Lawrence Berkeley National Laboratory
How massive can stars be? While theory predicts that stars up to a thousand times more massive than the sun have formed in the early Universe, the largest stars in our Galaxy are only 150 times the mass of our sun. It is well established that 8-20 solar mass (Msol) stars fuse progressively heavier elements in their centers and develop a core of inert iron, which then gravitationally collapses, leading to the explosion of the star as an ordinary supernova that leaves behind a remnant of either a neutron star or black hole. Stars as massive as 50-100 Msol have been shown to explode in a similar manner. Stars with M > 140 Msol develop heavy evolved cores, composed mostly of oxygen, which achieve high temperatures at relatively low densities. Conversion of energetic, pressure-supporting photons into electron-positron pairs occurs prior to oxygen ignition, and leads to a violent contraction that triggers a catastrophic thermonuclear explosion. Here I will present the discovery of the first convincing case for one of these "pair-instability" supernova, SN 2007bi.