Édouard Jean-Marie Stephan (31 August 1837 – 31 December 1923) was a Frenchastronomer. 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 ofNiort) and attended theEcole Normale Superieure, and graduated at the top of his class in 1862.[1]
He was the director of theMarseille Observatory from 1864 to 1907 (until 1872 he was subordinate toUrbain le Verrier).[2] In the early part of his career there, he had limited opportunities to do observations because he was preoccupied with improving the observatory.[1] He discovered theasteroid89 Julia in 1866.[1] In 1867 he used the new telescope to observe atransit of Mercury.[1]
Between 1870 and 1875, Stephan systematically studiednebulae, precisely recording their positions and discovering many new ones.[1] His goal was to enable the exact measurement of stellarproper motions by creating a reference system of fixed objects.[1]
In 1873, Stephan was the first person to attempt to measure theangular diameter of astar usinginterferometry, converting the 80 cm telescope at Marseille Observatory into an interferometer.[3] He did this by obscuring the reflector with a mask containing two vertical slits.[4]The star he chose to perform this experiment wasSirius.[5] He did not succeed in resolving any stellar disks,[4] 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).[6]
In 1881 he discoveredNGC 5, and he discovered the galaxyNGC 6027 the following year using the 80 cm reflector.[7]
Among others, he discoveredStephan's Quintet, also known as "Arp 319", a group of fivegalaxies.[8] Stephan made this discovery with the first telescope equipped with a reflection coated mirror.[9]
In 1884 theFrench Academy of Sciences awarded him theValz Prize (Prix Valz).[10] His name is associated with the periodiccomet38P/Stephan-Oterma, althoughJérôme Coggia saw it first.
He became a Chevalier of theLégion d'honneur in 1868 and an Officier of the Légion d'honneur in 1879.
| 89 Julia | 6 August 1866 |
| 91 Aegina | 4 November 1866 |
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