Secchi, Pietro Angelo (1818–1878)

Italian astronomer and astrophysicist. He classified stellar spectra into four classes based on their colour and spectral characteristics. He was the first to classify solar prominences, huge jets of gas projecting from the Sun's surface.

Secchi was born in Reggio nell'Emilia, Lombardy, and became a Jesuit priest, lecturing in physics and mathematics at the Collegio Romano from 1839. In 1848 he was driven into exile for being a Jesuit and went first to Stonyhurst College, England, then to Georgetown University in Washington, DC. He returned to Italy 1849 as director of the observatory at the Collegio Romano and professor of astronomy.

With English astronomer William Huggins, Secchi was the first person to adapt spectroscopy to astronomy in a systematic manner and he made the first spectroscopic survey of the heavens. He proposed that the differences in stellar spectra reflected differences in chemical composition. His classification system of 1867 is the basis of the modern system.

Secchi was among the first to use the new technique of photography for astronomical purposes. By 1859 he had a complete set of photographs of the Moon.