In the Primordial Nucleosynthesis or the Big Bang Nucleosynthesis (BBN), it is a period when nucleus first formed around hadron particle or particles (hadrons are protons and neutrons), but during BBN, the universe was still too hot for electrons to bond with these earliest atoms, hence these atom were ionized.
So this type of nucleosynthesis, is where ionized hydrogen nuclei, deuterium nuclei, helium nuclei and lithium nuclei were all formed when the universe was still very young - atoms without electrons.
It was only when the universe cooled down considerably, during the Recombination Epoch (started around 379,000 years after the Big Bang that electrons for the first time, bonded with these ionized atoms, that they became neutral stable atoms.
This bonding of electrons with nuclei, also cause the universe to be transparent for the first time, and photons that decoupled from nuclei, photons were able to travel to freely through space. And the world know these photons, the earliest detectable EM radiations, which astrophysicists called Cosmic Microwave Background Radiation (CMBR).
This CMBR was first detected and measured in 1964, by radio antenna built by Arno Penzias and Robert Wilson, but were first predicted by Ralph Alpher and Robert Herman in 1948 (Alpher also co-predicted BB Nucleosynthesis with George Gamow in 1948).
The Big Bang Nucleosynthesis is a different type of nucleosynthesis than other types -
- Stellar Nucleosynthesis
- Supernova Nucleosynthesis
Both of these types of nucleosynthesis related to how a star will fuse light atoms (hydrogen atoms) to form into heavier atoms.
There are many types of Stellar Nucleosynthesis, but two of these are common:
- proton-proton chain reaction nucleosynthesis, and
- CNO cycle nucleosynthesis
The proton-proton is where it take about 6 hydrogen nuclei to fuse into one helium nuclei.
The “CNO” stand for carbon, nitrogen and oxygen, where more massive and hotter stars hydrogen nuclei will fuse each of these heavier atomic nuclei in succession, and recycle again. This type of nucleosynthesis is more complex to understand, so I would suggest that you look up CNO Cycle, to better understand this type of Stellar Nucleosynthesis.
The hydrogen into helium nucleosynthesis (proton-proton chain reaction) occur in our star’s core - the Sun.
It doesn’t matter which of these two or other Stellar Nucleosynthesis occurred, but it take both gravity and pressure to cause thermonuclear fusion.
We even have the technology of producing hydrogen-into-helium fusion, through thermonuclear explosion, eg the hydrogen bomb.
Supernova Nucleosynthesis occurred when stars explode in supernova event, which can create atoms heavier than helium, carbon, nitrogen and oxygen atoms, from everything up to iron.
It is these Supernova Nucleosynthesis that produce all the materials needed to create planets, planetesimals, asteroids, meteors, comets, etc.
There are high probability that were couple of supernova explosions near here that were responsible for leaving materials behind that allow our Solar System to form.