Hubble found that some galaxies are difficult to put in the context of the tuning fork diagram. "S0," or lenticular galaxies, are in the transition zones between ellipticals and spirals and bridge these two types. An "SBc" is thus a loosely wound barred spiral galaxy. Barred spirals have a "B" in their classification. The spiral arms in barred spirals usually start at the end of the bar instead of from the bulge. The most important difference between these two groups is the bar of stars that runs through the central bulge in barred spirals. In fact the spiral galaxies are sub-divided into two groups - normal spirals and barred spirals. There are indications pointing to a very close connection between the bulges of certain galaxies (Hubble types "S0", "Sa" and "Sb") and elliptical galaxies. Also it is worth noting that the sizes of the round central regions in spirals - the so-called bulges - increase in size the more tightly the spiral arms are wound. "Sa" spirals, for example, are tightly wound whereas "Sc" spirals are more loosely wound. The spirals were assigned letters from "a" to "c," which characterize the compactness of their spiral arms. Hubble gave the ellipticals numbers from zero to seven, which characterize the ellipticity of the galaxy - "E0" is almost round, "E7" is very elliptical. The diagram is roughly divided into two parts: elliptical galaxies (ellipticals) and spiral galaxies (spirals). Although this scheme, also known as the Hubble tuning fork diagram, is now considered somewhat too simple, the basic ideas still hold. As one of the first steps towards a coherent theory of galaxy evolution, the American astronomer Edwin Hubble, developed a classification scheme of galaxies in 1926. Some are simple, while others are very complex in structure.
Galaxies are very important fundamental building blocks of the Universe.