Édouard Jean-Marie Stephan (31 August 1837 – 31 December 1923) was a French

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

. His surname is sometimes spelled Stéphan in some literature, but this is apparently erroneous. He was born in Sainte Pezenne (today one of the districts of the town of

Niort Niort (; Poitevin: ''Niàu''; oc, Niòrt; la, Novioritum) is a commune in the Deux-Sèvres department, western France. It is the prefecture of Deux-Sèvres. The population of Niort is 58,707 (2017) and more than 177,000 people live in the u ...

) and attended the Ecole Normale Superieure, and graduated at the top of his class in 1862. He was the director of the

Marseille Observatory Marseille Observatory (french: Observatoire de Marseille) is an astronomical observatory located in Marseille, France, with a history that goes back to the early 18th century. In its 1877 incarnation, it was the discovery site of a group of gal ...

from 1864 to 1907 (until 1872 he was subordinate to

Urbain le Verrier Urbain Jean Joseph Le Verrier FRS (FOR) HFRSE (; 11 March 1811 – 23 September 1877) was a French astronomer and mathematician who specialized in celestial mechanics and is best known for predicting the existence and position of Neptune using ...

). In the early part of his career there, he had limited opportunities to do observations because he was preoccupied with improving the observatory. He discovered the

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

89 Julia in 1866. In 1867 he used the new telescope to observe a

transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...

Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

. Between 1870 and 1875, Stephan systematically studied

nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...

e, precisely recording their positions and discovering many new ones. His goal was to enable the exact measurement of stellar

proper motions Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dist ...

by creating a reference system of fixed objects. In 1873, Stephan was the first person to attempt to measure the

angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...

of a

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

using

interferometry Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...

, converting the 80 cm telescope at Marseille Observatory into an interferometer. He did this by obscuring the reflector with a mask containing two vertical slits. The star he chose to perform this experiment was

Sirius Sirius is the list of brightest stars, brightest star in the night sky. Its name is derived from the Ancient Greek language, Greek word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinisation ...

. He did not succeed in resolving any stellar disks, but by 1874 had obtained an upper limit to stellar diameters of 0.158 " (the true angular diameter of Sirius is 0.0059 arcseconds, and for comparison, the angular diameter of Alpha Centauri and Betelgeuse are 0.0145 and 0.05 arcseconds respectively). In 1881 he discovered

NGC 5 NGC commonly refers to: * New General Catalogue of Nebulae and Clusters of Stars, a catalogue of deep sky objects in astronomy NGC may also refer to: Companies * NGC Corporation, name of US electric company Dynegy, Inc. from 1995 to 1998 * Nati ...

, and he discovered the galaxy NGC 6027 the following year using the 80 cm reflector. Among others, he discovered Stephan's Quintet, also known as "Arp 319", a group of five

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

. Stephan made this discovery with the first telescope equipped with a reflection coated mirror. In 1884 the

French Academy of Sciences The French Academy of Sciences (French: ''Académie des sciences'') is a learned society, founded in 1666 by Louis XIV of France, Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific me ...

awarded him the

Valz Prize The Valz Prize ''(Prix Valz)'' was awarded by the French Academy of Sciences, from 1877 through 1970, to honor advances in astronomy. History The Valz Prize was established in June 1874 when the widow of astronomer Benjamin Valz, Marie Madeleine J ...

(''Prix Valz''). His name is associated with the periodic

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...

38P/Stephan-Oterma, although Jérôme Coggia saw it first. He became a Chevalier of the

Légion d'honneur The National Order of the Legion of Honour (french: Ordre national de la Légion d'honneur), formerly the Royal Order of the Legion of Honour ('), is the highest French order of merit, both military and civil. Established in 1802 by Napoleon ...

in 1868 and an Officier of the Légion d'honneur in 1879.

References

Obituary

JO 7 (1924) 9
{{DEFAULTSORT:Stephan, Edouard 19th-century French astronomers Discoverers of asteroids 1837 births 1923 deaths People from Niort Officiers of the Légion d'honneur