Nuclear Fusion and mass defect?

Question

If 4 Hydrogen nuclei fuse to form a Helium atom, there is a mass defect. I.e. the mass of the Helium atom is leass than that of the 4 Hydrogen atoms. Now, I understand that this is because some of the mass is converted into the energy needed to keep the helium nucleus together. Thus that accounts for the missing mass - if this is the case then where does the energy come from that is given off in fusion??

Answer 1

If you check out this Wikipedia Page it will make more sense, especially the graph of binding energy v. # nucleons.

Basically, the idea is that when elements fuse, energy must be put in in order to separate the protons and neutrons in the nucleus, but if they're put together in an arrangement that is more stable, it will give off more energy than you put in. The most stable element is Fe, so you can fuse elements together all the way up to Fe (H and up), and can Fission atoms all the way down to Fe (the largest elements down to Fe). So to answer your question, the energy comes from putting the elements in a more stable form. He is more stable than 4H atoms, so ultimately, more energy is given off than put into it.

Check out the source if I didn't make sense.

Answer 2

As I understand it, none of the converted mass is used to hold the helium nucleus together. The nucleus is held together by the strong nuclear force. The strong nuclear force holds the quarks of a proton/neutron together and a 'residual' of that holds the nucleus together.

The energy release from fusion comes from the binding energy... the mass defect you mention.