This is the biography of James E. Keeler (1857–1900), a distinguished pioneer of astrophysics, the application of the methods of physics to understanding the nature of the stars, nebulae, planets, comets, and other objects that populate the universe. Keeler was an outstanding scientist, and his fellow astronomers and physicists at the end of the nineteenth century considered him the leading astronomical spectroscopist of his generation. His career was closely linked with that of George Ellery Hale, founder of Yerkes Observatory. Keeler himself was the first astronomer at Lick Observatory, and the story of his life tells much of the early history of these two early American ‘big-science’ research institutions.