Here L is total energy emitted by a glowing body per unit area .
T is the temperature and s is a constant.
In 1866 father Peitro Angelo Secchi sorted stars into four broad categories and noted that the spectral lines correlated with color. E.C. Pickering at Harvard began an all-out effort to obtain photographic spectra from thousands of stars. For this project, he employed an objective prism , a large prism filter fitted on the front end of a refracting telescope. With this device, all stars images projected onto the photographic plate are spread into spectra, and hundreds of stars can be cataloged and classified from one exposure. These stars were assigned the classification O .