Pietro Angelo Secchi (June 29, 1818 - February 26, 1878) was an Italian astronomer. At the age of 16, he entered the Jesuit Order in Rome. He continued his studies at the Roman College, and demonstrated great scientific ability. In 1839, he was appointed tutor of mathematics and physics at the College. In 1841, he became Professor of Physics at the Jesuit College in Loreto. In 1844, he began theological studies in Rome, and was ordained a priest in 1847. He became head of the Observatory of the College at age 32 and remained there as head for 28 years. He was a pioneer in astronomical spectroscopy, and was one of the first scientists to state authoritatively that the Sun is a star. He discovered three comets, including Comet Secchi. He proved that the solar corona and coronal prominences observed during a solar eclipse were part of the Sun, and not artifacts of the eclipse. He developed the first system of stellar classification, the five Secchi classes. Though his system has been replaced by the Harvard system, he is still considered the discoverer of the principle of stellar classification. Besides astronomy he was active in oceanography, meteorology, and physics. He died in 1878 at the age of 59.