Obituary: Professor Franz Kahn

FRANZ KAHN was a distinguished astrophysicist, a world leader in theoretical studies of the physics and the dynamics of the interstellar medium.

His special gift was the ability to illustrate the essentials of a complicated problem by the formulation of models which were both mathematically tractable and which retained the essentials of the system under study. His lifelong activity in Manchester's Department of Astronomy as colleague to the late Professor Zdenek Kopal ensured that Manchester theoretical studies would complement the radio observations emanating from Jodrell Bank, with which he in particular established close links.

Kahn was of German-Jewish stock. His parents Siegfried and Grete Kahn lived in Nuremberg, where Siegfried was a successful manufacturer of children's toys, including Trix model railways, of quality similar to the British Hornby class. Recognising the way Germany was developing in the Thirties, the Kahns organised their affairs so that in 1938 they could bring Franz and his elder sister Charlotte to England. Franz was enrolled at St Paul's School, London, where he soon showed his mettle, even winning the form prize for English after only a couple of years in England.

In 1944 he went up to Queen's College, Oxford, gaining a first in Mathematics in 1947. The following year he transferred to Balliol College while working as a research student under the supervision of Sydney Chapman, a pioneer of cosmical electrodynamics, and in particular of solar-terrestrial relations. Kahn's DPhil thesis was on the expulsion of ions and electrons from the Sun by solar flares.

In 1949, Kahn moved to Manchester as Assistant Lecturer in Mathematics. In 1952 he transferred as Research Fellow to the newly forming Department of Astronomy headed by Kopal; there he remained for the rest of his life, becoming successively Lecturer, Senior Lecturer, Reader and in 1966 Professor. He was given the title Emeritus Professor on his retirement in 1993. He was an excellent supervisor of research students, many of whom have gone on to leading positions round the world.

Kahn worked in many areas in astronomy, and was particularly well known for his contributions to the physics and dynamics of the interstellar medium. A crucial question is that of the energy sources that maintain the highly supersonic motions of the interstellar gas clouds. Sir Arthur Eddington, Bengt Stromgren and others had earlier pointed out that the ultra-violet part of the spectrum of radiation emitted by newly formed, hot stars would ionise neighbouring hydrogen (the "photo-electric effect"), so increasing the temperature by a factor of a hundred or more.

Jan Oort of Leiden noted that the associated high pressure of such an "HII-zone" would inevitably accelerate gas clouds, and, in one of his earlier papers, Kahn gave an elegant mathematical study of the process. In another paper he pointed out that, equally, the input of heat due to the dissipation of the kinetic energy of clouds by mutual collision must be included in temperature estimates, especially of the cooler, neutral "HI" clouds. In a seminal paper he showed that in estimating the efficiency of collisions between rarified ionised clouds, a "collective", many-particle treatment is essential.

In 1958, he was the joint winner of a competition, set by the German Society of Scientists and Physicians, on "Star Formation through the Condensation of Diffuse Matter", his essay demonstrating that his skill as a synthesiser of ideas matched his physical understanding and his analytical expertise.

Kahn's later papers include important contributions to our understanding of "planetary nebulae", emitted during the late stages of stellar evolution; the remnants of exploding stars ("supernovae"); and to the electrodynamics of "pulsars" - rapidly rotating collapsed magnetic stars.

His many other publications include a discussion of "Life in the Universe", and a joint paper in Nature, with his wife Carla, summarising a collection of letters from Alfred Einstein to the Dutch astronomer Willem de Sitter, discovered in the Leiden archives by Carla during one of their several visits to the Netherlands. They were written mainly during the First World War, when the publication of Einstein's relativistic theory of gravitation had led to the first papers on modern cosmology - the study of the Universe as a whole.

The paper makes especially interesting reading today, when Einstein's "cosmical repulsion", opposing gravity and becoming large at large distances - described later by Einstein himself as his "biggest mistake" - has nevertheless again surfaced in cosmological discussion.

Kahn had married Carla Copeland, whom he had met shortly after coming to Manchester, in 1951. She was herself a graduate in Mathematics from Bedford College, London. They were a very happy couple, enjoying each other's company and that of their two sons and two daughters. Once her children were at school, Carla returned to mathematics teaching. But in 1981, when she was applying for a new post as warden in a university hall of residence, she quite unexpectedly suffered a stroke and died the next day, aged only 52. Franz Kahn showed exceptional strength of character in adapting to the new life that had been forced on him. In the words of his children, he was grateful for the happy life he had had with Carla, and felt it would be impertinent to ask for more.

In the introduction to his paper on the dynamics of the galactic fountain, published in a Festschrift for Zdenek Kopal, he wrote: "It is a commonplace that life can be cruel. But fortunately fate only rarely administers so devastating a blow. I dedicate this paper - the first scientific work that I have attempted since Carla's death - to her memory."

Franz Kahn was an engaging companion, both professionally and socially, wearing his erudition lightly. Like all academics, he appreciated recognition, but never let delay in its appearance worry him unduly. He was a happy man, enjoying each stage of his life as it came. His end was, like his wife's, sudden - he had a heart attack at a petrol station while travelling from one son to another.

Franz Daniel Kahn, astrophysicist: born Nuremberg, Germany 13 May 1926; Assistant Lecturer in Mathematics, Manchester University, 1949- 52, Turner Newall Fellow 1952-55, Lecturer in Astronomy 1955-58, Senior Lecturer 1958-62, Reader 1962-66, Professor of Astronomy 1966-93 (Emeritus); President, International Astronomical Union Commission on Inter- stellar Matter 1970-73; Chairman, SERC Astronomy Theory Panel 1976-79; FRS 1993; married 1951 Carla Copeland (died 1981; two sons, two daughters); died Bourne End, Buckinghamshire 8 February 1998.