The measure of how much "stuff" something has, mass determines the inertia of an object (its resistance to being accelerated by a force) and how much gravitational force it exerts on another object. In pre-Einsteinian physics mass was conserved, neither created nor destroyed. Einstein discovered that mass can be converted into energy and vice versa. The conservation of mass is still a good approximation since mass-energy conversions generally involve relatively small amounts of mass. The mass of astronomical objects is often measure in terms of the Sun's mass. The solar mass is 2 × 1033 grams.