The Tuning Fork Diagram

Galaxies may be viewed as the basic building blocks for the large-scale visible structure of the Universe. There are many kinds of galaxies, having rather diverse features. Therefore, it is useful to have a way to classify galaxies into different types. Edwin Hubble introduced the classification scheme illustrated in the following figure. The Hubble classification separates most galaxies into elliptical, normal spiral, and barred spiral categories, and then subclassifies these categories with respect to properties such as the amount of flattening for elliptical galaxies and the nature of the arms for spiral galaxies. The galaxies that do not fit into these categories are classified separately as irregular galaxies.

This diagram is termed the Hubble classification scheme, or (because of its shape) the "tuning fork diagram".