|MadSci Network: Astronomy|
The strength of gravity depends on the total mass/emergy available. If a nucleus emits a particle or a photon, the gravity decreases. If particles join up, there's more mass and more gravity. Note, however, that gravity is extremely (really really really extremely!) weak compared to the electrical and nuclear forces within an atom, and can be ignored. Gravity is the weakest of all the physical forces. We only notice gravity because we're sitting right next to a giant lump of electrically neutral matter called the Earth. During the lifetime of the Sun, it will radiate away a total energy equal to its energy emitted per second times its lifetime. This amount of energy corresponds to about 10 to the 28th power kilograms being completely converted to energy. The mass of the Sun is about 200 times larger than this, so the energy loss produces a tiny change to the gravity. During the Big Bang, nuclear and electrical forces were far more important than gravity when the universe was at densities like today's nuclei - so just as in atoms, gravity is a very weak force and can be ignored in the calculations.
Try the links in the MadSci Library for more information on Astronomy.