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Consumption of Hydrogen in the Sun

by Granite

Nuclear fusion in the Sun consumes about 5 million tonnes of hydrogen per second. Despite this enormous consumption of hydrogen, the sun has enough hydrogen to burn at this rate for the next 4 billion years.  The Sun is currently composed of about 90 percent hydrogen, 9 percent helium, and only 1 percent of all the other elements such as carbon, nitrogen, oxygen, silicon, and iron.  https://history.nasa.gov/EP-177/ch3-2.html#:~:text=In%20this%20way%20the%20Sun,least%20another%204%20billion%20years

The temperature in the core of the Sun is about 15 million degC, the density 150 times that of water and the pressure over 200 billion times greater than atmospheric pressure on earth. https://www.wwu.edu/astro101/a101_sun.shtml

The mass of the sun s 1.99 x 1027 tonnes – about 333,000 times the mass of the Earth

The mass of the final helium-4 atom is is less than the mass of the four protons – about 0.7 percent of the mass of the original protons has been lost. This mass has been converted into energy, in the form of kinetic energy of produced particles, gamma rays, and neutrinos released during each of the individual reactions. The total energy yield of the conversion of 4 protons to 1 helium nucleus is 26.73 MeV which is equivalent to 4.28 x 10-12 Joules.

About 99 % of solar energy is produced by the proton-proton chain reaction .  The first process in the proton-proton chain reaction, is for two protons combine to make deuterium (and an electron).  The likelihood of two protons fusing is very very low process under the temperature and pressure conditions in the Sun.  Nevertheless enough fusions occur for the proton-protons chain reaction to generate the immense energy output of the Sun.

In the second process on the proton-proton chain reaction, an additional proton is absorbed to make helium-3.  This process is much quicker that the first process in the chain.  Each helium-3 nucleus exists for only about 400 years before it is converted to helium-4.

There are several paths to go from helium-3 to helium-4.  In the Sun, the dominate path (83%) is for two helium-3 nuclei combine to produce helium-4 plus two protons.

About 1 % of solar energy is produced by the CNO cycle where fusion is catalysed by successive proton captures in carbon-12,  nitrogen-14  and oxygen-15. The final proton capture in nitrogen-15 produces carbon-12 and helium.  For stars heavier than about 1.3 solar masses, the increased temperature and pressure means that energy production is dominated by the CNO process.

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