A hydrogen nucleus has one proton; and a helium nucleus has two protons and two neutrons. We can "fuse" four hydrogen nuclei into one helium nucleus, which means that we need to convert two of the four protons into two neutrons. The main difference between proton and neutron is that, proton carries charge +1, while neutron carry charge zero. So when one proton becomes a neutron, it needs to emit a positron which also carries charge +1. This process also has a bonus effect: in addition to positron, it also emits one neutrino, which carries no mass or charge, so it is harder to detect. Indeed, the neutrino was detected much later in history.

It is actually not so easy for this fusion process to happen, precisely because all the protons carry the same charge, and same charges will repel each other due to Coulomb repulsion. Hence this process only happens when the protons travel randomly with very high speed. With high speed the protons can collide with each other so strongly that their distance can become small enough to allow the nuclear force to dominate the Coulomb repulsion. Notice that the nuclear force ties the protons and neutrons together, and it is much stronger than the Coulomb repulsion at very small distance. This is precisely the fusion process that happens in the sun. This process creates huge nuclear energy which powers the entire solar system, and it is where the solar neutrinos come from.

The general process of combining atomic nuclei is called, straightforwardly, nuclear fusion.

Step 1: Two pairs of protons combine to form two deuterium atoms (+ released energy and 2 positrons). Tracking all of the particles involved: 4H+ -> 2 deuterium + 2 positrons ( Deuterium = hydrogen with a nucleus of one proton and one neutron instead of only a single proton; positrons are essentially positive electrons. )

Step 2: Each deuterium atom combines with another proton to form a helium-3 atom; 2 deuterium + 2H+ -> 2He (+ released energy).

Note that while each produced He has 2 protons in its nucleus (as they must, since the number of protons dictates the element), each has only 1 neutron in its nucleus instead of the 2 in the most stable isotope of He. Thus, these are called He-3 atoms (3 because the nucleus has 2 protons + 1 neutron = 3 nucleons).

Step 2 alternate: The 2 deuteriums combine to form a single, stable He-4 (He with 2 protons + 2 neutrons, the most stable isotope), and released energy. This alternate step seems to be less likely than the "main" Step 2 simply because the number of free protons is much greater than the number of deuterium atoms, such that the "deuterium + proton" reaction occurs more frequently.

Step 3: The two He-3 atoms combine to form a single beryllium-6 atom; He-3 + He-3 -> Be-6 + released energy. (6 because 4 protons + 2 neutrons = 6 nucleons).

Step 4: The Be-6 atom, which is unstable, breaks apart into a stable helium atom and 2 protons; Be-6 -> He + 2H+ + released energy.

This page describes two other paths which occur after Step 2, but with less frequency.

Some other sets of reactions which produce helium from hydrogen are deuterium-deuterium, where 2 deuterium combine to form a helium-3 and a free neutron; and deuterium-tritium, where one deuterium and one tritium combine into a single helium and a free neutron (tritium = hydrogen isotope with 2 neutrons in the nucleus).

As a slight extension of the answer, a primary difficulty in using the proton-proton chain for producing power from fusion on Earth is getting the charged nuclei close enough together to fuse. Within stars, gravity greatly increases the density such that relatively low energies (often given as temperatures) are required to surpass the electrostatic repulsion between the same-charged nuclei. However, gravity on Earth is much lower, meaning much larger energies (temperatures) are required to force the nuclei close enough to combine. The temperatures required are ~6x greater than in the core of the sun, which far exceeds the capabilities of known materials.

More promising terrestrial fusion schemes are actually the deuterium-deuterium and deuterium-tritium reactions, with those reactions being 10²⁴ times more reactive than standard hydrogen and easier to contain than the proton-proton chain. Research to enable appropriate conditions for these reactions has focused on magnetic and laser inertial confinement. And despite the theoretical benefits to nuclear fusion reactors, there are still potential drawbacks, as explored in this (lengthy) article.

[The Stellar Physics section in Unit 2 of this class website has several helpful pages. These two questions on ScienceLine are related to this one and may interest readers.]

There are many kinds of reactions of elements all around us--chemical reactions, physical changes of state, nuclear reactions, etc. To change an element, one of the basic forms of matter, into another one, we have to conduct a nuclear reaction, because we are changing the properties of the nucleus of that atom. In the case of turning hydrogen into helium, we have to somehow combine two hydrogen atoms to make one helium atom.

There are two kinds of nuclear reactions--fusion and fission.

Fusion is the fusing of two or more elements to make a new one. Fission is the opposite reaction--breaking apart a nucleus to make two or more elements. If we want helium from hydrogen, we have to fuse two hydrogen atoms together, that is, do nuclear fusion. Both nuclear fusion and fission are high energy processes, because it is very difficult to add or remove protons and neutrons to a nucleus. With high temperatures and pressures, however, it is possible.

The sun can do fusion in its core, and it converts hydrogen into helium due to the immense pressures and temperatures that exist there. We can think of the sun as the biggest nuclear reaction in the solar system! We can also do fusion on earth in scientific labs, but we can't make very much of any one element because it takes so much energy to do fusion. On the other hand, humans are good at doing fission, which is how nuclear reactors work to produce energy.

Hydrogen nuclei can become a helium nucleus through nuclear fusion. This happens naturally in the centers of main-sequence stars (not red giants or dying stars), and happens artificially in thermonuclear explosions.

Fusion is the process that converts hydrogen to helium. It takes a lot of energy!

"Hydrogen is converted into helium by a succession of nuclear reactions that change four protons into a helium nucleus, two positrons, and two neutrinos. (A positron is a particle like an electron but with a positive charge; a neutrino is a particle with no charge and negligible mass.) Chemical element,Processes producing. heavier elements.