David Stanley Evans (1916-2004)
We are sorry to have to report the death on 14 November 2004 in Austin, Texas, of David S. Evans who, for over twenty years, played an important and stimulating role in South African astronomy. When he came to South Africa in 1946, fundamental positional determinations and photometry dominated the astronomical scene. By the time he left, the Radcliffe Observatory and the Royal Observatory Cape were active in astrophysics and the efforts that eventually led to the establishment of SAAO were well under way. He played an important role in these developments.
David Stanley Evans was born in Cardiff, Wales, on 28 January 1916, the second son of Arthur Cyril Evans and his wife Kate (née Priest). He was educated at the Cardiff High School for Boys and King's College, Cambridge, where he was a "Major Scholar" in 1934. He obtained a First Class in the Mathematics Tripos Part II in 1936 and a Distinction in Part III in 1937. He was a PhD student at the Cambridge Observatory in 1937-8 under the supervision of R. v. d. R. Woolley. He obtained the degrees of MA and PhD in 1941, his thesis topic being "The formation of the Balmer Series of Hydrogen in Stellar Atmospheres". He worked as a medical physicist with Kurt Mendelssohn during World War II.
He held an appointment as a research assistant at the University Observatory in Oxford from 1938 to 1946, where he worked on solar spectroscopy and had as a colleague Madge Adam. He also acted as scientific advisory editor of the magazine Discovery, a precursor of the New Scientist. From October 1941 to December 1945 he was one of the editors of The Observatory.
From 1946 to 1951 he was Second Assistant at the Radcliffe Observatory in Pretoria, where Harold Knox-Shaw was the Radcliffe Observer and only other member of the staff, apart from three unskilled labourers. The mechanical parts of the 74-inch telescope had been erected in 1938 but the primary and the first secondary mirror were only ready for use in 1948. Evans had to assist with their installation, including a modification to the main cell that was necessary because the mirror was sligtly thinner than planned. Although it was the largest telescope in the southern hemisphere, the use of the Radcliffe reflector was for the first two years confined to photography at the Newtonian focus. Evans therefore worked on photographic photometry of elliptical galaxies and published a photographic catalogue of planetary nebulae in collaboration with A.D. Thackeray, who had been appointed as Chief Assistant in 1948. On the technical side, he constructed an aluminising plant capable of coating mirrors up to 15 inches diameter.
In 1949 he married Betty Hall Hart, daughter of Lt-Col R.C.H. Hart and Annie (née Hall).
In March 1951 Evans joined the Royal Observatory, Cape of Good Hope, as Chief Assistant, under R.H. Stoy. At this time, an agreement was struck between the Admiralty and the Radcliffe Trustees under which, in exchange for a subsidy, the Royal Observatory received access to a third of the time on the 74-inch. Evans was in charge of the programmes which made use of the Cape share. Their main purpose was to obtain photometry and spectroscopy of the nearby stars.
Among other activities, he obtained Newtonian photographs of southern galaxies which were published in a limited, photographically-reproduced, "Cape Photographic Atlas of Southern Galaxies". He became involved in early occultation measurements of stellar angular diameters using a photoelectric photometer and high-speed recording. His results suggested that Antares was not symmetrical or was spotted. Although this work was received with scepticism at the time, more recent observations have shown that he was right. In addition, he designed and had built a Newtonian spectrograph for the Radcliffe telescope. This was used to obtain galaxy redshifts (partly in collaboration with Stuart Malin). He was also involved in the selection of the Sutherland site for what was to become the South African Astronomical Observatory.
In 1965-66 he was a National Science Foundation Senior Visiting Scientist Fellow at the University of Texas at Austin. On 4 October 1968 he resigned from the Royal Observatory, where he had by then attained the UK Civil Service rank of SPSO (Senior Principal Scientific Officer), and moved to Texas permanently. There he became a Professor and the Associate Director of Macdonald Observatory. In Texas, he worked with Nather and Warner, among others, on high-speed photometry, particularly of occultations. In 1973 he organised an eclipse expedition to Mauretania to obtain the general relativistic deflection of apparent stellar positions caused by the solar gravitational field. In Texas, he had as students Bernard W. Bopp, Thomas J. Moffett, Francis C. Fekel and C.H. Lacy.
An important result of significance to the cosmic distance scale, arising from his occultation interests, was the establishment of the Barnes-Evans relation which connects the surface brightness of a star with its V–R colour. This enables the radii and distances of Cepheid pulsating variables to be determined from light, colour and radial velocity measurements around their pulsation cycles.
In 1971 he received the ScD degree. During the years 1977-86, he was Jack S. Josey Centennial Professor in the University of Texas. He became an emeritus Professor in 1986.
Evans was a highly sociable person and was a member of many clubs and societies, professional and otherwise. He held the positions of President of the the Radial Velocity Commission of the International Astronomical Union and Secretary of its Galaxy Commission. He received the Tyson medal and a Raleigh prize from Cambridge University and the Gill Medal and Macintyre award from the Astronomical Society of Southern Africa. On retirement, he and his wife were made honorary citizens of Texas by the Governor.
He is survived by his wife and two sons, Jonathan Gareth Weston Evans of Nashville, Tennessee and Barnaby Huw Weston Evans of Austin, Texas. Also his brother Arthur Evans of Cardiff, Wales. A memorial service was held on 18 November at the University of Texas Campus Club.