# Hydrogen burning vs Hydrogen fusing

Does the term "Hydrogen burning" mean the same as "Hydrogen fusing" in astronomy? If not, then what is the product of "Hydrogen burning"? Assume the product of "Hydrogen fusing" is helium.

• – uhoh
May 22, 2021 at 2:50
• chemical burning is chemistry, nuclear burning is nuclear physics. Words get reused often in science May 24, 2021 at 13:51

In stellar astrophysics, "burning" means nuclear fusion, not chemical combustion. So a star burns hydrogen to helium. (Incidentally, normal chemical burning of hydrogen in air produces water).

This might seem to be confusing terminology, but it's not an issue in practice because the regions in stars where fusion takes place (the core, and shells surrounding the core, in big old stars) are far too hot for molecules to exist, so chemical processes simply cannot occur there.

Hydrogen fusion in a star core is not a single nuclear reaction. There are two (known) sets of reactions: the proton–proton chain, and the catalytic CNO (carbon-nitrogen-oxygen) cycle. From Wikipedia:

The proton–proton chain, also commonly referred to as the p-p chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun, whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 times that of the Sun.

Hydrogen burning is the main way that young stars produce heat and light. (Older stars can burn heavier elements, if they have sufficient mass). But hydrogen burning is also important because it's the main way that neutrons are produced: the burning of heavier elements generally does not change the number of neutrons. (Neutrons are also created when a heavy star collapses into a neutron star).

In a Sun-like star, hydrogen burning is a very slow process. It takes a lot of heat and pressure to overcome the electrical repulsion between two protons and fuse them into a diproton. However, the diproton is extremely unstable, with a half-life of much less than a nanosecond. Usually, a diproton just splits up into two protons, but sometimes one of its protons transforms into a neutron, converting the diproton to a deuteron (a deuterium nucleus). The probability of diproton to deuterium conversion is very small, in the order of $$10^{-26}$$. Various p-p chain reactions convert deuterons to helium nuclei.

The slowness of deuterium production is a good thing: it means our Sun will continue to burn hydrogen for billions of years. OTOH, it also means plain hydrogen will never be a practical fuel for an artificial fusion reactor.

The density in the Sun's core is ~150 g/cm³, which is much greater than any terrestrial material (eg, the density of lead is only 11.3 g/cm³). So a cubic metre of solar core matter has a mass around 150 metric tons. But the rate of hydrogen fusion is so slow that it only produces around 276.5 watts of power, which is similar to the heat production of a cubic metre of compost, as mentioned on Wikipedia. Of course, a 1 m³ compost heap won't keep pumping out heat for billions of years. ;)

Wikipedia has a good collection of articles on the various stellar nuclear fusion processes, including:

Yes, it's the same thing.

Usually the phases a star goes through (if big enough) are the

• hydrogen burning
• helium burning
• ...

and the phase is named after the element which is being fused into a heavier element.

Thus hydrogen burning is the phase where hydrogen is fused into helium. Helium burning is the phase where three helium are fused into carbon; carbon burning where two carbon cores are fused into either magnesium, neon, sodium or oxygen etc.