Henry Norris Russell (born Oct. 25, 1877,Oyster Bay, N.Y., U.S.—died Feb. 18, 1957,Princeton, N.J.) was an Americanastronomer—one of the most influential during the first half of the 20th century—who played a major role in the establishment of modern theoreticalastrophysics by makingphysics the core of astrophysical practice. Bearing his name is theHertzsprung-Russell diagram, a graph that demonstrates the relationship between astar’sintrinsic brightness and its spectral type and that represents Russell’s theory of the way stars evolve.

The first of three sons born to Alexander Gatherer Russell, a liberal Presbyterian minister, and Eliza Hoxie Norris, his proud, mathematically adept mother, Russell entered Princeton Preparatory School in 1890 and thenPrinceton University in 1893, from which he graduated in 1897 with highest honours. Other than his family, the primaryintellectual influences on Russell were the astronomerCharles Augustus Young and the mathematician Henry B. Fine. He obtained his Ph.D. from Princeton in 1900 with a thesis—an analysis of the way that Mars perturbs the orbit of theasteroid Eros—that was very much within traditional mathematicalastronomy. After a year as a special student at theUniversity of Cambridge, Cambridgeshire, England, where he attended the lectures of the English astronomer and mathematical physicistGeorge Darwin on orbit theory anddynamics, Russell spent almost two years at the Cambridge University Observatory, developing one of the first photographicparallax programs for determining distances tostars.

When he returned to Princeton as an instructor in 1905, Russell was already firmly convinced that the future of astronomical practice lay not in open-ended data-gathering programs but in problem-oriented research in which theory and observation worked synergistically. He also had the good fortune at Princeton to escape theenvironment common at major observatories of the day, where research was largely instrument-based and defined by the interests of the observatory director. At Princeton neither Young, who directed the university observatory until 1905, nor his successor, the mathematician E.O. Lovett, established large-scale observing programs requiring a narrowly trainedlabour force. Russell, therefore, was free to search out new and exciting problems and to apply his considerable mathematical talents to their solution.

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Russell spent nearly his entire professional life at Princeton. He rose quickly, gaining a professorship in 1911 and becoming director of the observatory a year later. Although he maintained these administrative responsibilities until his retirement in 1947, his chief activity was always research; the details of managing the observatory, as well as much of the teaching, were left to others. Because Russell generally shunned administrative and academic responsibilities, the observatory grew little in staff and equipment during his longtenure. Among his few but notable students wereHarlow Shapley, who became director of Harvard College Observatory, Cambridge, Massachusetts, in 1921, Donald Menzel, who followed Shapley to Harvard in the 1930s to establish a major training program in astrophysics, andLyman Spitzer, Jr., who succeeded Russell as observatory director at Princeton.

Until 1920 Russell’s research interests ranged widely in planetary and stellar astronomy and astrophysics. He developed quick and efficient means for the analysis of the orbits ofbinary stars. Most notable were his methods for calculating the masses and dimensions ofeclipsing variable stars—that is, binary stars that appear to move in front of each other as they orbit about their commoncentre of gravity and thus show characteristic variations in brightness. He also developed statistical methods for estimating the distances, motions, and masses of groups of binary stars. Russell generally employed aheuristic, intuitive style to all his areas of interest, one that was accessible to his widening circle of astronomical colleagues, few of whom were mathematically adept. Russell’s strength was in analysis, and he soon found that observational astronomers, if properly approached, were more than happy to have their hard-won data managed, and showcased, by a bright theorist.

In his stellar parallax work at Cambridge, Russell had applied his study ofbinary stars to what they could reveal about the lives and evolution of stars and stellar systems. After choosing stars that might test which of several competing theories of stellar evolution was correct, he used his parallax measurements to determine theintrinsic, or absolute, brightnesses of these stars. When he compared their brightness to their colours, or spectra, Russell found, as had the Danish astronomerEjnar Hertzsprung several years earlier, that among the majority of the stars in the sky (the dwarfs), blue stars are intrinsically brighter than yellow stars and yellows are brighter than reds. Nevertheless, a few stars (the giants) did not follow this relationship; these were exceptionally bright yellow and red stars. Later, by plotting brightnesses and spectra in a diagram, Russell pictorialized the definite relationship between a star’s true brightness and itsspectrum. He announced his results in 1913, and the diagram, which came to be known as theHertzsprung-Russell diagram, was published the next year.

Russell aimed to confirm a theory of stellar evolution suggested by the astronomical spectroscopistJoseph Norman Lockyer and the mathematical physicist August Ritter, and to interpret the theory in terms of thegas laws. His diagram was the best way he knew to illustrate the viability of the theory. According to Russell, stars begin their lives as vastly extended,tenuous globes of gas, condensing through gravitational contraction out of thenebulous mists. As they contract, they heat up and pass through a colour change from red to yellow to blue, eventually achieving densities that cause them to deviate from theperfect gas laws. Further contraction toward the dwarf state, therefore, is accompanied by a cooling phase, in which the stars reverse their colour change, going from blue to red, and finally become extinct. Set firmly within thecontext of gravitational contraction as the source of energy of the stars, this description became known as Russell’s theory ofstellar evolution and enjoyed considerable popularity until the mid-1920s. When the English astronomerArthur Stanley Eddington found that all stars demonstrate the same relationship between their masses and intrinsic brightnesses and, therefore, that dwarfs were still in the perfect gas state, Russell’s theory lost its theoretical underpinning. It was not replaced by a substantially different theory until the mid-1950s.

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After 1920, the year in which the Indian astrophysicistMeghnad Saha announced his theory of ionizationequilibrium, Russell focused much of his energies on spectrum analysis, in which he applied laboratory methods to the study of stellar conditions. Saha’s theory confirmed that the spectrum of any star was governed mainly by temperature, secondarily by pressure, and in a small way by the relative abundance of the chemical elements in the star’scomposition. This realization, that the physical state of a star could be quantitatively analyzed through its spectrum, proved to be a major turning point in Russell’s career. His shift to spectrum analysis was also influenced by his new association withGeorge Ellery Hale, who made Russell a senior Carnegie research associate with annual residence atMount Wilson Observatory near Pasadena, California. Russell was thus given the best laboratory and astronomical spectroscopic data in the world, and he eagerlyexploited this to refine and extend Saha’s theory not only to the physics of stars but also to the structure of matter as studied in laboratories on Earth.

From 1921 until the early 1940s Russell spent several months each year atMount Wilson helping Hale’s solar and stellar spectroscopic staff exploit their vast stores of accumulated astrophysical data. He also formed numerous ad hoc networks of physical laboratory and observatory groups to work on term analysis—the description and evaluation of the line structure of complex spectra. Through these networks and his close association with Hale, Russell became one of the most influential astronomers of his day.

Russell extended his influence through his efforts as a promulgator and arbiter of astronomical knowledge. For 43 years, starting in 1900, Russell wrote for the lay publicationScientific American. Although at first a simple column that accompanied a night sky map, his writings soon became aforum on the status and progress of astronomy. Russell was a frequent commentator on astronomy for the professional journalScience and was constantly asked to referee papers in broad fields of spectroscopic and stellar astronomy for leading astrophysical publications. He also used his two-volume textbook,Astronomy (1926–27), coauthored with two Princeton colleagues, as a vehicle for the latest theories on the origin and evolution of stars, to stimulate growth in astrophysics.

Russell was a liberal Christian thinker. As a Princeton faculty member, he echoed thephilosophy of James McCosh, a former president of the school (then the College of New Jersey), in his public and student lectures on a “scientific approach to Christianity.” He ardently preached on the relationship ofscience and religion, arguing that science could strengthen religion in modern society by revealing theunity of design in nature. Russell was also a family man, marrying in 1908 and fathering four children.