The Italian Angelo Secchi (1818-1878) is considered a pioneer in the field of astrophysics and spent his entire life working at the Vatican Observatory, where he focused on the atmosphere of the sun and stars. Through his research on their chemical composition, he established himself as an influential trailblazer in spectral analysis and left a lasting impact that continues to this day. This work is a comprehensive overview, as it thoroughly and clearly addresses the general and physical conditions of stars and outer space as a whole. It is a carefully prepared reprint of the original edition from 1878.