Build your own model of a fusion reaction: start with a regular red lego (a proton). Wrap a rubberband around it to be the strong nuclear force. Then connect a small blue lego (an electron) to the red lego to make a hydrogen atom. Make four of these hydrogen atoms.
Now imagine that your hydrogen atoms are being heated up so hot that they come apart into eight pieces (and four rubberbands). Don’t really heat your Legos or they’ll melt! Just take the atoms apart again.
And put them back together into one helium atom. First, build two neutrons by stacking a blue electron on top of a red proton. Then connect your two neutrons to two more red protons – that’s the nucleus of the helium atom. Wrap one rubberband around the nucleus to be the strong nuclear force. Then fasten the two remaining blue electrons to the outside of the nucleus, and you’ve got a helium atom, and you’ve used up all your pieces.
But you have three rubberbands left over – that’s the extra energy that shoots off into space as sunshine and starshine.