Final answer:
Spiral galaxies are divided into two sub-classes: Sa and Sc. Sa sub-class consists of galaxies with a large central bulge and tightly coiled arms, while Sc sub-class consists of galaxies with a small central bulge and loosely wound arms.
Step-by-step explanation:
Spiral galaxies are divided into two sub-classes: Sa and Sc. The Sa sub-class represents spiral galaxies with a large central bulge and tightly coiled arms, resulting in a lens-like appearance. On the other hand, the Sc sub-class consists of spiral galaxies with a small central bulge and loosely wound arms, where luminous stars and emission nebulae are more prominent.