Stellar Spectral Types: OBAFGKM . ... Credit Copyright: KPNO 0.9-m Telescope , AURA , NOAO , NSF Explanation: Astronomers divide stars into different spectral types . First started in the 1800s, the spectral type was originally meant to classify the strength of hydrogen absorption lines . ... The seven main spectral types OBAFGKM are shown above with the spectrum of a single "O" star at the top followed by two spectra each from the progressively cooler designations, respectively. ...