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Physical astronomy (the study of the physical composition of celestial bodies) grew considerably from the 1860s on, following the discovery of Kirchhoff’s laws, making possible the interpretation of celestial spectra. Before becoming « astrophysics » in the 20th century, physical astronomy was practiced by a community of astronomers and physicists working at the margin of classical astronomy (the study of star positions and celestial mechanics). This paper aims at describing this little-known French community between 1860 and 1890, by following the career of three scientists involved in this new field: Jules Janssen (1824-1907), Alfred Cornu (1841-1902), and Georges Rayet (1839-1906). Their education, their social network, and their scientific and technical approaches allow us to understand the particularities of the construction of French physical astronomy. This new research field was pioneered by experimental physicists, under various forms, and developed in different places such as observatories and the laboratories of the grandes écoles. Janssen focused his work on the production of new celestial pictures (spectra, photographs). Cornu gave primary importance to the measurement of spectral lines, while Rayet saw physical astronomy as a method making it possible to renew the inventory of celestial objects. Among these various approaches, that of Cornu seems to have been the most productive in structuring the discipline.