54 UTC)

The interiors of stars are indeed very strange places! It gets even worse when you look at things like white dwarfs and neutron stars where the matter in the core is so dense that it actually becomes degenerate....

Posted by Megan on Thursday 07th Jan 2010 (07:30 UTC)

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