September 08, 2011
Most heavy chemical elements are formed in nuclear fusion reactions in stars. Also in the centre of our Sun, hydrogen is “burned” to create helium, thereby releasing energy. Heavier elements are then produced from helium if the star is more massive than our Sun. This process, however, only works up to iron; further fusion reactions do not yield any net energy gain. Therefore heavier elements cannot be produced in this fashion. Instead, they can be assembled when neutrons are captured onto “seed”-nuclei, which then radioactively decay.
This involves two main processes: the slow neutron capture (s-process), which takes place at low neutron densities inside stars during their late evolution stages, and the rapid neutron capture (r-process), which needs very high neutron densities. Physicists know that the r-process is responsible for producing a large fraction of the elements much heavier than iron (those with nuclear mass numbers A > 80), including platinum, gold, thorium, and plutonium. However, the question of which astrophysical objects can accommodate for this r-process remains to be answered.
“The source of about half of the heaviest elements in the Universe has been a mystery for a long time,“ says Hans-Thomas Janka, senior scientist at the Max Planck Institute for Astrophysics and within the Excellence Cluster Universe. ”The most popular idea has been, and may still be, that they originate from supernova explosions that end the lives of massive stars. But newer models do not support this idea. “
Violent mergers of neutron stars in binary systems (see background information on neutron stars) offer an alternative scenario, when the two stars collide after millions of years of spiralling towards each other. For the first time, scientists at the Max Planck Institute for Astrophysics and the Free University of Brussels have now simulated all stages of the processes occurring in such mergers by detailed computer models. This includes both the evolution of the neutron star matter during the relativistic cosmic crashes and the formation of chemical elements in the tiny fraction of the whole matter that gets ejected during such events, involving the nuclear reactions of more than 5000 atomic nuclei (chemical elements and their isotopes (see background information on isotopes)).