Particle masses in fusion

[James_23]

Hi everyone, I'm currently taking an astronomy class and I was reading about the fusion reactions in stars and it says that the mass of particles are different in each element. For example the book says helium has a lower mass per nuclear particle than hydrogen. Does this mean that the mass of a proton isn't constant? Or are the masses of particles averaged like the atomic masses of the elements?

[clem]

The mass of a nucleus includes the binding energy of its constituents, which makes it less than the sum of the masses of the constituent nucleons. The masses of the protons and neutrons don't change.

[bcrowell]

Hi everyone, I'm currently taking an astronomy class and I was reading about the fusion reactions in stars and it says that the mass of particles are different in each element. For example the book says helium has a lower mass per nuclear particle than hydrogen. Does this mean that the mass of a proton isn't constant? Or are the masses of particles averaged like the atomic masses of the elements?

Just to amplify on what clem said, this is an example of Einstein's E=mc2. When you lift a rock against the opposing force of gravity, you're storing potential energy in it (and that PE is transformed into kinetic energy when you release the rock). The rock-earth system has more PE when its two constituents are far apart, less when they're closer together. Exactly the same thing applies to chemical reactions, where the forces are electrical, and nuclear reactions, where the forces are nuclear. In all three cases, the differences in potential energy are measurable as mass differences, due to E=mc2. The energies involved in nuclear reactions are very large, so the mass difference is easier to measure compared to the mass differences caused by, say, chemical reactions.