Arthur Eddington

Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English mathematician and astrophysicist. The Eddington limit—the maximum brightness a star can have, or the radiation produced when matter falls onto a dense object—bears his name. In a 1920 paper titled "The Internal Constitution of the Stars," he correctly guessed that stars are powered by the fusion of hydrogen into helium, a process that was still completely unknown at the time. He also led an expedition to the island of Príncipe to observe the solar eclipse on 29 May 1919, which gave one of the first experimental proofs of Einstein's general relativity.

Eddington was born on 28 December 1882 in Kendal, Westmorland (now Cumbria), to Quaker parents. His father, Arthur Henry Eddington, was headmaster of a Quaker school; his mother was Sarah Ann Shout. After his father died in the 1884 typhoid epidemic, his mother raised him and his sister on a small income. The family moved to Weston-super-Mare, where Eddington was first taught at home, then attended a preparatory school. They lived at 42 Walliscote Road in a house called Varzin, which now bears a plaque noting his scientific work.

In 1893 he entered Brynmelyn School, excelling in mathematics and English literature. His performance won him a scholarship to Owens College, Manchester (now the University of Manchester) in 1898. After a general first year, he focused on physics for three years, influenced by teachers Arthur Schuster and Horace Lamb. He lived at Dalton Hall, where the Quaker mathematician J. W. Graham left a lasting impression. He graduated with first-class honours in physics in 1902. That same year, his record earned him a scholarship to Trinity College, Cambridge. There, his tutor was Robert Alfred Herman, and in 1904 he became the first second-year student ever ranked Senior Wrangler. After earning his M.A. in 1905, he briefly researched thermionic emission at the Cavendish Laboratory and taught mathematics to engineering students. On the recommendation of E. T. Whittaker, he then took a post at the Royal Observatory, Greenwich, beginning his career in astronomy—a field he had been drawn to since childhood, when he would often "try to count the stars."

In January 1906, Eddington became chief assistant to the Astronomer Royal at the Royal Observatory, moving from Cambridge to Greenwich the following month. He analysed photographic plates of the asteroid 433 Eros to measure its parallax, a project started in 1900. He devised a statistical method using the apparent drift of two background stars, winning the Smith's Prize in 1907. This prize brought him a fellowship at Trinity College, Cambridge. After George Darwin died suddenly in December 1912, Eddington was appointed Plumian Professor of Astronomy and Experimental Philosophy in early 1913. Later that year, Robert Ball, holder of the theoretical Lowndean chair, also died, and Eddington became director of the Cambridge Observatory in 1914. He was elected a fellow of the Royal Society in May 1914, received the Royal Medal in 1928, and gave the Bakerian Lecture in 1926.

Eddington also studied the interiors of stars theoretically, developing the first real understanding of stellar processes. He began in 1916 by investigating possible physical explanations for Cepheid variable stars, extending Karl Schwarzschild's earlier work on radiation pressure in Emden polytropic models. These models treated a star as a gas sphere held up by internal thermal pressure against gravity. Eddington's key addition was showing that radiation pressure was necessary to keep the sphere from collapsing. He built his model even though he lacked firm knowledge of opacity and energy generation inside stars. His results allowed calculation of temperature, density, and pressure at every point inside a star, and he argued that the theory was so useful for further astrophysical research that it should be kept despite not resting on fully accepted physics. James Jeans contributed the important idea that stellar matter would be ionized, but that ended their collaboration; the two became known for their lively debates. Eddington defended his approach by pointing to the utility of his results, especially his mass–luminosity relation, which unexpectedly showed that nearly all stars—both giants and dwarfs—follow the same pattern.