MacTutor
Franz Daniel Kahn
RAS obituary
Fellow and Councillor of the RAS, distinguished theoretical astrophysicist, gifted teacher and valued colleague.
Franz Kahn was one of the most widely liked and respected theoretical astro-physicists of his generation. Although he worked in various areas, he is perhaps best known for his fundamental work in cosmical gas dynamics. Under him the Astronomy Group at Manchester University became world famous for such studies. The characteristic feature of his work was his ability to reduce complex problems to deceptively simple forms through a rare combination of physical insight and the mathematical skills he first acquired as an undergraduate at Oxford. His influence ranged far beyond his specific research areas, and many astrophysicists used Franz as a sounding-board for their ideas. Approval from Franz was a fair guide to the validity of ideas!
Franz was born into a German-Jewish family on 13 May 1926, the son of Siegfried and Grete Kahn of Nuremberg. His father was a toy maker, who made, amongst other items, the scooters Franz used to ride around the city walls of Nuremberg.
The influence of his father's occupation seemed to have lasted all his life, although Franz used to try to explain his many visits to Hamley's toy shop in London as being for the benefit of his children and grandchildren. In 1938 his parents took the decision to leave Germany, and the family, including his elder sister Charlotte, settled in London. Franz became a pupil at St Paul's School and rapidly showed his academic qualities when he won the form prize for English after only two years in England.
From St Paul's School, Franz proceeded to Queen's College, Oxford, where he graduated with a first class degree in mathematics in 1947. He then studied at Balliol College under the late Sydney Chapman, who was one of the great pioneering names in the area of solar-terrestrial relationships. His DPhil thesis was an investigation into the production of fast particles in solar flares and their subsequent interaction with the Earth's atmosphere. He once commented with evident amusement that even such a distinguished scientist as Chapman had not noticed that in his thesis he had used incorrect values for the statistical weights of certain energy levels of sodium!
The defining events of his professional and personal life took place following his graduation. In 1949, he took up an assistant lectureship in mathematics at Manchester University. The following year he met, and within five days got engaged to, Carla Copeland. Their marriage was an exceptionally happy one that lasted until Carla's tragically early death in 1981.
Manchester University was at that time establishing itself as a major world centre for astronomy. Initially, the emphasis was in the area of radio astronomy under Sir Bernard Lovell. Lovell was very much aware that radio astronomy needed close liaison with other areas of astronomy, and in 1951 he was instrumental in attracting the late Prof. Zdenek Kopal from Harvard College Observatory to take up the first Chair of Astronomy at Manchester. Kopal started to set up his department of astronomy, and Franz transferred into it in 1952 as a Turner and Newall research fellow. He remained there for the rest of his life, becoming in rapid succession lecturer, senior lecturer, reader and then professor of astronomy in 1967. He took over as head of the Astronomy Group when Kopal retired in 1981. Franz became professor emeritus when he retired in 1993 and that year he was elected a fellow of the Royal Society. He never stopped working, and right up to his death was busy writing papers, supervising research students and working with collaborators from around the world, including China and Portugal. He was also active as an editor for Monthly Notices. Franz served on many scientific committees, including those of the then SERC, and was president of Commission 34 (Interstellar Matter) of the International Astronomical
Union 1970-73. He was a Council member of the RAS from 1967-70. One of the distinctions which he received and which gave him much pleasure and amusement was to have an asteroid, Kahnia, named after him in 1991.
Franz worked in many areas of theoretical astrophysics and also in theoretical plasma physics. During his career he produced significant papers on radiation from cosmic-ray air showers, from pulsar magnetospheres and from flares on the surfaces of stars. He also wrote about instabilities of plasma waves, galactic-scale dynamics of molecular clouds, and galactic fountains. An excellent example of Franz's style is an early paper he wrote on the interpretation of 21 cm absorption-line data where he very simply and elegantly showed that the inferred temperature of the absorbing gas has to be interpreted as a geometric mean temperature. In 1958 Franz won a prestigious prize from the German Society of Scientists and Physicians for an essay entitled The Formation of Stars through the Condensation of Diffuse Matter. (Since the submission had to be made under a pen-name to maintain anonymity for the judging, he chose the rather appropriate one of "Canopus".) In spite of all the advances, both real and apparent, in this area over the last 40 years, the essay is still worth reading for its insight and the clarity of the treatment.
However, the area with which he (and the Manchester Group) will always be associated is that of cosmical gas dynamics. As Bruce Balick remarked in a lecture given at the meeting held in Manchester in 1993 to mark Franz's retirement, talking about cosmical gas dynamics in Manchester was "a real example of carrying coals to Newcastle!". Three particular examples stand out.
In the 1930s, several astronomers, in particular Bengt Strömgren in Sweden, had pointed out that the hard UV radiation from early type stars would create hot (temperature of about 10 K) regions of ionized gas (later called "Strömgren spheres") around them. This work concerned only the heating and spatial extent of these regions but did not take into account the dynamical effects resulting from the fact that the hot gas was vastly overpressurized with respect to the cool neutral surroundings. In 1954, Franz produced one of the classic papers of 20th-century astrophysics in which he laid down the basic ideas of ionization-front propagation, and in subsequent work he used these ideas to treat the dynamics of HII region expansion. He introduced the now famous classification of ionization fronts into two kinds, R-type ("rarefied") and D-type ("dense"). (This 1954 paper inspired two major papers in the early 1960s on ionization-front propagation by Allin Goldsworthy and ionization-front structure by Ian Axford, who at that time were both in the mathematics department at Manchester.) The work initiated by Franz is central to an understanding of areas such as star formation, planetary nebula dynamics, and even galactic superwind generation. It is interesting to note that the interpretation of some of the most spectacular Hubble Space Telescope data (e.g. the "Pillars of Creation" in M16 and the proplyds in the Orion Nebula) is largely based on Franz's work. Franz was also the first person to explore the dynamics of the dusty envelopes around recently formed massive stars. In 1974 he produced a model for cocoon stars where the effects of the stellar radiation field on both the gas and dust components of the accreting gas were taken into account.
Later, he became interested in the global dynamics of planetary nebulae. He was particularly concerned with the way the shapes of these nebulae are determined by the interaction of the fast wind from the hot central stars with the non-spherical envelopes ejected during the red-giant and asymptotic giant branch phases of the central star's evolution. In 1985, he wrote (with Kym West) a particularly elegant paper where he reproduced the expansion dynamics of the envelopes by considering a model based on simple angular momentum considerations. Although present studies of such interactions are now carried out by heroic numerical studies, this paper anticipated most of their believable results. He also (with Dieter Breitschwerdt) wrote the first papers which explored the dynamical effects (e.g., the importance of Rayleigh-Taylor instabilities) produced when the interactions of time-dependent fast winds are considered. Incidentally, Franz was probably one of the last theoretical astrophysicists to use a slide rule, which he used almost until retirement. He reluctantly gave this up for a pocket calculator which was given to him by his children.
Franz was an excellent teacher who enjoyed supervising research students (although he certainly made them work!). It is no coincidence that some of his research students (e.g., Wal Sargent, John Hazlehurst, and Leon Lucy) also had distinguished careers in astronomy. Franz had the great gift of making scientific criticism both helpful and courteous. He was a generous person whose ideas given at meetings, seminars, or even in the local pub must far outnumber the many contributions published actually in his name. He was a delightful companion and a very sociable person who was fond of good company, food, and, in particular, malt whisky (of which his knowledge was as extensive as his knowledge of astrophysics).
Franz was a devoted family man, and his family was a constant source of happiness to him. He and Carla had four children; they, and, later, their seven grandchildren, were probably his proudest achievement. Carla's early death hit him very badly. It coincided with the time he took over the group at Manchester, and when he was in the process of compiling a "Festschrift" to mark Zdenek Kopal's retirement. The introduction to his contribution on "The Galactic Fountain" in that volume very movingly recounts that this paper contained the first work he had attempted since Carla died, and the paper is dedicated to her memory. Franz never remarried, but in later years he found great friendship with Junis Davis, which lasted until his very sudden death from a heart attack while he was visiting his family in London on February 8 of this year. Franz left behind a legacy of scientific achievement and many friends who mourn him. The old phrase "a scholar and a gentleman" could have been coined just for him.
John Dyson
Franz was born into a German-Jewish family on 13 May 1926, the son of Siegfried and Grete Kahn of Nuremberg. His father was a toy maker, who made, amongst other items, the scooters Franz used to ride around the city walls of Nuremberg.
The influence of his father's occupation seemed to have lasted all his life, although Franz used to try to explain his many visits to Hamley's toy shop in London as being for the benefit of his children and grandchildren. In 1938 his parents took the decision to leave Germany, and the family, including his elder sister Charlotte, settled in London. Franz became a pupil at St Paul's School and rapidly showed his academic qualities when he won the form prize for English after only two years in England.
From St Paul's School, Franz proceeded to Queen's College, Oxford, where he graduated with a first class degree in mathematics in 1947. He then studied at Balliol College under the late Sydney Chapman, who was one of the great pioneering names in the area of solar-terrestrial relationships. His DPhil thesis was an investigation into the production of fast particles in solar flares and their subsequent interaction with the Earth's atmosphere. He once commented with evident amusement that even such a distinguished scientist as Chapman had not noticed that in his thesis he had used incorrect values for the statistical weights of certain energy levels of sodium!
The defining events of his professional and personal life took place following his graduation. In 1949, he took up an assistant lectureship in mathematics at Manchester University. The following year he met, and within five days got engaged to, Carla Copeland. Their marriage was an exceptionally happy one that lasted until Carla's tragically early death in 1981.
Manchester University was at that time establishing itself as a major world centre for astronomy. Initially, the emphasis was in the area of radio astronomy under Sir Bernard Lovell. Lovell was very much aware that radio astronomy needed close liaison with other areas of astronomy, and in 1951 he was instrumental in attracting the late Prof. Zdenek Kopal from Harvard College Observatory to take up the first Chair of Astronomy at Manchester. Kopal started to set up his department of astronomy, and Franz transferred into it in 1952 as a Turner and Newall research fellow. He remained there for the rest of his life, becoming in rapid succession lecturer, senior lecturer, reader and then professor of astronomy in 1967. He took over as head of the Astronomy Group when Kopal retired in 1981. Franz became professor emeritus when he retired in 1993 and that year he was elected a fellow of the Royal Society. He never stopped working, and right up to his death was busy writing papers, supervising research students and working with collaborators from around the world, including China and Portugal. He was also active as an editor for Monthly Notices. Franz served on many scientific committees, including those of the then SERC, and was president of Commission 34 (Interstellar Matter) of the International Astronomical
Union 1970-73. He was a Council member of the RAS from 1967-70. One of the distinctions which he received and which gave him much pleasure and amusement was to have an asteroid, Kahnia, named after him in 1991.
Franz worked in many areas of theoretical astrophysics and also in theoretical plasma physics. During his career he produced significant papers on radiation from cosmic-ray air showers, from pulsar magnetospheres and from flares on the surfaces of stars. He also wrote about instabilities of plasma waves, galactic-scale dynamics of molecular clouds, and galactic fountains. An excellent example of Franz's style is an early paper he wrote on the interpretation of 21 cm absorption-line data where he very simply and elegantly showed that the inferred temperature of the absorbing gas has to be interpreted as a geometric mean temperature. In 1958 Franz won a prestigious prize from the German Society of Scientists and Physicians for an essay entitled The Formation of Stars through the Condensation of Diffuse Matter. (Since the submission had to be made under a pen-name to maintain anonymity for the judging, he chose the rather appropriate one of "Canopus".) In spite of all the advances, both real and apparent, in this area over the last 40 years, the essay is still worth reading for its insight and the clarity of the treatment.
However, the area with which he (and the Manchester Group) will always be associated is that of cosmical gas dynamics. As Bruce Balick remarked in a lecture given at the meeting held in Manchester in 1993 to mark Franz's retirement, talking about cosmical gas dynamics in Manchester was "a real example of carrying coals to Newcastle!". Three particular examples stand out.
In the 1930s, several astronomers, in particular Bengt Strömgren in Sweden, had pointed out that the hard UV radiation from early type stars would create hot (temperature of about 10 K) regions of ionized gas (later called "Strömgren spheres") around them. This work concerned only the heating and spatial extent of these regions but did not take into account the dynamical effects resulting from the fact that the hot gas was vastly overpressurized with respect to the cool neutral surroundings. In 1954, Franz produced one of the classic papers of 20th-century astrophysics in which he laid down the basic ideas of ionization-front propagation, and in subsequent work he used these ideas to treat the dynamics of HII region expansion. He introduced the now famous classification of ionization fronts into two kinds, R-type ("rarefied") and D-type ("dense"). (This 1954 paper inspired two major papers in the early 1960s on ionization-front propagation by Allin Goldsworthy and ionization-front structure by Ian Axford, who at that time were both in the mathematics department at Manchester.) The work initiated by Franz is central to an understanding of areas such as star formation, planetary nebula dynamics, and even galactic superwind generation. It is interesting to note that the interpretation of some of the most spectacular Hubble Space Telescope data (e.g. the "Pillars of Creation" in M16 and the proplyds in the Orion Nebula) is largely based on Franz's work. Franz was also the first person to explore the dynamics of the dusty envelopes around recently formed massive stars. In 1974 he produced a model for cocoon stars where the effects of the stellar radiation field on both the gas and dust components of the accreting gas were taken into account.
Later, he became interested in the global dynamics of planetary nebulae. He was particularly concerned with the way the shapes of these nebulae are determined by the interaction of the fast wind from the hot central stars with the non-spherical envelopes ejected during the red-giant and asymptotic giant branch phases of the central star's evolution. In 1985, he wrote (with Kym West) a particularly elegant paper where he reproduced the expansion dynamics of the envelopes by considering a model based on simple angular momentum considerations. Although present studies of such interactions are now carried out by heroic numerical studies, this paper anticipated most of their believable results. He also (with Dieter Breitschwerdt) wrote the first papers which explored the dynamical effects (e.g., the importance of Rayleigh-Taylor instabilities) produced when the interactions of time-dependent fast winds are considered. Incidentally, Franz was probably one of the last theoretical astrophysicists to use a slide rule, which he used almost until retirement. He reluctantly gave this up for a pocket calculator which was given to him by his children.
Franz was an excellent teacher who enjoyed supervising research students (although he certainly made them work!). It is no coincidence that some of his research students (e.g., Wal Sargent, John Hazlehurst, and Leon Lucy) also had distinguished careers in astronomy. Franz had the great gift of making scientific criticism both helpful and courteous. He was a generous person whose ideas given at meetings, seminars, or even in the local pub must far outnumber the many contributions published actually in his name. He was a delightful companion and a very sociable person who was fond of good company, food, and, in particular, malt whisky (of which his knowledge was as extensive as his knowledge of astrophysics).
Franz was a devoted family man, and his family was a constant source of happiness to him. He and Carla had four children; they, and, later, their seven grandchildren, were probably his proudest achievement. Carla's early death hit him very badly. It coincided with the time he took over the group at Manchester, and when he was in the process of compiling a "Festschrift" to mark Zdenek Kopal's retirement. The introduction to his contribution on "The Galactic Fountain" in that volume very movingly recounts that this paper contained the first work he had attempted since Carla died, and the paper is dedicated to her memory. Franz never remarried, but in later years he found great friendship with Junis Davis, which lasted until his very sudden death from a heart attack while he was visiting his family in London on February 8 of this year. Franz left behind a legacy of scientific achievement and many friends who mourn him. The old phrase "a scholar and a gentleman" could have been coined just for him.
John Dyson
Franz Daniel Kahn's obituary appeared in Journal of the Royal Astronomical Society 39;5 (1998), 32-33.