Hubble's Classification

Edwin Hubble noted a gradation in galaxy types from perfect spheres, through elliptical lenses, to flattened disks. Within the disk galaxies, he noted the presence or absence of bars and the tightness of the spiral arms. Other galaxies that didn't fit this scheme he classified as irregular.

Class		Description
Elliptical Galaxies
E0		Perfect sphere
E1
E2
E3
E4
E5
E6
E7		Flattened Lens
Disk Galaxies
S0		No spiral arms
Normal	Barred
Sa	SBa	Tightly wound arms
Sb	SBb
Sc	SBc	Loosely wound arms