He was the director of the Marseille Observatory from 1864 to 1907 (until 1872 he was subordinate to Urbain le Verrier). 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 89 Julia in 1866. In 1867 he used the new telescope to observe a transit of Mercury.
Between 1870 and 1875, Stephan systematically studied nebulae, 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 of a star using interferometry, 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. 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).
He became a Chevalier of the Légion d'honneur in 1868 and an Officier of the Légion d'honneur in 1879.
|89 Julia||6 August 1866|
|91 Aegina||4 November 1866|
- Steinicke, Wolfgang (2008). Nebel und Sternhaufen: Geschichte ihrer Entdeckung, Beobachtung und Katalogisierung - von Herschel bis zu Dreyer's "New General Catalogue" (in German). Hamburg. pp. 296–298. ISBN 978-3-8370-8350-7.
- Hutchins, Roger (2008). British university observatories, 1772-1939. England: Ashgate Publishing Lmiited. p. 248. ISBN 978-0-7546-3250-4.
- North, John David (2008). Cosmos: an illustrated history of astronomy and cosmology. University of Chicago Press. p. 590. ISBN 978-0-226-59440-8.
- Dorminey, Bruce (2002). Distant wanderers: the search for planets beyond the solar system. New York: Springer-Verlag. p. 123. ISBN 0-387-95074-5.
- McLean, Ian S. (2008). Electronic imaging in astronomy: detectors and instrumentation. Springer. p. 67. ISBN 978-3-540-76582-0.
- Richichi, Andrea; Delplancke, Francoise (2008). The power of optical/IR interferometry: recent scientific results and 2nd generation instrumentation. Springer. p. 9. ISBN 978-3-540-74253-1.
- "Seyfert's Sextet at SEDS.org". Archived from the original on 2007-02-27. Retrieved 2007-04-06.
- Plotner, Tammy (2009). The Night Sky Companion: A Yearly Guide to Sky-Watching 2009. Springer Science. p. 303. ISBN 978-0-387-79508-9.
- Plotner, Tammy; Barbour, Jeff (2006). What's Up 2006 - 365 Days of Skywatching. Universe Today. p. 274. ISBN 978-1-4116-8287-0.
- "Prizes given by the French Academy". The American Naturalist. U. of Chicago Press. 18: 751. 1884. doi:10.1086/273730.
- JO 7 (1924) 9 (in French)