``Tuning Fork''

Edwin Hubble, in the 1920s, devised a classification scheme for galaxies based on their appearance. According to his scheme, galaxies are distributed in three classes: elliptical, spiral and irregular. Hubble's classification scheme follows a so called ``tuning-fork'' diagram as shown in Figure 1. The splitting of the diagram is because of the barred and the ordinary spiral galaxies (Zeilik, Gregory & Smith 1992).

 

Figure 1: Tuning Fork Diagram