Nuclear Astrophysics

The light emitted by the stars is a direct result of the nuclear reactions that naturally occur when massive clouds of gas collapse and heat, whether it be the constant shine of our Sun shown above, or the newly observed flares of gamma-ray bursters. The abundances of the elements here on Earth are the result of nuclear reactions that have occurred in previous generations of stars. The understanding of nucleosynthesis processes and of the energy generation in astrophysical objects is the subject of Nuclear Astrophysics studied experimentally by the Edinburgh Nuclear Physics Group.

Explosive Hydrogen Burning

Accretion from a hydrogen rich envelope of one star onto the electron degenerate surface of a compact evolved companion can generate the conditions required for a thermonuclear runaway. In a binary system such a case might be when material from the outer layers of a red giant is transferred on to the surface of a white dwarf. Such an event is what we observe as a classical nova, and understanding the sequence of nuclear reactions that occur is the key to understanding the observed light curves and elemental abundances in the ejecta. Similarly, X-ray bursters can be interpreted as accretion on to a neutron star. In this case the deeper gravitational potential wells lead to hotter temperatures and higher pressures, and consequently the nuclear reaction pathways follow faster, more extreme routes. The rate of key nuclear reactions in explosive nucleosynthesis can be directly determined using radioactive beams.

Measuring Explosive Reactions here on Earth

Explosive reactions involve radioactive nuclear species. A major advance in the last decade has been the development of accelerators capable of producing intense beams of radioactive isotopes. This is the only direct way to measure these key reactions that control explosive astrophysical events. To the left is shown the pioneering two cyclotron accelerator complex at Louvain la Neuve, Belgium.

Hot Cycling in Stars

To the right is shown one of the hot CNO cycles that ignites on the surface of white dwarfs in novae explosions. The rate of energy generation in such processes critically depends on reactions involving radioactive nuclear species.