HUBBLE’S TUNING-FORK CLASSIFICATION SYSTEM
1 Sc:open spiral These contain more Population I stars and interstellar gas. They have loosely wound arms and a small central bulge. These consist of ten per cent gas and dust, compared with two per cent in Sa spirals.
2 Sb:intermediate spiral
Found between Sa and Sc, intermediate is the most common form of spiral galaxy. All spirals are rich in gas and dust and contain young Population I stars, along with older Population II stars.