How would you describe nuclear astrophysics?
Nuclear Astrophysics: Element Synthesis in the Universe
In the Big Bang, only the light elements hydrogen and helium and, in small amounts, lithium, beryllium and boron were produced. All heavier substances were only produced by stars afterwards. Nuclear astrophysics deals with the responsible nuclear physical processes.
Nuclear physical processes play a crucial role in the evolution of the universe. Nuclear structure effects and the dynamics of nuclear reactions are reflected directly in the various stages of development of stars, in the course of star explosions and in the frequency distribution of the elements in the universe. Nuclear astrophysics today addresses a number of key issues. This includes the formation of the chemical elements, the physics of stellar explosions, the nuclear and mixing processes inside the stars as well as the understanding of compact objects such as white dwarfs and neutron stars. Thermonuclear explosions on the surfaces of these objects, which are noticeable as novae or X-ray bursts, are also of central importance for modern astrophysics. In Germany, several experimental and theoretical groups are working on these questions.
Star evolution and supernovae
Stars like our sun generate energy by fusing hydrogen to helium. In about six billion years our daytime star will expand into a red giant and burn helium to carbon and oxygen. In order to model the development of stars in their various development phases, it is necessary to know the cross-sections of the nuclear reactions at the energies relevant in the star's interior. In this area, it has been possible in recent years to use various methods to determine nuclear reaction cross-sections for the relevant energies. In addition, microscopic models were developed to describe the astrophysically important nuclear reactions.
After burning hydrogen and helium, massive stars go through a sequence of further burning phases in which they burn carbon, neon, oxygen and finally silicon. This fusion stops with the production of elements in the mass range of iron. Then the star center collapses and a type II supernova is created. The star sheds its outer shell and hurls the nuclides that have been produced during the various hydrostatic combustion phases into space. The dynamics of the collapse are largely determined by the electron capture rate at nuclei. Their measurement and theoretical determination is one of the primary tasks of nuclear astrophysics today.
Nucleosynthesis in the s, r and p process
All elements that are heavier than iron are created by the successive capture of neutrons and subsequent beta decays. A distinction is made between two reaction paths: s- and r-process. The s-process (Slow Neutron Capture Process) occurs in environments with relatively low neutron densities, such as red giants. There the beta half-lives are shorter than the neutron capture times. In principle, the observation of the relative abundances of elements that arise here allows conclusions to be drawn about the properties of the astrophysical environment - such as temperature, neutron density or convective mixing. Such analyzes often fail because of inadequate experimental cross sections for neutron capture or because of ignorance of the half-lives of excited nuclear states. Only recently has it been possible to measure many of the important neutron capture rates with sufficient accuracy.
The second pathway, the r-process (Rapid Neutron Capture Process), occurs in areas with extreme neutron density, such as in type II supernovae. The reaction path runs through very neutron-rich nuclei, many of which physical properties are not known experimentally. Lately it has at least been possible to measure the half-life of some important r-process cores in the mass region around 130 and to determine the masses of relevant cores for the first time.
There is a third process path, the p-process. The proton-rich nuclei between selenium and mercury are formed in it. This process occurs in Type II supernovae when high-energy gamma photons break up heavy nuclei from the s and r processes. The process network is extremely complex and includes around 2000 cores and more than 20,000 reactions. Because of this large number, the necessary reaction rates must be estimated theoretically, for example in the context of a static model. It is therefore of the utmost importance to put these reaction rates on a solid footing with experimental data on key reactions. With the help of accelerators it has already been possible to measure important cross-sections and properties of mostly stable cores.
Neutron stars and white dwarfs
Nuclear astrophysics is also concerned with the structure of neutron stars. These are compact remnants of stars with a radius of about ten kilometers, in which around one solar mass is united. The structure of these exotic celestial bodies is still fairly unknown. This applies in particular to the central area. A novel form of quark matter is conceivable there. Theoretical work on neutron stars and dense matter is carried out by different groups.
Another current area is the explosive burning of hydrogen on the surface of compact objects. It is triggered by the mass flow of a nearby companion onto the compact star. If this is a white dwarf, it repeatedly sheds its outer shell (nova); a neutron star causes repeated X-ray bursts (X-Ray Burst). The dynamics of these explosive events are partly self-reinforcing nuclear reactions and reaction chains (Thermonuclear Run Away) certainly. In order to model these, it is necessary to know the masses, half-lives and, in particular, reaction cross-sections of proton-rich nuclei up to the limit of stability.
In the future, the FAIR facility at the GSI in Darmstadt will make it possible to experiment with many astrophysically important, short-lived nuclei and to determine their properties. These globally unique opportunities will also help to develop and test core models. They are necessary to predict how core processes will take place under the extreme conditions often encountered in astrophysical objects. The continuation of the successful work at other research institutions is also essential. Key hydrostatic burning reactions need to be measured with greater accuracy. This requires low-energy accelerators with high beam intensity and quality. Precision experiments with electrons and photons at the SDalinac at the TU Darmstadt can indirectly contribute to better understanding the reactions of the p-process. These developments must be accompanied by theoretical work.
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