Hubble sequence

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented.

Hubble’s scheme divides regular galaxies into 3 broad classes – ellipticals, lenticulars and spirals – based on their visual appearance (originally on photographic plates). A fourth class contains galaxies with an irregular appearance. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.

On the left (in the sense that the sequence is usually drawn) lie the ellipticals. Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity in the sky. By convention, n is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as e = 1 − b/a for an ellipse with semimajor and semiminor axes of lengths a and b respectively. The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle). Observationally, the most flattened elliptical galaxies have ellipticities e = 0.7 (denoted E7). This is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles.

...
Wikipedia