Franz Kahn

Guardian obituary

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Under Franz Kahn, who has died aged 71, Manchester University's department of astronomy became a world-famous centre for the study of astrophysics. Hie was one of his generation's most widely respected theoretical astro-physicists and an authority on the study of the complex gas flows which occur around and between the stars in a galaxy such as our own.

He could reduce complex problems to deceptively simple forms through insight and mathematical analysis. Many astrophysicists used him as a sounding board and approval from Kahn was approval indeed.

He was horn in Nuremberg, into a German-Jewish toymaking family which settled in London in 1938. After only two years'residence, be won the form prize for English at St Paul's boys'school. In 1947, he graduated from Queen's College. Oxford, with first- class honours in mathematirs, and then studied at Balliol under Sydney Chapman, one of the great pioneers of solar- terrestrial relationships. Kahn's doctorate Investigated the production of fast particles in solar Dares and their subse- quent interaction with the Earth's atmosphere.

In 1949. he took up an assistant mathematics lectureship at Manchester University, then establishing Itself in world astronomy. Initially, the emphasis was on radio astron- omy under Professor Bernard Lovell, who was aware that his field needed dose liaison with other areas of astronomy. In

1951. the late Professor Z Kopal took up the first astronomy chair at Manchester and, in

1952, Kahn transferred to this new department, where he remained for the rest of his academic life, becoming professor of astronomy in 1967. and retiring in 1993.

Kahn worked In theoretical astrophysics and In theoreti- cal plasma physics. He pro- duced significant papers on the radiation produced by en- ergetic particles entering the earth's atmosphere and by the motion of ultra-relativistic particles in the atmospheric magnetic fields of pulsars -- rotating ultra-dense neutron stars -- and the radio emission from dares on the sur- faces of stars.

In 1958. he won a prestigious German prize for an essay. The Formation of Stars through the Condensation of Diffuse Matter. In spite of all the advances made in this area over the last 40 years, the essay is still worth r eadin g. Elegance and simplicity were his trademarks. Under his leadership, Manchester Uni- versity and cosmical gas dy- namics became synonymous for astronomers worldwide. In the 1930s, several astronomers had pointed out that energetic radiation from young massive stars would heat the gas be- tween tiie stars --the "inter- stellar medium" --from a temperature of about 100 degrees above absolute zero, to a temperature of about 10.000 degrees. But the early work neglected the fact that the gas pressure increased by about the same factor and that the hot gas and any contigu- ous unheated gas would be accelerated.

In 1954, Kahn produced one of contemporary astrophysics' classic papers, laying down the basic theory for the way in which stellar radiation sweeps across interstellar gas, heat- ing it and setting it in motion. This paper is central to an understanding of star forma- tion. the late stages of evolu- tion of intermediate mass stars and phenomena in the nuclei of active galaxies. Recent Hubble photographs of regions of massive star forma- tion, like the now famous Pil- lars of Creation in the Eagle Nebula, rely on the basic Kahn theory for their interpretation.

Kahn wrote many other significant papers. Planetary neb- ulae are the radiation-heated envelopes surrounding very hot stars, which are rap- idly evolving to the white dwarf stage. The envelopes are ejected when the parent stars -- which generally are a few times more massive than the sun -- go through the red giant stage after leaving the main sequence. Their shapes are complex but there are some regularities.

Kahn realised that the hot envelopes are shaped by the interaction of a fast wind from the hot central star, with non-spherical envelopes ejected in the red giant stage. He produced an elegant analytic model for an interaction which Is now normally tackled with super-computers. (He was probably one of the last theoretical astrophysicists to use a slide rule, which he relinquished only a few years before retirement his chil- dren gave him a calculator).

Kahn also did important work on the interaction of exploding stars (supernovae) with their surroundings. He was particularly interested in the collective effects of super-novae in galaxies, where the energy input from a succes- sion of supernovae drives hot gas well above the galactic plane, a "galactic fountain".

He made scientific criti- cism helpful, courteous and generous. His ideas given in conversation, meetings and seminars probably far out- number his many published contributions. In 1993. he be- came professor emeritus, and a fellow of the Royal Society. He was also an editor for tbe Royal Astronomical Society's main journal. Monthly Notices. He never stopped working.

In 1950, be met and, within five days, got engaged to Carla Copeland. Their excep- tionally happy marriage last- ed until Carla's death in 1981. His family were a constant source of bappiness. their four children and seven grandchildren, were probablv his proudest achievement. He never remarried but later found great friendship with Junis Davis.