Latest  three-dimensional computer simulations are closing in on the solution of  an decades-old problem: how do massive stars die in gigantic supernova  explosions? Since the mid-1960s, astronomers thought that neutrinos,  elementary particles that are radiated in huge numbers by the newly  formed neutron star, could be the ones to energize the blast wave that  disrupts the star. However, only now the power of modern supercomputers  has made it possible to actually demonstrate the viability of this  neutrino-driven mechanism. 

  Supernovae are among  the brightest and most violent explosive events in the Universe. They  are not only the birth sites of neutron stars and black holes; they also  produce and disseminate heavy chemical elements up to iron and possibly  even nuclear species heavier than iron, which could be forged during  the explosion. Understanding the explosion mechanism of massive stars is  therefore of fundamental importance to better define the role of  supernovae in the cosmic cycle of matter. 

  Stars with more than  about eight times the mass of our sun evolve by "burning" nuclear fuel  to successively heavier chemical elements, thus converting hydrogen to  helium, carbon, oxygen, sulfur and silicon, until a dense, degenerate  core mostly made of iron builds up in the center. At this stage no  further energy gain by nuclear fusion is possible, because neutrons and  protons in iron nuclei possess the highest nuclear binding energies. 

  For more than 30  years there had been hope that ever more improved computer models would  finally be able to demonstrate that this "core-bounce shock" is able to  trigger a successful supernova explosion by reversing the infall of the  outer stellar layers. However, the opposite turned out to be the case:  Better models showed that the energy losses of the bounce shock are so  dramatic that its outward propagation comes to a halt still well inside  of the iron core. It became clear that something has to help reviving  the stalled shock. Some mechanism has to supply the shock with fresh  energy so that it reaccelerates and expels the stellar mantle and  envelope in the supernova blast. 

Fig. 1:  Sequence of volume-rendering images that show the violent non-spherical  mass motions that drive the evolution of the collapsing 20 solar-mass  star towards the onset of a neutrino-powered explosion. The whitish  central sphere indicates the newly formed neutron star, the enveloping  bluish surface marks the supernova shock. (Visualization: Elena  Erastova and Markus Rampp, Max Planck Computing and Data Facility  (MPCDF)); copyright (2015) by American Astronomical Society).   

Fig. 2.—  Cross-sectional entropy distributions (in kB per nucleon) for the 3D  models without (3Dn; upper row) and with strangeness contributions (3Ds;  bottom). The bottom row clearly shows stronger SASI  activity in model 3Ds (180 ms, 250 ms), whose traces are still imprinted  on the ejecta geometry after the onset of the explosion (530 ms; note the di erent scale). 

     Already in the 1960's it was speculated (in a seminal publication by  Stirling Colgate and Richard White) that neutrinos might be involved.  Myriads of these high-energy elementary particles are radiated by the  extremely hot, newly formed neutron star. If less than one percent of  them gets absorbed in the matter behind the stalled shock, a healthy  supernova explosion will be the consequence (see MPA research highlight 2001).  This was shown, in principle, already in the mid 1980's with first  sufficiently detailed numerical simulations by Jim Wilson and  interpretative work by Wilson and Hans Bethe. 

However, many aspects  of the involved physics were still too crude and too approximate to be  realistic. In particular, with the observation of Supernova 1987A it  became clear that stellar explosions are highly asymmetric phenomena and  non-spherical plasma flows must play an important role already at the  very beginning of the explosion. Early multi-dimensional computer models  ---mostly still in two dimensions, i.e., assuming rotational symmetry  around a chosen axis for reasons of computational efficiency--- indeed  showed that convection and non-radial mass motions provide crucial  support to the neutrino-heating mechanism and enhance the energy  deposition by neutrinos. Thus explosions could be obtained although  spherical models did not find shock revival and did not lead to  explosions (see MPA press release 2009). 

   Nature, however,  has three spatial dimensions and therefore these early successful models  were critisized to be unrealistic and not reliable. In fact, not only  the assumed axial symmetry is artificial, also the physics of turbulent  flows differs in two dimensions compared to the 3D case. 

  Only very recently  the increasing power of modern supercomputers has now made it possible  to perform supernova simulations without artificial constraints of the  symmetry. A new level of realism in such simulations is thus reached and  brings us closer to the solution of a 50 year old problem. 

  The stellar collapse  group at the Max Planck Institute for Astrophysics (MPA) plays a  leading role in the worldwide race for such models. With all relevant  physics included, in particular using a highly complex treatment of  neutrino transport and interactions, such computations are at the very  limit of what is currently feasible on the biggest available computers.  The model simulations are performed on 16,000 cores (equivalent to a  similar number of the fastest existing PCs) in parallel, which is the  largest share of SuperMUC at the Leibniz-Rechenzentrum (LRZ) in Garching  (Fig. 1) and of MareNostrum at the Barcelona Supercomputing Center  (BSC; Fig. 2) that the MPA team is granted access to. Nevertheless, one  full supernova run, conducted over an evolution time of typically half a  second, consumes up to 50 million core hours and takes more than 1/2  year of project time to be completed. 

     For a little more insight into the project see this：http://arxiv.org/pdf/1504.07631v2.pdf