American astronomer (1877–1957)
Russell, the son of a Presbyterian minister, was born in in Oyster Bay, New York. A brilliant scholar at Princeton, he graduated in 1897 and obtained his PhD in 1899. He spent the period 1902–05 as a research student and assistant at Cambridge University, England, returning then to Princeton where he served as professor of astronomy from 1911 to 1927 and director of the university observatory from 1912 to 1947. He was also a research associate at the Mount Wilson Observatory in California (1922–42) and, after his retirement, at the Harvard and Lick observatories.
Russell's great achievement was his publication in 1913 of a major piece of research contained in what is now called the Hertzsprung–Russell diagram (H–R diagram). The same results had in fact been published earlier and independently by Ejnar Hertzsprung with little impact. Russell's work was based upon determinations of absolute magnitudes, i.e., intrinsic brightness, of stars by the measurement of stellar parallax. His measurement technique was developed in collaboration with Arthur Hinks while he was at Cambridge and involved photographic plates, then a fairly recent scientific tool. He found that values of absolute magnitude correlated with the spectral types of the stars. Spectral type was derived from the Harvard system of spectral classification as revised by Annie Cannon and indicated surface temperature.
A graph of absolute magnitude versus spectral type produced the H–R diagram and showed that the majority of stars lie on a diagonal band, now called the ‘main sequence’, in which magnitude increases with increasing surface temperature. A separate group of very bright stars lie above the main sequence. This meant that there could be stars of the same spectral type differing enormously in magnitude. To describe such a difference the now familiar terminology of ‘giant’ and ‘dwarf’ stars was introduced into the literature.
The most obvious feature of the diagram for Russell, however, was that it was not completely occupied by stars. This led him to propose a path of stellar evolution, which he put forward in 1913 at the same time as the diagram. He argued that stars evolve from hot giants, pass down the main sequence and end as cold dwarfs. The mechanism driving the change was that of contraction. The bulky giants of spectral type M contract and with the resulting rise of temperature move leftward in the diagram, gradually becoming B-type dwarfs. But at some stage the contraction and density become too great for the gas laws to apply and the star cools, slipping down the main sequence and evolving finally to an M-type dwarf.
By 1926, however, Arthur Eddington could talk confidently of the overthrow of the ‘giant and dwarf theory’; it was too simple to fit the growing data on the distribution of mass and luminosity among the different spectral types of stars. Although Russell's evolutionary theory quickly fell from favor the H–R diagram has continued to be of enormous importance and the start for any new theory of stellar evolution.
Eclipsing binary stars, such as Algol, the ‘winking demon’, were also of great interest to Russell. He devised methods by which both orbital and stellar size could be determined and which became widely used. He also analyzed the variations in light output of a large number of eclipsing binaries, which again became invaluable to later researchers.
Another major line of research for Russell was his investigation of the solar spectrum, which began as a result of the publication in 1921 of the ionization equation of Meghnad Saha. The Saha equation was tested and modified by Russell, using the solar spectrum, and was then used by him to calculate the abundance of the chemical elements in the Sun's atmosphere. He realized that the abundances in other stars could also be calculated from their spectra. He showed that the abundance of elements within the Sun itself could be found and in 1929 published the first reliable determination of this, demonstrating surprisingly that 60% of the Sun's volume was hydrogen. Although this was an underestimate, as Donald Menzel was later able to show that a figure of over 80% was more accurate, it did pose the problem as to why the Sun, and presumably other stars too, should contain so much hydrogen. The answer to this question was given in the version of the big-bang theory proposed by George Gamow.
