US astronomer who deduced that spiral nebulae were galaxies that produced a cloud of debris which accumulated in the plane of the galaxy.
Curtis was born in Muskegan, Michigan, and studied classics at the University of Michigan. At the age of 22, he became professor of Latin at Napa College, California. Access to the small observatory there changed his career, and in 1897 he became professor of mathematics and astronomy at the University of the Pacific. He subsequently worked at a number of US observatories and 1906-09 in Chile, but most of his research was done at the Lick Observatory, Mount Hamilton, California, between 1898 and 1920.
Curtis worked on a programme for the measurement of stellar radial velocities 1902-09. For the next 11 years he concentrated his efforts on spiral nebulae, trying to establish whether they were distant star clusters or clouds of debris. From studying photographs he concluded that they were both. If such a cloud of debris had also gathered outside our own Galaxy, this would explain why none appeared in the plane of the Milky Way. Spiral nebulae in that position would simply be obscured by dust.