A subclass of spirals contains the barred spirals. Ordinary spirals have a nucleus which is approximately spherical, while barred spirals have an elongated nucleus which looks like a bar. The nucleus of an Sc galaxy is smaller than in an Sa galaxy, and the arms of the Sc are wrapped more loosely. Elliptical galaxies are classified according to the relative sizes of their apparent major and minor axes. All elliptical galaxies have n between 0 and 7.
Irregular galaxies have no obvious spiral or elliptical structure.
It is thought that many irregulars were once spiral or elliptical, but that a close encounter with a larger galaxy disrupted the organization of the material by gravitational forces. I's are resolvable into individual stars, and Irr II's are not. Not all galaxies are easily classified. Quasars are the bright, superluminal cores of very distant active galaxies. These galaxies are so distant in fact, that the quasars look like stars in most images.