Suggests mass in outer parts of galaxy - unseen. Gas orbiting rapidly. Radio observations consistent with hypothesis. Black hole most likely explanation of observations. Sa, Sb, Sc Depends on nuclear bulge, tightness of spiral, amount of gas, dust.
Arms originate in bar.
Intermediate between E, spirals. No obvious nucleus, spiral arms. Formation of hot stars triggers further star formation. Most galaxies in clusters. Mostly E, S0 galaxies.