US astronomer who was the first to put in graphic form what became known as the Hertzsprung-Russell diagram 1913.
Russell was born in Oyster Bay, New York, and studied at Princeton, where in 1905 he was made professor and director of the observatory. In 1921 he moved to the Mount Wilson Observatory, California.
Like Ejnar Hertzsprung, Russell concluded that stars could be grouped in two main classes, one much brighter than the other. He used Annie Cannon's system of spectral classification, which also indicated surface temperature. Most of the stars were grouped together in what became known as the 'main sequence', but there was a group of very bright stars outside the main sequence. Russell put forward the theory that all stars progress at one time or another either up or down the main sequence, depending on whether they are contracting (and therefore becoming hotter) or expanding (thus cooling), but the progression he proposed was discredited within a decade.
Russell's lifelong study of binary stars resulted in a method for calculating the mass of each star from a study of its orbital behaviour. He pioneered a system using both orbits and masses in order to compute distance from Earth.