Jesuit priest and astrophysicist who made the first survey of the spectra of stars and suggested that stars be classified according to their spectral type.
He entered the Society of Jesus in 1833 and became lecturer in physics and mathematics at the Jesuit College in Loreto Italy in 1839.
He returned to Rome in 1844 where he completed his theological studies and lectured at the Collegio Romano.
When the Jesuits were expelled from Rome in 1848 Secchi went to Stonyhurst College Clitheroe Lancashire and then to George-town University Washington D.C.
Because of his reputation as an astronomer he was allowed in 1849 to return to Rome where he became professor of astronomy and director of the observatory at the Collegio Romano.
He rected a new observatory in which he carried out his research in stellar spectroscopy, terrestrial magnetism and meteorology.
From his survey of stellar spectra Secchi concluded that the stars could be arranged in three classes according to the type of spectra they display.
Subsequently he expanded his system to four classes and placed all surveyed stars in the following four groups: (1) stars such as Sirius with bright hydrogen spectral lines, (2) stars similar to the Sun with numerous fine lines, (3) stars of the ac Herculis type with broad bands sharply defined toward the red end of the spectrum and (4) carbon stars with bands sharply defined toward the violet end of the spectrum.
These divisions were later expanded into the Harvard classification based on a simple temperature sequence on which modern systems separate stars into temperature and brightness (luminosity) classes.
Secchi was also active in solar observation.
He proved that prominences seen during an eclipse of the Sun are features on the Sun itself and he discovered many aspects of their behaviour and of the behaviour of the finer prominence-like jets of gases now known as spicules.