Sb galaxies are intermediate between Sa and Sc. Our own galaxy is an Sb galaxy, as is M31. M51, which has a smaller bulge, is an Sc galaxy. In a barred spiral galaxy, the spiral arms wind away from an elongated central bar rather than from a spherical central bulge. The fact that S0 galaxies lack both gas and spiral arms has led to the hypothesis that interstellar gas is necessary for spiral arms to form.
Elliptical galaxies are actually the most common type of galaxy.
Randomly select a large number of galaxies. Consequently, there is no current star formation in elliptical galaxies. The stars in elliptical galaxies are old Population II stars. Unlike the disks of spiral and S0 galaxies, elliptical galaxies are NOT rotating rapidly. Elliptical galaxies were given their name because they appear elliptical on the sky.