Dr. Tony Mezzacappa

September 26, 2016

Colloquium by Dr. Tony Mezzacappa - The University of Tennessee, Knoxville

Title: “From Stars to Nuclei and Back: Our Cosmic Origin and the Exascale Challenge to Find It”


We learn in elementary school that the elements in the Periodic Table are the building blocks of our world, including our very bodies. But from where do the elements come? This is among the most basic questions we can ask, yet the precise answer remains elusive. We witness the cycle of life in our daily lives, everywhere on Earth. This is no less true in the Universe. With the exception of the lightest elements such as hydrogen and helium, elements are made in stars. As stars evolve and die, these elements pepper the interstellar medium, from which new stars, and planets, – in particular, our solar system – form. We understand the essential elements of this cycle – from stellar birth, life, and death, to the formation of the elements, to the formation of new stars and planets including those elements, to ultimately the origin of our solar system and life on Earth given those elements. But pieces of the puzzle are missing. We do not yet understand how certain stars that are factories for many of the elements, die, nor do we know the precise origin of half the elements heavier than iron, although we have narrowed down the list of possible sites. Today’s colloquium will focus on the death of massive stars in catastrophic explosions known as core collapse supernovae. Such supernovae provide the lion’s share of the elements between oxygen and iron, and are considered a potential site for the origin of half the elements heavier than iron. Arguably, they are the single most important source of elements in the Universe. Such supernovae present us with a general relativistic, radiation magneto-hydrodynamic – i.e., a multi-physics – environment to model. Further richness and complexity is added by the fact that the macroscopic evolution of such a system is governed in no small part by the high-density, neutron-rich, nuclear matter at the core of the supernova and by the microscopic interaction of radiation in the form of neutrinos with this stellar core nuclear material. I will discuss specific examples of how the macroscopic and microscopic worlds intertwine to produce such a supernova. It is intuitively obvious that such a multi-physics arena might wind up among the set of exascale challenges that must be met in order to advance our understanding of the world and our ability to use it for the common good. Indeed, as I will discuss, core collapse supernovae do in fact present us with a sustained exascale challenge. I will provide concrete examples to illustrate why. The cost to meet such a challenge, in human effort and in the provisioning of leadership-class computing platforms and their use, is high, but the scientific return on investment is significant. Progress, particularly within the past decade, has been rapid. A new and related branch of astronomy – gravitational wave astronomy – has recently been born. We are beginning to see the goal. We stand on a foundation of accumulating knowledge and experience. And we are provided with ever-more-capable computational and observational instruments as we endeavor to reach that goal. These are exciting times to be a core collapse supernova modeler, and I look forward to sharing my excitement with you.