The degree of flattness subdivides the classification into ellipticity groups. The observed classes range from E0 to E7 . No known elliptical galaxies are flatter than E7. However, a certain group of galaxies display a central region that is flatter than E7 - but these same galaxies invariably show an outer region of low surface brightness which resembles a thin, fundamental plane. Such galaxies are classified as SO , as an intermediate group between the ellipticals and the spirals.
Elliptical galaxies are the most numerous type of galaxy.
Most of these are faint, elliptical dwarf galaxies that are dim, small, and hard to observe. Now lets move onto the spiral galaxies. Spirals Our own Galaxy, and our nearest large neighbor, M31, are typical spiral galaxies. A spiral consists of a nucleus, a disk, a halo, and spiral arms. Interstellar material is usually observed in the spiral arms.