Elliptical galaxies are subcategorized as E0 through E7, E0 being nearly spherical without internal structure to E7 being lenticular without internal structure. Spiral galaxies are S0,Sa,Sb and Sc. S0 is a spiral galaxy with an outer ring of stars. Our Milky Way galaxy is an Sb type. Sc galaxies are usually two-armed galaxies.
Irregular galaxies exhibit no uniform structure overall and have no discernable arms.
Structure densities vary greatly. Also many exhibit evidence of black hole formation in their nuclei. BlackHoles exist when matter density is great enough to cause the gravitational retraction of matter and radiation. Our galaxy appears to have a black hole in it's nucleus. Many earlier galaxies contain Quasars in their cores.