Proton-proton fusion?

[Artlav]

I've been thinking about nuclear fusion process in the sun, and stumbled upon something weird:

Basically, why there is a net energy from PP fusion?

We have an input of 4 protons and output of 2 protons and 2 neutrons, now, a mass of a neutron is slightly larger than a mass of proton, so the result appears to be more massive, than the input!

Some online research shown that it doesn't go just that simple, but the PP-chain process have the same problem - the input is 6 protons, the output is 2 neutrons and 4 protons, which is again more massive than the input, and above that there are 2 positrons, neutrinos and gamma-quants of net output.

Where does the energy come from?
Kinetic energy of proton collisions?
If yes, what is the point of fusion, if it only reemit parts of the heat that make it go in the first place?

Something does not add up, where am i wrong?

[malawi_glenn]

I thought one ended up with a He4 nucleus, and not 2p + 2n ;-)

And the mass of He4 is smaller than mass(2p + 2n)

[Artlav]

I thought one ended up with a He4 nucleus, and not 2p + 2n ;-)

And the mass of He4 is smaller than mass(2p + 2n)

Hm, and He4 nucleus is made out of 2p and 2n, which raises the question, why is it lighter?
Are there different protons and neutrons in the atoms, than in hydrogen and free-flying neutrons?

[malawi_glenn]

This is basic nuclear physics, the mass of the nucleus has lower mass than its constituent particles due to the (negative)binding energy. Same holds for atoms aswell, an atomic nucleus + electrons has higher mass then the atom as a whole.

http://en.wikipedia.org/wiki/Binding_energy

http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/procyc.html

http://en.wikipedia.org/wiki/Nuclear_fusion

http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/fission.html

Enjoy your reading, if you have any more questions regarding this, please ask hera again :-)

[Artlav]

Thank you for the links, that made some sense although somewhat counter-intuitive at first - energy having "negative mass"...

[stevebd1]

While it takes energy to turn a proton into a neutron, there's also energy created when the positron produced from the beta decay of the proton as it turns into a neutron reacts with an electron creating an additional 1.022 MeV of energy. So while you have energy 'left over' when the constituent particles bind to create a larger particle, there's also a little extra from the electron-positron reaction.

http://en.wikipedia.org/wiki/Proton-proton_chain