We can’t leave the nucleosynthesis period without having a good look at it. So let’s see a bit more about helium and its consequences.
Helium; how was it made?We know that helium is the second element on the periodic table. The first one is hydrogen with one proton and one electron. In fact, we can say that the decay of a neutron (after 15 minutes) produces hydrogen because the ejected electron stays captured around the new proton. Beside this event, if there were free protons at the beginning of the nucleosynthesis, a proton could capture an electron and then become a hydrogen atom much before 15 minutes.
So what about helium, that second produced element?
Firstly, we have to know that there’s an “intermediate production phase” between the hydrogen atom and the production of helium.
That phase is the capture of one or two neutrons by a hydrogen atom, which transforms it in deuterium or tritium. So the “evolution” of the first atoms was the result of merging of particles available at the time: Protons, neutrons and electrons.
The following drawing represents this “intermediate production phase” involved:

So these were the ingredients available to make the first helium atom. Note that the “real” first atom to exist is the deuterium; because it is the first atom to join all available ingredients in the universe of that time. Hydrogen didn’t integrate a neutron.
As we can see, both the fusion of two deuterium and the fusion of a tritium with hydrogen will produce a helium-4 atom:

An atom of helium-4 is made of 2 protons + 2 neutrons + 2 electrons.
So, no wonder that the most numerous kind of helium in the universe is the helium-4.
Afterward, producing a stable heavier atom needs to integrate a deuterium to a lighter atom; which, in the end, gives the total periodic table.
The two most frequent helium atoms are helium-4 and helium-3
With two protons, two neutrons, and two electrons, helium-4 has an overall spin of zero,
making it a boson; as for, with one fewer neutron, helium-3 has an overall spin of one half, making it a fermion.
We also know that after the hydrogen was “created”, having one proton + one electron, the fusion of two hydrogen element was impossible since positive proton particles repulse themselves. So a neutron had first to be introduced “between” two positive protons in order to stabilize the nucleus. Introducing a neutron produced the deuterium composed of 1 proton + 1 neutron + 1 electron.
Now by fusing two deuterium atoms, we clearly get a helium-4 atom (the same condition is answered by the fusion of hydrogen and tritium). And since these elements appeared before heavier elements, it’s easy to understand that these were the processes producing helium-4.
What do we know about helium-4?Helium-4 is described as a non-radioactive isotope of the element helium. But I don’t agree at all that helium-4 is an isotope since an isotope is an atom that doesn’t have the same amount of protons and neutrons; which is not the case for a helium-4 atom.
Most helium-4 in the Sun and in the universe is thought to have been produced by the Big Bang, and is referred to as "primordial helium".
Helium-4 makes up about one quarter of the ordinary matter in the universe with almost all of the rest being hydrogen.
The total spin of the helium-4 nucleus is an integer (zero), and therefore it is a boson. Which destroys the concept of bosons being “force/interaction carriers” because helium -4 is not a “force carrier” and the universe doesn't make "concessions" as easily as people do.
The nucleus of the helium-4 atom is identical to an alpha particle projected in an alpha decay process.
The stability and low energy of the electron cloud of helium is responsible for helium's chemical inertness (the most extreme of all the elements), and also the lack of interaction of helium atoms with each other. The reason is that their valence orbital is “full”; meaning that it already contains two electrons and cannot contain more.
Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis. Note that helium-3 is also a stable element even if it contains more protons than neutrons; but its valence orbital can contain one more electron, which permits easy chemical bounding producing compounds (molecules).
The end of formation of helium-4 has left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4), with nearly all the neutrons in the universe trapped in helium-4.
At low temperatures (about 2.17 K), helium-4 undergoes a phase transition, a fraction of it becoming a superfluid. Such a mechanism is not available for helium-3 atoms, which are fermions.
All heavier elements had to be produced afterward in stars
which were hot enough (surprising note since temperature was higher when lighter elements were produced in early universe) to fuse not just hydrogen (for this produces only more helium), but to fuse helium itself. Which really means that since the temperature was lower than when helium was produced, there has to be another reason than “hot temperature” for fusing heavier elements. We will have to check this further, one day.
Helium-4 makes up about 23% of the universe's ordinary matter. In fact, nearly all the ordinary matter that isn't hydrogen.
Although there are nine known isotopes of helium, only helium-3 and helium-4 are stable.
The process of alpha decay produces alpha particles which consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus.
Alpha particles, like helium-4 nuclei, have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles generally have a kinetic energy of about 5 MeV, and a velocity in the vicinity of 5% the speed of light.
The helium-4 nuclei (alpha particle) that form 9% of cosmic rays are also usually of much higher energy than those produced by nuclear decay processes, and are thus capable of being highly penetrating. According to NASA, cosmic rays therefore come equally from all directions of the sky; 90% are hydrogen nuclei; which means “protons”. The remaining 1% are heavier atoms nuclei. The mechanisms of cosmic ray production continue to be debated (so we will debate it eventually).
The smallest nuclei that have to date been found to be capable of alpha emission are the lightest nuclides of tellurium (element 52), with mass numbers between 106 and 110 (with the exception of beryllium-8).
In contrast to beta decay, the fundamental interactions responsible for alpha decay
are a balance between the electromagnetic force and nuclear force (to me it’s gravitation). Alpha decay results from the Coulomb repulsion
between the alpha particle and the rest of the nucleus, which both have a positive electric charge, but which is kept in check by the nuclear force (gravitation). In classical physics, alpha particles do not have enough energy to escape the potential well from the strong force inside the nucleus (this well involves escaping the strong force to go up one side of the well, which is followed by the electromagnetic force causing a repulsive push-off down the other side).
However, the quantum tunnelling effect allows alphas to escape even though they do not have enough energy to overcome the nuclear force. This is allowed by the wave nature of matter, which allows the alpha particle to spend some of its time in a region so far from the nucleus that the potential from the repulsive electromagnetic force has fully compensated for
the attraction of the nuclear force (evidently:
gravitation). From this point, alpha particles can escape, and in quantum mechanics, after a certain time, they do so. (Ô dear God! Bless the magic of forces!!!)
The energy of alpha particles emitted varies, with higher energy alpha particles being emitted from larger nuclei, but most alpha particles have energies of between 3 and 7 MeV (mega-electron-volts), corresponding
to extremely long and extremely short half-lives of alpha-emitting nuclides, respectively.
This energy is a substantial amount of energy for a single particle, but their high mass means alpha particles have a lower speed (with a typical kinetic energy of 5 MeV; the speed is 15,000 km/s, which is 5% of the speed of light)
than any other common type of radiation (β particles, neutrons, etc.)
In 2011, was detected the antimatter partner of the helium-4 nucleus, also known as the anti-alpha.
But let’s come back to production of elements:
Nuclear fusion converts hydrogen into helium in all stars; but this is the only reaction that takes place in stars less massive than the Sun. In stars more massive than the Sun (but less massive than about 8 solar masses), further reactions that convert helium to carbon and oxygen, take place in successive stages of stellar evolution. In the very massive stars, the reaction chain continues to produce elements like silicon up to iron.
So where do come from, all the elements heavier than iron?
The answer is in supernovae. In a supernova explosion, “neutron capture” reactions take place (this is not fusion), leading to the formation of heavy elements. And since t
his is not fusion, tell me
what else than "gravitation" can
capture a "neutral" neutron to join it to a nucleus?
To go into technical details, there are two processes of neutron capture, called rapid process (r-process) and slow process (s-process); these lead to formation of different elements.
Slow process means that it takes hundreds to thousands of years between successive neutron captures. In this situation, a seed nucleus will slowly capture neutrons, followed by beta decay transforming neutrons in protons, thus building up new heavier nuclei up to a certain critical point when it doesn’t work anymore.
The s-process, while an elegant theory of nucleosynthesis, cannot explain some basic features of element abundances, as actinides elements.
In the
rapid process, “neutron capture” is very swift. It happens within a time much shorter than the average beta-decay half-lives. The r-process is fast enough to break past the region of alpha-instability beyond 208Pb. The stable actinides may be produced directly from a neutron-rich precursor, or from alpha-decay of even heavier elements.
As an example, let’s see where uranium comes from.
Uranium was produced in one or more supernovae. The main process concerned was the rapid capture of neutrons on seed nuclei at rates greater than disintegration through radioactivity. The neutron fluxes required are believed to occur during the catastrophically explosive stellar events called supernovae. Gravitational compression of iron and sudden collapse in the center of a massive star, triggers the explosive ejection of much of the star into space, together with a flood of neutrons. Once these neutrons are captured, they take about 15 minutes to transform into protons and produce new heavier elements.
We can calculate the abundances of U-235 and U-238 at the time the Earth was formed. Knowing further that the production ratio of U-235 to U-238 in a supernova is about 1.65, we can calculate that if all of the uranium now in the solar system were made in a single supernova, this event must have occurred some 6.5 billion years ago. This 'single stage' is, however, an oversimplification. In fact, multiple supernovae from over 6 billion to about 200 million years ago were involved.
So these are all the processes that produced the natural elements of the periodic table. Once hydrogen and helium (plus a small quantity of few heavier elements) had appeared, the universe had to occupy itself in “creating” stars with what was available, before producing heavier atoms.