Megan's Blog

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

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