Catching a Supernova in the Act
Saturday, May 24th, 2008A Supernova is the explosive death of a massive star, and is also one of the coolest (not in the temperature sense!) things in the universe. Stars are basically defined by the fluctuating battle in their interiors between their own crushing weight forcing atomic nuclei to fuse and the tremendous thermal pressure generated by the energy released from that nuclear fusion. As it happens, more massive atomic nuclei are more energetically stable than less massive nuclei. This means that the mass of, say, a carbon nucleus is less than the sum of the masses of the three helium nuclei that fuse to form it inside the core of a star. How can that be? It be. Einstein famously showed the equivalence between mass and energy, so mass is a form of potential energy in the same way that the height of a boulder on a hillside represents potential energy. The boulder at the top of the hill has gravitational potential energy that is released when it rolls down the hill. Protons and neutrons, the constituents of atomic nuclei, have mass potential energy that is released when they are brought close enough together for the nuclear strong force to bind them together into a larger nucleus. Just as the rolling boulder manifests energy in a new form (rolling and falling faster) as its gravitational potential energy is released, so do the products of nuclear fusion in stars release energy that comes from its mass potential energy. In the case of nuclear fusion, that energy is released in the form of gamma ray (high energy) photons, neutrinos, and the kinetic energy of the occasional positron.
Up to a point: it turns out that it is only energetically advantageous for nuclei to fuse elements as massive as iron or less. Fusing more massive nuclei, such as silver and gold, is like rolling the boulder back up the hill: it takes more energy rather than releasing it. That’s where supernovae come in. Once the core of a massive star has fused into iron, it’s not energetically advantageous for any further fusion to take place. The iron core gets more and more massive as fusion in shells around the core dumps more heavy elements onto it. At some point the electron degeneracy pressure holding the core up cannot support its own weight, and it collapses suddenly and violently. Now we’re back to the boulder rolling down the hill: gravitational potential energy is converted to kinetic energy as the core collapses from something the size of the Earth to just a few kilometers across. This release of energy, which quickly bounces back outward into space, is a supernova. And it is in this very high energy event that atomic nuclei more massive than iron are created. Earlier this year astronomers serendipitously caught the early X-ray emission from a supernova in the galaxy NGC 2770 about 100 million light years from here. Because the bright X-ray flash is the first light to escape from the exploding star, catching it allowed the astronomers, led by Alicia Soderberg of Princeton, to alert the rest of the astronomical community to observe the supernova, providing an unprecedented record of the event from its earliest stages. Usually supernovae are not observed until days after the initial core collapse, because the brightness of the explosion in the visible part of the spectrum grows over the course of many days. They were fortunate enough to catch this one in the act because they were observing a supernova that had taken place in the same galaxy earlier. Perhaps the coolest thing about supernovae is that you probably have a fair amount of material made in an ancient supernova, whether it is a gold ring or silver necklace, every piece of that was blasted into space by a supernova explosion more than 4.6 billion years ago.