The more massive a planet or star is, the stronger the gravitational force it exerts. It is this force that allows a planet or star to hold other objects in their orbit. This is summed up in Isaac Newton's Universal Law of Gravitation, which is an equation for calculating the force of gravity. Newton's Universal Law of Gravitation is a formula for understanding the relationship of gravity between two objects. The equation is "F = G(M1)(M2)/R," where "F" is the force of gravity, "G" is the gravitational constant, the "M"s are the masses of the objects being considered, and "R" is the radius of the distance between the two objects. Thus, the more massive either object is, and the closer they are together, the stronger the force of gravity.