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
