A classification scheme for galaxies; also known as the Yerkes system. It is based primarily on the spectrum of the galaxy's central regions, which correlates with the degree of concentration of light towards the centre. A notation is also given for the simple structural form of the galaxy. Lower-case letters af, f, fg, g, gk, and k indicate the spectral type of star that most closely matches the spectrum of the galaxy's nuclear region. Structural forms are E (elliptical), S (spiral), B (barred spiral), I (irregular), D (dustless elliptical with extended envelope of stars), L (low surface brightness), and N (small brilliant nucleus on faint background). An index from 1 (face-on) to 7 (edge-on) shows the inclination of the plane of the galaxy to the line of sight. For example, M33 is classified as fS3. The scheme was devised by W. W. Morgan.
Subjects: Astronomy and Astrophysics.