The process is called nucleosynthesis
Gravity took over and eventually these atoms were pulled together into massive clouds gas in the vastness of space. Once these clouds got large enough they were drawn together by gravity with enough force to actually cause the atomic nuclei to fuse together, in a process called . The result of this fusion process is that the two one-proton atoms have now formed a single two-proton atom. In other words, two hydrogen atoms have begun one single helium atom. The energy released during this process is what causes the sun (or any other star, for that matter) to burn.
Our Sun is currently burning, or fusing, hydrogen to helium.
In 1920, Arthur Eddington, on the basis of the precise measurements of atoms by F.W. Aston, was the first to suggest that stars obtained their energy from nuclear fusion of hydrogen to form helium. In 1928, George Gamow derived what is now called the Gamow factor, a quantum-mechanical formula that gave the probability of bringing two nuclei sufficiently close for the strong nuclear force to overcome the Coulomb barrier. The Gamow factor was used in the decade that followed by Atkinson and Houtermans and later by Gamow himself and Edward Teller to derive the rate at which nuclear reactions would proceed at the high temperatures believed to exist in stellar interiors.
The subsequent nucleosynthesis of the heavier elements requires the extreme temperatures and pressures found within and . These processes began as hydrogen and helium from the Big Bang collapsed into the first stars at 500 million years. Star formation has occurred continuously in galaxies since that time. Among the elements found naturally on Earth (the so-called ), those heavier than boron were created by and by . They range in from Z=6 () to Z=94 (). Synthesis of these elements occurred either by (including both and multiple neutron capture) or to a lesser degree by followed by .
Helium is made from hydrogen by nuclear fusion in the core of stars.
The formation of nuclides present in the universe by various nuclear reactions. Theories of the origin of the elements involve synthesis with charged and neutral elementary particles (neutrons, protons, neutrinos, photons) and other nuclear building blocks of matter, such as alpha particles. The theory of nucleosynthesis comprises a dozen distinct processes, including big bang nucleosynthesis, cosmic-ray spallation in the interstellar medium, and static or explosive burning in various stellar environments (hydrogen-, helium-, carbon-, oxygen-, and silicon-burning, and the, -, -, -, γ-, and ν-processes). Acceptable theories must lead to an understanding of the cosmic abundances observed in the solar system, stars, and the interstellar medium. The curve of these abundances is shown in . Hydrogen and helium constitute about 98% of the total element content by mass and more than 99.8% by number of atoms. There is a rapid decrease with increasing nuclear mass number , although the abundance of iron-group elements like iron and nickel are remarkably large. The processes of nucleosynthesis described in this article attempt to explain the observed pattern.
Why did the Universe start off with Hydrogen, Helium, …
When the universe cools off a bit more, these high-energy photons become rare enough that it becomes possible for deuterium to survive. At this point, a race begins. These deuterium nuclei can keep sticking to more and more protons and neutrons, forming nuclei of helium-3, helium-4, lithium, and beryllium. This process of element-formation is called "nucleosynthesis". The denser protons and neutrons are at this time, the more of these light elements will be formed. As the universe expands, however, the density of protons and neutrons decreases and the process slows down.
Big Bang nucleosynthesis - ScienceDaily
within exploding stars by fusing carbon and oxygen is responsible for the abundances of elements between (atomic number 12) and (atomic number 28). Supernova nucleosynthesis is also thought to be responsible for the creation of rarer elements heavier than iron and nickel, in the last few seconds of a event. The synthesis of these heavier elements absorbs energy () as they are created, from the energy produced during the supernova explosion. Some of those elements are created from the absorption of multiple neutrons (the ) in the period of a few seconds during the explosion. The elements formed in supernovas include the heaviest elements known, such as the long-lived elements uranium and thorium.