Nuclear Astrophysics

Nuclear astrophysics is the study of nuclear processes in stars. It aims to understand how energy is generated in stars and the origin of the chemical elements. When the Universe formed, the big bang only created isotopes of the light elements hydrogen, Z=1, and helium, Z=2 (with traces of other light elements like lithium, Z=3). One of the goals of nuclear astrophysics is to show how all the other elements, up to uranium, Z=92, and beyond, were created in stars.

The birth, life and death of stars…

Stars are born in huge clouds of gas and dust, which are mostly made up of hydrogen and helium. In regions of higher density the gas and dust begins to gravitationally attract. If enough material is attracted this can result in an infall of matter, releasing gravitational energy, and raising the temperature of the gas and dust. As more and more material from the surrounding cloud is attracted the temperature and density at the centre becomes so high that nuclear reactions begin to occur and a new star is born. If the energy generated by the nuclear reactions balances the gravitational attraction of all the matter, the star becomes stable.

The Orion Nebula is a stellar nursery; observations indicate there are approximately 700 stars in various stages of formation within the nebula.
(Credit: NASA, ESA)

For most of a stars life it will generate energy by hydrogen fusion to form helium. For stars like our Sun the dominant nuclear energy process is called the proton-proton chain. This is a sequence of nuclear reactions, which take two hydrogen nuclei (i.e. two protons) and produces helium-4. Stars like our Sun can generate energy this way for about 10 billion years. We estimate that the Sun has used about half the hydrogen in its centre and has almost reached middle age.

When the hydrogen at the centre of a star runs out gravity finally wins and the star begins to collapse. This drives up the temperature and pressure in the centre of the star allowing helium fusion to occur. This requires more energy than hydrogen fusion as there are more positively charged protons in helium nuclei to repel each other. Helium fusion produces carbon, Z=6, and oxygen, Z=8, the elements essential for life as we know it. Helium fusion produces enough energy to stabilise the star again and it becomes a red giant.

How a star dies depends on how massive it is. For stars less than eight times the mass of our Sun, when the helium in the centre runs out, the star begins to collapse again. The temperatures and pressures cannot get high enough to fuse elements heavier than helium so the star becomes unstable, the outer layers blow away leaving a dense hot core behind called a white dwarf star.

The Crab Nebula, a supernova remnant. In the nebula's centre lies a pulsar: a neutron star that rotates about 30 times each second.
(Credit: NASA, ESA, J. Hester, A. Loll (ASU))

For stars with masses more than 8 times the mass of the Sun heavier elements can fuse in the centre and carbon, oxygen, neon, Z=10, and silicon, Z=14, fusion creates elements up to iron, Z=26. Unfortunately for the star, the fusion of iron does not produce energy so once the star has an iron core it begins to collapse again. The outer layers fall rapidly into the star and bounce back off the dense iron core in a powerful shockwave. The resulting explosion is called a supernova and leaves behind a neutron star or a black hole.

In the explosive environments found in a supernova much more dramatic nuclear reactions can occur to produce the heavy elements. One such process is called the r-process, which is a sequence of rapid neutron captures by nuclei like iron in the star. The resulting neutron-rich nuclei then beta decay to produce a new element. This process is thought to occur in supernova and to be responsible for the creation of about half the neutron-rich nuclei heavier than iron. The supernova explosion also blows these newly made elements out into space where they can get caught up in the next generations of stars, perhaps forming planetary systems and life forms.

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