As many of the Exercises show, the conversion of energy from one form to 
another to another happens throughout the universe, in almost every kind
of astronomical system.  When the energies of such systems are in balance,
they can stay stable for long periods of time - long enough to form such
complicated things like galaxies, stars, planets, and even life.  This kind
of long-term balance is called equilibrium. 

Some particularly important energy conversions include changing gravitational
potential energy into kinetic energy, luminosity, or heat.  In stars, the
conversion of matter into energy is particularly important.