We all know that
matter is made of atoms, but the question is where did these atoms come from
and how and when were they formed?
At first glance, the
formation of these atoms does not seem so difficult. You only need a few
neutrons and protons for the nucleus, then you have electrons around it so that
they are all electrically neutral. It seems very simple but it is not so.
The electron is the
simplest part. When you get a highly charged nucleus of an atom, it will
attract electrons and the electrons will automatically fall into shells around
the nucleus. But making the nucleus of an atom is not so easy. The problem with
this whole process is that all the protons have a positive charge
and they repel each other. Now if you bring them close together, the strong
nuclear force will pull them together and if we add enough neutrons to it, then
the nucleus will be formed. But to bring the protons very close together we
would need very high temperatures of about a million degrees Kelvin (10,000,000
degrees Kelvin), the same temperatures that existed at the beginning of the universe,
i.e., a few hours after the Big
Bang.
However, at that time
there was a very high density of matter everywhere in the universe.
It was a fluid of tiny particles of atoms called plasma, which had no
structure. There is no atomic nucleus in this juice, it is only a compound of
the constituents of the nucleus. When that plasma expands and cools, some of
these particles manage to stick together. In this way, the first atomic nucleus
was formed, which then controlled the electrons to form atoms. From this, we
found hydrogen, helium, and some other chemical elements along with their
isotopes whose atomic number was up to four.
This process of
making nuclear nuclei is called nucleosynthesis and the process of nuclear
chemical synthesis that happened a few minutes after the big
bang is called big bang nucleosynthesis. But after the Big
Bang, the diffusion of plasma happened so fast that only light nuclear
nuclei could be formed in this process. It took millions of years for
the formation of heavy nuclear nuclei. During this period, the universe
continued to expand, but the light nuclei continued to gather under the pull of
gravity and the early stars were born. As a result of the gravitational pull on
these stars, the temperature once again increased. Eventually, the temperature
increased to such an extent that the nuclear nuclei were fused, resulting in
the formation of large nuclear nuclei.
This nuclear fusion
produces energy and causes stars to be hot and bright. The process of nuclear
fusion in stars continues up to atomic number 26, after which it stops. The
reason for this is that the element with atomic number 26 is iron and it is the
strongest of the chemical elements. Its binding energy is the highest, so if
you combine smaller nuclei, you will continue to gain energy until you get to
the iron element, after which the nuclei have to give the energy to connect
them. So, from nuclear fusion inside stars, we get elements with the atomic
number of irons. But where do the heavy elements after the atomic number of
irons come from? They are obtained from neutron capture.
Some nuclear
reactions produce free neutrons, and since neutrons have no charge,
they enter a nuclear nucleus in a much shorter time
than protons. And after entering the nuclear nucleus, they decay into protons
and electrons and electrons into anti-protons.
If they do, then they produce heavy elements. Most of the nuclei produced in
this process are unstable isotopes, but they continue to break apart until they
achieve stable isotopes.
The neutron capture
process in stars happens randomly. Over time,
old stars produce elements heavier than iron, but eventually, they run out of
nuclear fuel and die. Most of them are destroyed and burst. These supernovae
split nuclear nuclei in galaxies or eject nuclear nuclei from galaxies.
Some of the light
elements that are around us today are created by cosmic
rays colliding with these heavy elements. However, in old stars, the process of
neutron capture is slow and the stars last much longer. This process does not
produce the same number of heavy elements present on our planet. To do this,
rapid neutron capture is required. For this, we need a very high-pressure
environment, in which many neutrons collide with a small nuclear nucleus. In
this process, some neutrons annihilate after entering the nuclear nucleus and a
proton is left behind, which gives rise to heavier elements.
For a long time,
astrophysicists believed that supernovae undergo a fast neutron capture
process, but this idea has been abandoned. Their calculations showed that
supernovae do not produce large numbers of neutrons very quickly. This theory
was not consistent with the observations. For example, if the heavy elements
observed by astrophysicists in some of the smaller galaxies known as dwarf
galaxies originated from supernovae, they would have required many supernovae
to tear the smaller galaxies apart. They could not be observed in any one place.
So, astrophysicists
believe that heavy elements did not originate in supernovae but as a result of
neutron star fusion. Neutron stars are the remnants of supernovae. From its
name, it can be guessed that it is composed of neutrons. They don't have
nuclear cores, they're just giant balls of dense nuclear plasma.
If they collide, the
nuclei will be ejected and conditions will be created that are favorable for
the rapid neutron capture process. It could have created all the heavy elements
that exist on Earth. Recent investigations into the light
emitted by neutron star mergers support this hypothesis as there is evidence
for the presence of some heavy elements in the light.
If you look at the periodic
table, you will notice that some of the heaviest elements are missing. This
is because they are unstable. Some of them turn into tiny nuclear nuclei in a
few thousand years and a few microseconds. The elements that formed the stars
have long since disappeared. We only know about their properties because they
have been created in laboratories by colliding tiny nuclei together at high
energies.
A question is
whether there are some such nuclear nuclei, which have not been discovered yet.
The simple answer is that it can.
It is a very old
hypothesis of nuclear physics that there are some heavy nuclear nuclei, which
contain a certain number of neutrons and protons, which have been in a stable
state for millions of years and we have not yet been able to create them.
Nuclear physicists call it the island of stability.
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