by F. Graham-Smith
© Oxford University Press 2004 All rights reserved
Kahn, Franz Daniel (1926-1998), astrophysicist and mathematician, was born on 13 May 1926 in Nuremberg, Germany, the only son and the younger child of Siegfried Kahn (1894-1967), a company director, and his wife, Grete, née Mann (1896-1980). His parents were German Jews; his father was a successful manufacturer of children's toys. In 1938 the family moved to England, where his father, with Jewish colleagues, had already set up a branch of their firm (Trix Ltd, manufacturers of toy trains). Kahn was educated at St Paul's School from 1940 to 1944, during its evacuation to Crowthorne. He won a form prize for English after he had been in England for less than two years, and in 1944 secured an open scholarship to Queen's College, Oxford. After graduating with first-class honours in mathematics in 1947 he continued at Oxford with research under Sydney Chapman, then moved in 1948 to Balliol College as a Skynner senior student. His DPhil thesis was a study of the expulsion of corpuscular streams in solar flares. He was awarded the degrees of MA and DPhil in 1950.
Kahn was appointed assistant lecturer in mathematics at the University of Manchester in 1949 and remained in Manchester for the rest of his life. On 22 March 1951 he married Carla Vivienne Copeland (1928/9-1981), a schoolteacher and the daughter of Benjamin Copeland, a textile merchant. It was a happy marriage, and they had two sons and two daughters. In the same year as Kahn's marriage, a new astronomy department was formed at Manchester, under Zdeneùk Kopal, complementing the development of radio astronomy at Jodrell Bank under Bernard Lovell. Kahn joined the new department in 1952 as a Turner and Newall fellow, and was appointed in succession lecturer (1955), senior lecturer (1958), reader (1962), and professor (1966). He was elected a fellow of the Royal Society in 1993 and retired in the same year as professor emeritus. He continued active in research until his death.
Kahn's DPhil dissertation on solar streams marked the beginning of his seminal theoretical work on astrophysical gas dynamics. His classic paper 'The acceleration of interstellar clouds', published in the Bulletin of the Astronomical Institute of the Netherlands in 1954, analysing the effect of radiation from hot stars on interstellar gas, was written during a year in Leiden, where he collaborated with Jan Oort, Henk van der Hulst, Lyman Spitzer, and Bengt Strömgren. Photoionization by ultraviolet light from hot stars occurs in narrow zones; Kahn introduced the widely used classification of these ionization fronts into four types, according to their velocities of propagation. Many papers on related subjects followed; a frequently quoted paper (published in Astronomy and Astrophysics in 1976) was on the temperature and cooling rate of supernova remnants. On the larger scale of the dynamics of whole galaxies, Kahn suggested that the collective effects of many supernova explosions may be to drive hot gas away from the disc of a spiral galaxy, creating a 'galactic fountain': this was the subject of an introductory lecture to a symposium of the International Astronomical Union (IAU), subsequently published with its proceedings in 1991. Other especially notable research topics were the formation of massive stars by accretion within dusty molecular clouds, the electrodynamics of pulsar magnetospheres, and the effect of intergalactic gas on the dynamics of the local group of galaxies.
In 1975 Franz and Carla Kahn wrote an important historical paper, published in Nature that year, on the correspondence between Einstein and de Sitter, which they found in the archives of the Leiden observatory during a sabbatical year. In a series of letters, dated 1916-18, Einstein asked de Sitter to make his relativity theory more widely known; in England de Sitter later published papers in the Monthly Notices of the Royal Astronomical Society, which attracted the attention of A. S. Eddington. The letters included discussion of cosmical repulsion during a period when Einstein was introducing the so-called cosmical constant.
Kahn was an inspiration to all astrophysicists, contributing at least as much by his quiet and thoughtful remarks at discussion meetings as in his formal papers. He was a council member of the Royal Astronomical Society (1967-70) and an editor of the Monthly Notices of the Royal Astronomical Society (1993-8). He was president of the IAU's commission on interstellar matter (1970-73), and the IAU named an asteroid, Kahnia, after him. He also served on committees of the Science and Engineering Research Council, chairing the theory panel (1976-9), the panel for allocation of telescope time (1979-81), and the astronomy II committee (1981-4). Following the death of his wife, Carla, in 1981 he developed a warm friendship with Junis Davis, but did not remarry. He died suddenly of a heart attack in Bourne End, Buckinghamshire, on 8 February 1998 and was buried in the south Manchester Jewish cemetery. He was survived by his four children.
F. GRAHAM-SMITH
Sources
J. E. Dyson and D. Lynden-Bell, Memoirs FRS, 45 (1999), 255-67
J. Dyson, 'Franz Daniel Kahn', Astronomy and Geophysics, 39 (1998), 32-3
The Independent (7 March 1998)
The Guardian (16 March 1998)
WWW
personal knowledge (2004)
private information (2004)
m. cert.
d. cert.
Likenesses
photograph, repro. in Memoirs FRS, 254
photograph, repro. in The Independent
Wealth at death
£261,068: probate, 13 May 1998, CGPLA Eng. & Wales
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