Secchi, who was born at Reggio in Italy, was a Jesuit who lectured in physics and mathematics. He spent some time abroad when the Jesuits were expelled from Rome, being at one time professor of physics at Georgetown University, Washington. He returned to Italy in 1849, becoming professor of astronomy and director of the observatory of the Roman College, which he rebuilt and reequipped.

He researched in stellar spectroscopy and his main work was done on spectral types. He introduced some order into the mass of new observations that was pouring in from the early spectroscopists. In 1867 he proposed four spectral classes. Class 1 had a strong hydrogen line and included blue and white stars; class 2 had numerous lines and included yellow stars; class 3 had bands rather than lines, which were sharp toward the red and fuzzy toward the violet and included both orange and red lines; finally, class 4 had bands that were sharp toward the violet and fuzzy toward the red and included red lines alone. These spectral types mark an important, and fairly straightforward, temperature sequence. Secchi's classification, as very much extended and modified by Edward Pickering and Annie Cannon, has become one of the basic tools of astrophysicists.