HUBBLE CLASSIFICATION

Hubble's system of classification for galaxies. Hubble, Edwin Powell born Nov. 20, 1889, Marshfield, Mo., U.S. died Sept. 28, 1953, San Marino, Calif. American astronomer who is considered the founder of extragalactic astronomy and who provided the first evidence of the expansion of the universe. Hubble's interest in astronomy flowered at the University of Chicago, where he was inspired by the astronomer George E. Hale. At Chicago, Hubble earned both an undergraduate degree in mathematics and astronomy (1910) and a reputation as a fine boxer. Upon graduation, however, Hubble turned away from both astronomy and athletics, preferring to study law as a Rhodes Scholar at the University of Oxford (B.A., 1912). He joined the Kentucky bar in 1913 but dissolved his practice soon after, finding himself bored with law. A man of many talents, he finally chose to focus them on astronomy, returning to the University of Chicago and its Yerkes Observatory in Wisconsin. After earning a Ph.D. in astronomy (1917) and serving in World War I, Hubble settled down to work at the Mount Wilson Observatory near Pasadena, Calif., and began to make discoveries concerning extragalactic phenomena. While at Mount Wilson, Hubble discovered (192224) that not all nebulae in the sky are part of the Milky Way Galaxy, the vast star system to which the Sun belongs. He found that certain nebulae contain stars called Cepheid variables, for which a correlation was already known to exist between periodicity and absolute magnitude. Using the further relationship among distance, apparent magnitude, and absolute magnitude, Hubble determined that these Cepheids are several hundred thousand light-years away and thus outside the Milky Way system and that the nebulae in which they are located are actually galaxies distinct from the Milky Way. This discovery, announced in 1924, forced astronomers to revise their ideas about the cosmos. Soon after discovering the existence of these external galaxies, Hubble undertook the task of classifying them according to their shapes (1926) and exploring their stellar contents and brightness patterns. In studying the galaxies, Hubble made his second remarkable discoverynamely, that these galaxies are apparently receding from the Milky Way and that the further away they are, the faster they are receding (1927). The implications of this discovery were immense. The universe, long considered static, was expanding; and, even more remarkably, as Hubble discovered in 1929, the universe was expanding in such a way that the ratio of the speed of the galaxies to their distance is a constant now called Hubble's constant. Although Hubble was correct that the universe was expanding, his calculation of the value of the constant was incorrect, implying that the Milky Way system was larger than all other galaxies and that the entire universe was younger than the surmised age of the Earth. Subsequent astronomers, however, revised Hubble's result and rescued his theory, creating a picture of a cosmos that has been expanding at a constant rate for 10 billion to 20 billion years. For his achievements in astronomy, Hubble received many honours and awards. Among his publications were Red Shifts in the Spectra of Nebulae (1934) and The Hubble Atlas of Galaxies (published posthumously, 1961, and edited by Allan Sandage). Hubble remained an active observer of galaxies until his death.