HOW DOES THE SUN CREATE HEAT?
There have been several publications that describe the nuclear fusion reactions in our Sun in great detail. One recent excellent article authored by Molly Wakeling in Astronomy Magazine—the October 2025 issue starting on page 46—describes the two predominant types of nuclear fusion reactions that occur in our Sun, and also in most other stars: Reaction Type #1 is a proton-proton chain (also known as the P-P cycle—see the diagram on page 48) that is the primary set of nuclear reactions in our Sun that starts with hydrogen atoms (or, more correctly—in the core of the Sun—hydrogen ions, i.e., protons in a solar plasma) and ends with helium ions—also known as alpha particles. Another published source that describes this P-P cycle is in a book titled, UNIVERSE, Haynes Owners’ Workship Manual, by David M. Harland; see page 80. (It shows the same P-P diagram.)
Reaction Type #2 involves other types of atoms in addition to hydrogen and helium; its cycle involves hydrogen, carbon, nitrogen, oxygen, and helium (also known as the CNO cycle—see the diagram on page 48 of the Astronomy article by Molly Wakeling) as being the primary set of nuclear reactions in larger stars, and starts with hydrogen atoms (again, more correctly—in the core of the Sun—hydrogen ions, i.e., protons in a solar plasma) and ends with helium ions—also known as alpha particles. The book titled, UNIVERSE, Haynes Owners’ Workship Manual, by David M. Harland also describes this CNO cycle; see page 81. (It shows the same CNO diagram.)
To be more clear, the Sun’s core is full of hydrogen ions—i.e., protons—in a plasma where the normal electrons have been ‘stripped’ away from their hydrogen nuclei because of the very hot and very dense physical conditions found in the core of a star. When a protostar (a large sphere of hydrogen gas that is massive enough to eventually become a shining star) begins to fuse its hydrogen and become a main sequence star, initially the final result is many helium ions, or helium nuclei. Such helium nuclei consist of two protons and two neutrons, which is the stable isotope of helium. This is also known as 4He2+. The majority of the hydrogen nuclei as the ‘nuclear fuel’ are in the form of a single proton, with no neutron in the nucleus of those hydrogen nuclei. This is also known as 1H1+. (Note that, outside the core of a star, the notations of 1H and 4He would include their electrons, thereby being a complete atom that is neutral in electrical charge.)
Here’s my point: most everyone has heard of Albert Einstein’s famous equation, E=mc2, and therefore, most people who think about how our Sun creates heat probably believe that a certain amount of mass is lost (or somehow consumed) to create energy during the process of fusing four protons (i.e., 1H1+) into helium. But the P-P and CNO diagrams in the above-identified publications do not give us an ‘energy equation’, nor does the text describe how, or where in the P-P or CNO cycles, mass is lost. Most people probably believe that the fusing of four protons into two protons and two neutrons (helium) loses some mass because neutrons are ‘lighter’ than protons. But the opposite is true—neutrons are slightly ‘heavier’ than protons. So where is mass lost in these P-P and CNO cycles?
To be sure, the P-P and CNO diagrams in the two above-identified publications show gamma rays and neutrinos being generated at certain portions of those cycles. But these are essentially massless particles. So where is mass being lost? Answer: it must be the positrons that are emitted at two places in the P-P and CNO cycles. And even at that, these publications do not describe what happens to those positrons. Well, since positrons are anti-matter, they will eventually come into contact with normal matter electrons (still inside the star), and both a positron and an electron will annihilate each other and create pure energy. When that happens, a nice little surge of energy will be created, and a corresponding amount of mass will be lost, according to Einstein’s famous equation. (Note: electrons and positrons are quite ‘light’ as compared to protons or neutrons; however, if they annihilate each other and create pure energy, this is an enormous amount of energy compared to their tiny masses; remember: E=mc2. The speed of light squared is a very large number.)
Assuming my description above is correct (note, I am an Electrical Engineer, not a Nuclear Physicist), then one must go outside the P-P and CNO diagrams to understand how the Sun (or any main sequence star) creates energy in the form of heat. Obviously, stars do not merely create heat per se; instead, stars shine with electromagnetic energy—i.e., photons—and those photons can travel through space and impart energy onto a surface, such as a planet called Earth. And some of our Sun’s photons are not in the form of visible light; the Sun also emits some infrared light and ultraviolet light—ever get a sunburn? We should be happy that the gamma rays emitted by the P-P and CNO cycles don’t all reach the Earth; most solar-created gamma rays interact with other matter within the Sun by raising electrons of those other atoms to greater energy orbits. When that occurs, a photon of a given wavelength will be emitted by that atom—and yet more electromagnetic energy is created, usually at lower energy levels (i.e., at longer wavelengths). Eventually, most of the electromagnetic energy that is released at the surface of the Sun is (thankfully) at much lower energy levels, such as visible light.
And finally, in my above analysis, the plethora of positrons being created in the P-P and CNO cycles would eventually deplete the electrons in the star, and we need electrons to somehow be generated by some physical process to keep the star electrically neutral. So (obviously) my above analysis still needs some work to be entirely accurate. It would really be nice if some Nuclear Physicist or an Astronomer would give us a more full description of how mass is converted into energy in a star—I’m sure this is described in advanced stellar physics textbooks. But I am merely trying to raise the issue, and hope for a reasoned response from a true expert.
FRED GRIBBELL, January 2026
Afterword:
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