gravitational binding energy
Imagine taking a planet apart and carrying every grain of it infinitely far away, against the gravity that wants to pull it all back together. The total work you would have to do is the gravitational binding energy — the energy debt that holds a self-gravitating object together. Equivalently, it is the energy released when scattered matter falls together to build the object in the first place.
Binding energy is larger for bodies that are more massive and more compact. For a uniform sphere of mass M and radius R it is roughly G M^2 / R (with a numerical factor of order one), so squeezing the same mass into a smaller radius deepens the gravitational well dramatically. Building the Sun from a diffuse cloud released of order 10^41 joules; for a neutron star, packing a Sun's worth of mass into a 10-km ball, the binding energy is a sizeable fraction of its entire rest-mass energy.
This released energy is not bookkeeping — it is a power source that lights up the universe. Gravitational collapse heats the cores of forming stars until fusion ignites; matter spiralling toward a black hole converts a large fraction of its binding energy into the blazing light of a quasar; and a supernova's spectacular energy comes in part from the binding energy unleashed as a stellar core collapses to a neutron star. To pull such an object apart again, you would have to pay all that energy back.
When a giant gas cloud collapses into a star, about half the released gravitational binding energy heats the new star and the other half is radiated away — the slow gravitational warmth that keeps a protostar shining before fusion even begins.
Gravity itself is a furnace: collapse pays out energy as heat and light.
Sign conventions trip people up: physicists often write the gravitational potential energy as negative (a bound system sits in an energy well), so 'binding energy' is the positive amount you must add to free the system — same physics, opposite sign.