David Stanley Evans
Encyclopedia
David Stanley Evans was a British astronomer, noted for his use of lunar occultations to measure stellar angular diameter
s during the 1950s.
, Wales
on 28 January 1916. He was first educated at the Cardiff High School for Boys. He obtained a First Class in the Mathematics Tripos Part II in 1936 and a Distinction in Part III in 1937 from King's College, Cambridge
and became a Ph.D. student at Cambridge Observatory
in 1937, where he was a student of Sir Arthur Eddington. His Ph.D. degree was awarded in 1941 for a dissertation on “The Formation of the Balmer Series
of Hydrogen in Stellar Atmospheres.” Being a conscientious objector to World War II
he spent the war years at Oxford with physicist Kurt Mendelssohn
where they worked on medical problems relating to the war effort. Over this period he was scientific editor of Discovery and editor of The Observatory.
, South Africa
when positional determinations and photometry were the main interests of the astronomical world, but when he left some twenty years later, the South African observatories had become active in astrophysics. Together with Harold Knox-Shaw
he aluminised and installed the mirrors in the 74 inches (1.9 m) telescope. He determined the angular diameter of Antares
and also wrongly came to the conclusion that Arcturus was elliptical in shape. This was later found to be an observational artifact, but the feasibility of measuring stellar diameters using lunar occultations, was soundly established. At this time Evans had become chief assistant at the Royal Observatory in Cape Town
, South Africa. He designed and oversaw construction of a Newtonian spectrograph
for the 74 inches (1.9 m) Radcliffe Telescope with which he measured the first southern galaxy redshifts.
in 1965-66, when he was National Science Foundation
Senior Visiting Scientist at The University of Texas and McDonald Observatory
. This visit led to a permanent move to Austin in 1968, and he became professor of astronomy and associate director of McDonald Observatory. At McDonald Observatory R. E. Nather with Brian Warner developed a photometer able to measure extremely rapid changes in brightness. Their technique led Evans to look afresh at his occultation research and, for the next two decades he and his co-workers calculated the angular diameters of late-type stars. He also wrote "Herschel
at the Cape,” and participated in measurements of the occultation of Beta Scorpii
by Jupiter
in 1972 and the apparent gravitational displacement of stars visually close to the Sun during a solar eclipse in 1973. The eclipse was observed from Mauritania
and once again confirmed Einstein’s predictions.
Evans and his fellow researchers studied late-type star
s showing large starspot
s, and those subject to flares. They also turned their attention to double stars and multiple star
s revealed by lunar occultation. Evans’ major contribution to astronomy was using the angular diameters of stars to calculate their surface brightness
. This relation extended to stars which lay away from the ecliptic
and could not be occulted by the Moon, as well as to Cepheid variables, yielding their distances. The relation between angular diameter and V-R colour index is termed the Barnes-Evans Relation, which is calibrated by using direct diameter observations of Cepheid variables. Using these relations the distance is calculated to delta Cephei
, and compared with an independent distance derived from trigonometric parallax measurements by the Hubble Space Telescope
- the two measurements agree to within a few percent.
Evans was appointed as Jack S. Josey Centennial Professor of Astronomy in 1984, a position he held until his retirement in 1986. He received the Gill Medal of the Astronomical Society of South Africa in 1988. He wrote eight books including an introduction to astronomy.
Evans died in Austin on 14 November 2004. At the time of his death, he had completed a book with Karen Winget on the eclipse expedition to Mauritania.
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
s during the 1950s.
Early life and education
Evans was born in CardiffCardiff
Cardiff is the capital, largest city and most populous county of Wales and the 10th largest city in the United Kingdom. The city is Wales' chief commercial centre, the base for most national cultural and sporting institutions, the Welsh national media, and the seat of the National Assembly for...
, Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
on 28 January 1916. He was first educated at the Cardiff High School for Boys. He obtained a First Class in the Mathematics Tripos Part II in 1936 and a Distinction in Part III in 1937 from King's College, Cambridge
King's College, Cambridge
King's College is a constituent college of the University of Cambridge, England. The college's full name is "The King's College of our Lady and Saint Nicholas in Cambridge", but it is usually referred to simply as "King's" within the University....
and became a Ph.D. student at Cambridge Observatory
Cambridge Observatory
Cambridge Observatory is an astronomical observatory at the University of Cambridge in the East of England. It was first established in 1823 and is now part of the site of the Institute of Astronomy...
in 1937, where he was a student of Sir Arthur Eddington. His Ph.D. degree was awarded in 1941 for a dissertation on “The Formation of the Balmer Series
Balmer series
The Balmer series or Balmer lines in atomic physics, is the designation of one of a set of six different named series describing the spectral line emissions of the hydrogen atom....
of Hydrogen in Stellar Atmospheres.” Being a conscientious objector to World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
he spent the war years at Oxford with physicist Kurt Mendelssohn
Kurt Mendelssohn
Kurt Alfred Georg Mendelssohn FRS was a German-born British medical physicist, elected a Fellow of the Royal Society 1951.He was a great-great-grandson of Saul Mendelssohn, the younger brother of philosopher Moses Mendelssohn...
where they worked on medical problems relating to the war effort. Over this period he was scientific editor of Discovery and editor of The Observatory.
South Africa
Evans left England in 1946 to work at the Radcliffe Observatory, PretoriaPretoria
Pretoria is a city located in the northern part of Gauteng Province, South Africa. It is one of the country's three capital cities, serving as the executive and de facto national capital; the others are Cape Town, the legislative capital, and Bloemfontein, the judicial capital.Pretoria is...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
when positional determinations and photometry were the main interests of the astronomical world, but when he left some twenty years later, the South African observatories had become active in astrophysics. Together with Harold Knox-Shaw
Harold Knox-Shaw
Harold Knox-Shaw was an English astronomer.He was born in St Leonards-on-Sea, Sussex as the oldest of four siblings. During his youth he earned scholarships to Wellington College in Berkshire and to Trinity Hall, Cambridge, from which he graduated in 1907 ranked as Sixth Wrangler...
he aluminised and installed the mirrors in the 74 inches (1.9 m) telescope. He determined the angular diameter of Antares
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...
and also wrongly came to the conclusion that Arcturus was elliptical in shape. This was later found to be an observational artifact, but the feasibility of measuring stellar diameters using lunar occultations, was soundly established. At this time Evans had become chief assistant at the Royal Observatory in Cape Town
Cape Town
Cape Town is the second-most populous city in South Africa, and the provincial capital and primate city of the Western Cape. As the seat of the National Parliament, it is also the legislative capital of the country. It forms part of the City of Cape Town metropolitan municipality...
, South Africa. He designed and oversaw construction of a Newtonian spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
for the 74 inches (1.9 m) Radcliffe Telescope with which he measured the first southern galaxy redshifts.
Austin, Texas
He and his family visited Austin, TexasAustin, Texas
Austin is the capital city of the U.S. state of :Texas and the seat of Travis County. Located in Central Texas on the eastern edge of the American Southwest, it is the fourth-largest city in Texas and the 14th most populous city in the United States. It was the third-fastest-growing large city in...
in 1965-66, when he was National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
Senior Visiting Scientist at The University of Texas and McDonald Observatory
McDonald Observatory
The McDonald Observatory is an astronomical observatory located near the unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Fowlkes and Mount Locke in the Davis Mountains of West Texas...
. This visit led to a permanent move to Austin in 1968, and he became professor of astronomy and associate director of McDonald Observatory. At McDonald Observatory R. E. Nather with Brian Warner developed a photometer able to measure extremely rapid changes in brightness. Their technique led Evans to look afresh at his occultation research and, for the next two decades he and his co-workers calculated the angular diameters of late-type stars. He also wrote "Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...
at the Cape,” and participated in measurements of the occultation of Beta Scorpii
Beta Scorpii
Beta Scorpii is a star system in the constellation Scorpius. It has the traditional names Acrab, Akrab or Elacrab, all come from al-'Aqrab, the Scorpion, for the whole constellation, as well as Graffias, a name it shares with Xi Scorpii...
by Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
in 1972 and the apparent gravitational displacement of stars visually close to the Sun during a solar eclipse in 1973. The eclipse was observed from Mauritania
Mauritania
Mauritania is a country in the Maghreb and West Africa. It is bordered by the Atlantic Ocean in the west, by Western Sahara in the north, by Algeria in the northeast, by Mali in the east and southeast, and by Senegal in the southwest...
and once again confirmed Einstein’s predictions.
Evans and his fellow researchers studied late-type star
Late-type star
In stellar classification, a late-type star is a star of class K or class M. The term was coined in the early 20th century, when there was a belief that stars began their history as early-type stars of class O, B, or A, and subsequently cooled to late-type stars....
s showing large starspot
Starspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...
s, and those subject to flares. They also turned their attention to double stars and multiple star
Multiple star
A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...
s revealed by lunar occultation. Evans’ major contribution to astronomy was using the angular diameters of stars to calculate their surface brightness
Surface brightness
The overall brightness of an extended astronomical object such as a galaxy, star cluster, or nebula, can be measured by its total magnitude, integrated magnitude or integrated visual magnitude; a related concept is surface brightness, which specifies the brightness of a standard-sized piece of an...
. This relation extended to stars which lay away from the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
and could not be occulted by the Moon, as well as to Cepheid variables, yielding their distances. The relation between angular diameter and V-R colour index is termed the Barnes-Evans Relation, which is calibrated by using direct diameter observations of Cepheid variables. Using these relations the distance is calculated to delta Cephei
Delta Cephei
Delta Cephei is a binary star system approximately 891 light-years away in the constellation of Cepheus . Delta Cephei is the prototype of the Cepheid variable stars, and it is among the closest stars of this type to the Sun...
, and compared with an independent distance derived from trigonometric parallax measurements by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
- the two measurements agree to within a few percent.
Evans was appointed as Jack S. Josey Centennial Professor of Astronomy in 1984, a position he held until his retirement in 1986. He received the Gill Medal of the Astronomical Society of South Africa in 1988. He wrote eight books including an introduction to astronomy.
Evans died in Austin on 14 November 2004. At the time of his death, he had completed a book with Karen Winget on the eclipse expedition to Mauritania.
Books
- Astronomy (Teach Yourself)- David Stanley Evans (Hodder & Stoughton Ltd 1975) ISBN 0340152486
- The Eddington Enigma - David Stanley Evans (Xlibris Corporation 1998) ISBN 0738801313
- Herschel at the Cape. Diaries and correspondence of Sir John Herschel 1834-1838 Evans, D. S., Deeming, T. J., Evans, B. H. & Goldfarb, S. (AA Balkema, Cape Town 1969) ISBN 292-78387-6.