Type S galaxy
A type S galaxy, also known as a spiral galaxy is a disk-shaped galaxy with a spiral pattern of arms, typically containing 1010 to 1012 M of stars, dust, and gas.
Spiral galaxies are divided into ordinary and barred spirals - ordinary spirals have a more S-shaped inner configuration. They usually contain a central bulge, often sharing many properties with elliptical galaxies, surrounded by a thin rotating disk containing whatever spiral structure there may be.
Spirals are subdivided into a sequence jointly defined by the winding and prominence of the spiral arms, and the relative importance of the central bulge. Sa galaxies have a bright bulge and tightly wound arms, while Sc galaxies have loosely wound arms and a relatively less important bulge. This sequence Sa-Sb-Sc-Sd has counterparts SBa-SBb-SBc-SBd in the barred spirals. As more detail was observed in some galaxies, intermediate substeps (Sab,Sbc,Scd,S0/a) were added when necessary.
