The spirit beneath the surface of nearly any astronomical phenomenon is gravitation. The reason why gravity is the motive force for much of the Universe is not hard to see. What primarily interests us about the Universe is its structure, including ourselves. And the physical reason for the existence of this structure is gravity. Even in the case of ourselves, it is the force of gravity in massive stars which drives their nuclear reactions to produce heavy elements, then eventually causes the star to explode and spew these elements throughout the galaxy. Some of them collect into new stars and planets, partly through the more gentle ministrations of gravity – and here we are! Of course, the mass of humanity, though important to ourselves, is only about 10-41 of the mass of the visible Universe. To put it another way, we contribute about 10-19 kms-1 Mpc-1 to the Hubble constant.

As our own origin, through star formation and evolution, was driven by gravity, so even more directly does gravity govern the dynamics of other astronomical structure: stellar clusters, the shapes and evolution of galaxies, and the motions of the entire system of galaxies.