When you looked at elliptical galaxies you saw that as the classification numbers progressed from E1 toward E7, their appearance was more and more flattened or elongated. Edwin Hubble observed other galaxies that were elongated, but they were different from elliptical galaxies because they had bright centers. He called the bright centers of galaxies "nuclei". He noted that many galaxies with bright nuclei also had "arms" spiraling out from the middle. He called these galaxies with bright nuclei "spiral galaxies".
Hubble named the galaxies that had bright nuclei but no spiral arms "S0" (S-zero) galaxies.
He classified galaxies with spiral arms into three sub-groups that described how tightly the arms were wound around the nucleus. Hubble called spirals with their arms wound tightly around the nucleus, type "Sa". He called spirals with their arms less tightly wound (that is, looser and looser) "Sb" and "Sc". Here are examples of these types of spiral galaxies.
S0, Sa, Sb, Sc
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