Research Group Gabriel Martinez-Pinedo
Theoretical Nuclear Astrophysics

Core-Collapse Supernovae

Core-collapse supernovae (CCSN) belong to the most luminous and powerful events in the universe, making a single star shine brighter than a whole galaxy for a short time [1]. It was already speculated by Walter Baade and Fritz Zwicky in 1934 that supernovae denote the transition of ordinary stars to neutron stars. Since then, CCSN belong to the most interesting topics of nuclear astrophysics as their study involves the understanding of all four fundamental forces: gravity, electroweak & strong interaction.

The canonical picture of CCSNe that we have today is that stars more massive than about 10 times the mass of our sun will fusion heavier and heavier elements during their different hydrostatic burning stages, starting from hydrogen and helium. Hydrostatic equilibrium is maintained due to the energy released by the nuclear fusion reactions and thereby makes the star stable against its own gravitational pull. Once iron is produced, any further fusion reaction doesn't release energy anymore. Hence, more and more “nonreactive” iron is accumulated in the core. With the decreasing pressure support from nuclear burning, the star will eventually become gravitationally unstable and start to collapse. During the collapse, the core of the star becomes incredibly dense, exceeding even the densities of an atomic nucleus until matter starts to encounter the short-range repulsive behaviour of the strong force, halting the collapse and forming a shock wave that travels outwards through the star. Together with the shock wave, some of the surrounding material is heated up and catapulted into space. During the explosion, heavy elements beyond iron are created.

A neutron star is born in its centre that in some cases will collapse further to a stellar-mass black hole. The supernova explosion itself is powered by the immense amount of gravitational energy that is released by shrinking the inner part of a star to an object of the size of just about 20 kilometers which cools down by emitting neutrinos that carry away 99% of the gravitational binding energy. A supernova also produces gravitational waves that hopefully will be observed in the near future.

From a theoretical point of view, the main task is to explain the mechanism that turns the implosion of the star into an explosion and to understand the underlying physics. Among other explanations, it is widely believed that the huge number of neutrinos that is created during the supernova, will be responsible for powering the explosion by interacting with the material behind the shock wave and thereby depositing energy. Weak interaction processes [2] play a fundamental role and especially how neutrinos are created, scattered or absorbed in a hot and dense environment. Neutrino flavor transformations can furthermore affect the dynamics and nucleosynthesis [3].