Where do heavy elements come from?

Where do heavy elements come from?

Remnants of Supernova Tycho courtesy of mpia.de

Stars are powered by nuclear fusion – any atom heavier than this is formed by this process. When a star starts running low on Hydrogen, it will begin 'burning' the Helium too, forming even heavier elements. Unlike Hydrogen being fused to Helium, this is a process of diminishing returns. Each new elemental fusion yields less energy than the previous. At a certain point, depending on the stars mass, this process grinds to a halt, and the star can go no further. If this star is small, like our sun, it will eventually burn out, as a red giant and eventually a white dwarf. When this occurs the star will retain the heavier elements it formed during its lifetime. However, if it is a bit bigger, though, its undergo core collapse when nuclear fusion suddenly becomes unable to sustain the core against its own gravity. The collapse may cause violent expulsion of the outer layers of the star resulting in a supernova - tremendous energy is suddenly released in the form of neutrinos and electromagnetic radiation. Such is the explosive energy unleashed during a Supernova event that even the poorest grade of fuel, the middle-weight elements like Iron, aren't safe from atomic infusion. Brief though the blast is, compared with the life of the star, a Supernova packs enough energy to force the creation of Uranium, and even more dense, and more unstable elements. The blast is also powerful enough to eject these ultra-dense atoms out into the surrounding cosmos.

Constituent elements of Supernova Tycho courtesy of sci.esa.int

Stars & Nuclear Fusion

Supernovae are a key source of elements heavier than oxygen. These elements are produced by nuclear fusion (for iron-56 and lighter elements), and by nucleosynthesis during the supernova explosion for elements heavier than iron. 

Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to energy which is released. Fusion is the process that powers active stars. The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy.

A substantial energy barrier of electrostatic forces must be overcome before fusion can occur. At large distances two naked nuclei repel one another because of the repulsive electrostatic force between their positively charged protons. If two nuclei can be brought close enough together, however, the electrostatic repulsion can be overcome by the strong nuclear force, which is stronger at close distances. This is one of the most important fusion process in nature. The net result is the fusion of four protons into one alpha particle, with the release of two positrons, two neutrinos (which changes two of the protons into neutrons), and energy, but several individual reactions are involved, depending on the mass of the star. For stars the size of the sun or smaller, the proton-proton chain dominates.

Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the Sun. In the CNO cycle, four protons fuse, using carbon, nitrogen and o