A piece of writing that best captures the beauty and mystery surrounding the dynamics of galaxies can be found in chapter 7 of the first volume of The Feynman Lectures on Physics. There, in a few simple sentences and by means of a picture of a globular cluster and a spiral galaxy, we are rapidly brought to the scales and structure of the systems involved and to the underlying limits that our physical knowledge must recognize. We are told that gravitation is the prime actor, stars rarely collide, and angular momentum leads to a contraction in a plane, but we are also reminded of the facts that “of course we cannot prove that the law here is precisely inverse square,” “what determines the shape of these galaxies has not been worked out,” and that we are dealing with systems that are enormously complex.

Approximately four decades earlier, through a decisive distance measurement, some nebulae had been recognized as huge systems millions of light years away; those were nearby galaxies, and galaxies were thus found to be the visible building blocks of the Universe. If we examine that discovery, we see that modern cosmology and quantum mechanics were developed at approximately the same time and well after the formulation of general relativity; they were also developed well after the basic equations that govern the motion of a self-gravitating stellar system were studied by Jeans.