Stellar Burning

Fig. 1: Star formation in the Large Magellanic Cloud (Hubble Heritage)

All naturally occurring chemical elements - apart from the primordial H, He, and Li abundances produced in the Big Bang - were and still are synthesised in stars. Soon after the formation of a star by gravitational collapse of an interstellar dust cloud, nuclear fusion processes are activated in the core when the temperatures become high enough. This leads to energy generation and the resulting radiation pressure stabilizes the star. These nuclear reactions are also responsible for the formation of light elements up to Fe (A<56).

In the first nuclear burning phase hydrogen is converted to helium. Depending on the central temperature, this happens, either via the pp-chain or by the CNO-cycle.

The pp-chain

Fig. 1: Reactions of the pp-chain.

In the pp-chain four protons are converted to helium. The reaction path and the probability of the various branches are summarized in Fig. 2. The reactions involved in the pp-chain are not only responsible for the energy generation but also for the production of solar neutrinos. Previously, it was discussed whether inaccurate reaction rates could solve the neutrino puzzle (detection of fewer solar neutrinos than expected). Nowadays, after the discovery of neutrino oscillations, the detection of neutrinos can be used to test the solar model, provided that the reaction rates are known with sufficient accuracy. At GSI we have investigated the 7Be(p,γ)8B reaction via the Coulomb dissociation method. This reaction is especially important since it leads to the formation of 8B and is, therefore, responsible for the production of high energy neutrinos, which can be measured in neutrino detectors such as SuperK or SNO.

The CNO-cycle

Fig. 3: The closed CNO-cycle (black lines) which is responsible for the energy production in stars more massive than the sun. At higher temperatures a break-out is possible and heavier nuclei can be produced. (Credit: The owner of the image)

In more massive stars, where the central temperatures are higher, the energy generation is provided by the CNO-cycle. Again, as in the pp-chain, four protons are converted to helium, with carbon, nitrogen, and oxygen acting as catalysts. When hydrogen is burning in the center of stars, the CNO-cycle remains closed, but in hotter environments, e.g. in Novae or X-ray bursts, a break-out of the reaction chain towards heavier nuclei becomes possible (Fig. 3). There are various ways for break-out from the cycle, all involving fusion reactions of radioactive nuclei, which are difficult to study. During the rp process, which is associated with the high temperatures in X-ray bursts, the break-out towards heavier nuclei occurs via the 15O(α,γ)19Ne reaction and subsequent proton captures and β+ decays. An alternative and competing reaction path could be the 2p-capture on 15O. It is planned to measure the 15O(2p,γ)17Ne reaction via the Coulomb dissociation method.