As their name implies, spiral galaxies [spiral galaxy] have outstretched, curving arms suggestive of a whirlpool or pinwheel. Hubble distinguished different sub-classes according to the tightness of the arms and the size of the nucleus. He called these Sa, Sb [Sb galaxy], and Sc [Sc galaxy]. In terms of the arms, Sa is the tightest wound while Sc [Sc galaxy] is the most open. In terms of the nucleus, Sa has the largest while Sc [Sc galaxy] has the smallest .