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The Universe is now 1 minute old, and all the anti-matter has been
destroyed by annihilation with matter. The leftover matter is in the
form of electrons, protons and neutrons. As the temperature continues
to drop, protons and neutrons can undergo fusion to form heavier atomic
nuclei. This process is called nucleosynthesis.
- to build higher mass nuclei requires time, but the Universe is still expanding and cooling
- the abundance of various light elements will be dependent on the number of protons avaliable, i.e. the mass
desnity or
 baryons |
Its harder and harder to make nuclei with higher masses (because more protons mean
more positive charge and more electrostatic repulsion). So the most common
substance in the Universe is hydrogen (one proton), followed by helium,
lithium, beryllium and boron (the first elements on the periodic table). Isotopes (nuclei with extra
neutrons) are formed, such as deuterium and tritium, because the neutron
has no charge and is absorbed by the nuclei. However, these elements are
unstable and decay into free protons and neutrons.
The rate of nucleosynthesis is set by the density of nuclei per cc in the early
Universe, in other words, the cosmic density parameter, . |
- current estimates place baryons at 0.02
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Note that this above diagram refers to the density parameter, , of
baryons, which is close to 0.02. However, much of the Universe is in the form
of dark matter.
A key point is that the ratio of hydrogen to helium is extremely sensitive
to the density of matter in the Universe (the parameter that determines if
the Universe is open, flat or closed). The higher the density, the more
helium produced during the nucleosynthesis era. The current measurements
indicate that 75% of the mass of the Universe is in the form of hydrogen,
24% in the form of helium and the remaining 1% in the rest of the periodic
table (note that your body is made mostly of these `trace' elements).
Note that since helium is 4 times the mass of hydrogen, the number of
hydrogen atoms is 90% and the number of helium atoms is 9% of the total
number of atoms in the Universe. |
- stellar thermonuclear fusion produces elements 4 to 26, heavier elements require supernova explosions
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There are over 100 naturally occurring elements in the Universe and
classification makes up the periodic table. The very lightest elements
are made in the early Universe. The elements between boron and iron
(atomic number 26) are made in the cores of stars by thermonuclear
fusion, the power source for all stars.
The fusion process produces  energy, which keeps the temperature of a
stellar core high to keep the reaction rates high. The fusing of new
elements is balanced by the destruction of nuclei by high energy
gamma-rays. Gamma-rays in a stellar core are capable of disrupting
nuclei, emitting free protons and neutrons. If the reaction rates are
high, then a net flux of energy is produced.
Fusion of elements with atomic numbers (the number of protons) greater
than 26 uses up more energy than is produced by the reaction. Thus,
elements heavier than iron cannot be fuel sources in stars. And,
likewise, elements heavier than iron are not produced in stars, so what is
their origin?. |
- neutron capture during a SN produces the rest of the periodic table
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The construction of elements heavier than involves nucleosynthesis by
neutron capture. A nuclei can capture or fuse with a neutron because the
neutron is electrically neutral and, therefore, not repulsed like the
proton. In everyday life, free neutrons are rare because they have shorthalf-life's before they radioactively decay. Each
neutron capture produces an isotope, some are stable, some are
unstable. Unstable isotopes will decay by emitting a positron and a
neutrino to make a new element. |
- neutron capture is rapid sensitive, faster allows elements to form before they decay
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Neutron capture can happen by two methods, the s and r-processes, where
s and r stand for slow and rapid. The s-process happens in the inert
carbon core of a star, the slow capture of neutrons. The s-process
works as long as the decay time for unstable isotopes is longer than the
capture time. Up to the element bismuth (atomic number 83), the
s-process works, but above this point the more massive nuclei that can
be built from bismuth are unstable.
The second process, the r-process, is what is used to produce very
heavy, neutron rich nuclei. Here the capture of neutrons happens in
such a dense environment that the unstable isotopes do not have time
to decay. The high density of neutrons needed is only found during a
supernova explosion and, thus, all the heavy elements in the Universe
(radium, uranium and plutonium) are produced this way. The supernova
explosion also has the side benefit of propelling the new created
elements into space to seed molecular clouds which will form new
stars and solar systems. |
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