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 on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

. 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''; ; ) 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 urban area. Geography T ...

) 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 () 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 galaxies known as Stephan's Quintet, ...

from 1864 to 1907 (until 1872 he was subordinate to

Urbain le Verrier Urbain Jean Joseph Le Verrier (; 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 only mathematics. ...

). 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—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

89 Julia 89 Julia is a large main-belt asteroid that was discovered by French astronomer Édouard Stephan on 6 August 1866. This was first of his two asteroid discoveries; the other was 91 Aegina. 89 Julia is believed to be named after Saint Julia of ...

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

transit of Mercury file:Mercury transit symbol.svg, frameless, upright=0.5 A transit of Mercury across the Sun takes place when the planet Mercury (planet), Mercury passes directly between the Sun and a superior planet. During a Astronomical transit, transit, Merc ...

. Between 1870 and 1875, Stephan systematically studied

nebula A nebula (; 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 regions, such as in the Pillars of Creation in ...

e, precisely recording their positions and discovering many new ones. His goal was to enable the exact measurement of stellar proper motions 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 separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual an ...

of a

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

using

interferometry Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...

, 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 brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated Canis Majoris, Latinized to Alpha Canis Majoris, and abbr ...

. 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 may refer to: Companies * NGC Corporation, the name of US electric company Dynegy, Inc. from 1995 to 1998 * National Gas Company of Trinidad and Tobago, a state-owned natural gas company in Trinidad and Tobago * National Grid plc, a former nam ...

, and he discovered the galaxy

NGC 6027 NGC 6027 is a lenticular galaxy which is the brightest member of Seyfert's Sextet, a compact group of galaxies. It was discovered by French astronomer Édouard Stephan on 20 March 1882. See also * NGC 6027a * NGC 6027b * NGC 6027c * NGC 6 ...

the following year using the 80 cm reflector. Among others, he discovered

Stephan's Quintet Stephan's Quintet is a visual grouping of five galaxies of which four form the first Galaxy group#Compact Groups, compact galaxy group ever discovered. The group, visible in the constellation Pegasus (constellation), Pegasus, was discovered by É ...

, also known as "Arp 319", a group of five

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and 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 Sys ...

. 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 (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific method, scientific research. It was at the forefron ...

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 ...

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

comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...

38P/Stephan-Oterma, although

Jérôme Coggia Jerome (c.347–420) was a priest, confessor, theologian and historian from Dalmatia. Jerome may also refer to: People Given name * Jerome (given name), a masculine name of Greek origin, with a list of people so named * Saint Jerome (disambiguat ...

saw it first. He became a Chevalier of the

Légion d'honneur The National Order of the Legion of Honour ( ), formerly the Imperial Order of the Legion of Honour (), is the highest and most prestigious French national order of merit, both military and Civil society, civil. Currently consisting of five cl ...

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 Officers of the Legion of Honour