NUCLEOSYNTHESIS!
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There are two main processes by which nuclei that
fill out the range of β-stable nuclei are formed. The most
important process (since it has to happen first) is the so-called
r process, an explosion which subjects the nuclei found in
old, large stars to a flood of neutrons. This spray of neutrons
instantly populates the neutron-drip line and then almost
immediately these highly unstable nuclei decay down into the
valley of β stability. The other important process is the
so-called s process, which occurs over long periods of
time in intermediate-mass stars, mainly in the so-called
Asymptotic giant branch (AGB). This slow neutron capture process
runs right down the valley of β stability; the nuclei formed are
close to the valley and decay into it before they have any chance
of capturing another neutron. The r process is also vital in
more exotic events, such as neutron-star collisions. An
imperfect knowledge of masses of nuclei far off the line of
stability is needed in detailed calculations describing r
processes.
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There are other processes of far less
importance, such as proton capture. Observation of stars that
formed very early in the history of the universe show an
abundance of elements characteristic of the r process alone.
While the types of supernovae that explosively compress the core
are the standard suspects for r-process events, it is
increasingly likely that scenarios involving collisions of
neutron stars with other dense or denser objects play a
significant role.
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NUCLEOSYNTHESIS IN THE
EARLY UNIVERSE
Between about 10 to 350
seconds the entire universe had the right temperature and
density, and comparable numbers of protons and neutrons, to do
fusion in the ordinary nuclear way, which produced the
primordial amounts of 4He, deuterium and 3He.
Very, very tiny amounts of 7Be and 7Li
were also generated. The incredibly rapid drop in density and
temperature, and the fact that the half-life of a free neutron
is only 10 minutes, made this era of Big Bang nucleosynthesis
very short and very limited. The astonishing fact that
interstellar gas which has never been inside a star is
already only 75% protons and the remaining 25% helium made it
very clear to researchers in the late 1950s that the
entire universe must at one time have been hot and dense
enough to fuse protons and neutrons into helium, and
that this would have had to occur over a time frame fairly
short compared to the free neutron lifetime.
Pioneers of Cosmology
Cosmology
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