Galaxy morphological classification

[9] To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.

[14] The de Vaucouleurs system retains Hubble's basic division of galaxies into ellipticals, lenticulars, spirals and irregulars.

For example, a weakly barred spiral galaxy with loosely wound arms and a ring is denoted SAB(r)c. Visually, the de Vaucouleurs system can be represented as a three-dimensional version of Hubble's tuning fork, with stage (spiralness) on the x-axis, family (barredness) on the y-axis, and variety (ringedness) on the z-axis.

[18] Thus, as a rough rule, lower values of T correspond to a larger fraction of the stellar mass contained in a spheroid/bulge relative to the disk.

The approximate mapping between the spheroid-to-total stellar mass ratio (MB/MT) and the Hubble stage is MB/MT=(10−T)2/256 based on local galaxies.