[The "Chacornac Affair": the Rise and Stigmatizatin of a State Astronomer]

This article traces the unusual astronomical career of Jean Chacornac (1823–1873) during the French Second Empire. This clerk in a bazaar in Marseille became in just a few years astronomer at the Paris imperial observatory, and was then brutally expelled from this prestigious institution. The «Chacornac affair», largely forgotten, was an asymmetric struggle between a self-taught astronomer and the most famous professional French astronomer of the time, Urbain Le Verrier. Through the study of this case, we want to shed a light on people and practices kept on the margins of science by the process of professionalization of astronomy. Although he was excluded from the institution, Chacornac tried to continue to be an astronomer, independently, «from below». But the construction of a new social identity for State astronomy, in particular by Le Verrier, was inseparable from the attribution of indelible social stigma, which made Chacornac an «obligatory amateur».