He was an American astronomer who is famed for debating Harlow Shapley on 26 Apr 1920 before the National academy of Sciences. He spoke for “island universes”—whereby spiral nebulae were composed of stars, and represented galaxies far outside the Milky Way. Shapley disagreed, believing that our galaxy was 300,000 light-years in diameter and included the spiral nebulae. By the end of 1924, Curtis was shown to be correct, when a paper from Edwin Hubble was read to the American Astronomical Society on 1 Jan 1925. After his early work measuring radial velocities of the brighter stars, but in 1910 he became active in nebular photography, trying to find evidence of their nature as isolated independent star systems.