Arago invented the polarimeter, an instrument that enables one to see whether light has a certain characteristic: its polarization. At the Observatory, he used this instrument to study the light emitted by white hot iron, and by molten platinum. He showed that it is polarized in both cases providing that one observes the emitting surface obliquely. He then examined the solar edge, and saw that its light was not polarized; he deduced that the solar surface was neither solid nor liquid but made up of an incandescent gas. Later on, he attributed this characteristic to stars. This was the first time that one was able to say something about the nature of a celestial body: Arago was thus the creator of astrophysics, the study of the physical properties of celestial bodies.

caption : Polarimeter of Arago - credits : Observatoire de Paris