Hubble, Edwin Powell (1889–1953)

Edwin Hubble

Edwin Hubble was an American astronomer whose observations proved that galaxies are "island universes," not nebulae inside our own galaxy. His greatest discovery was the linear relationship between a galaxy's distance and the speed with which it is moving, now known as the Hubble law.


The general outward movement of galaxies and clusters of galaxies resulting from the expansion of the universe is known as the Hubble flow. It occurs radially away from the observer and obeys the Hubble law. Galaxies can overcome this expansion on scales smaller than that of clusters of galaxies. The clusters, however, are being forever driven apart by the Hubble flow.


Upon graduation from the University of Chicago, Hubble won a Rhodes scholarship and earned a law degree at Oxford University. After obtaining his doctorate he spent his career, aside from army service in both world wars, at Mount Wilson Observatory where he used the 100-inch (2.5-meter) telescope.


In 1923 to 1925 he identified Cepheid variables in the "nebulae" NGC 6822, M31 (the Andromeda Galaxy), and M33 (the Triangulum Galaxy) and proved conclusively that they are outside the Galaxy. His investigation of these and similar objects, which he called extragalactic nebulae and which astronomers today call galaxies, led to his now-standard classification system of elliptical, spiral, and irregular galaxies, and to the proof that they are distributed uniformly out to great distances. Hubble measured distances to galaxies and, with Milton Humason, extended Vesto Slipher's measurements of their redshifts. In 1929 he published the velocity-time relation which, taken as evidence of an expanding universe, is the basis of modern cosmology; the constant in this relation is called the Hubble constant.


The Hubble Space Telescope is also named in his honor.


Hubble law

The Hubble law is the empirical rule that governs the rate of expansion of the universe. It is simply that the velocity of recession of a galaxy, v, is directly proportional to its distance from the observer, r. The Hubble constant is the proportionality factor in this law.


Hubble radius

The Hubble radius is the distance from the observer at which the cosmological recession velocity of a galaxy would equal the speed of light – in mathematical terms, c/Ho, where c is the velocity of light and Ho is the Hubble constant. Roughly speaking, the Hubble radius is the radius of the observable universe, and is believed to about 13.7 billion light-years.