Galaxies are described according to the Hubble tuning fork diagram which classifies galaxies according to there shapes. S0 is an extra category that Hubble used to describe the shape of galaxies that were about halfway between elliptical galaxies and spiral galaxies. E = Elliptical, S = Spiral, SB = Barred Spiral.

E - Elliptical Galaxies These are the first group on the fork they are denoted by the letter E. The range is from is a perfect circle and 7 is more cigarette shaped. This is an example of an elliptical Galaxy An elliptical galaxy shows no spiral structure and can vary from almost round (what Hubble called E0) to almost cigar shaped (called E7). This classification is based on our perspective from Earth and not on the actual shape.An elliptical galaxy shows no spiral structure and can vary from almost round (what Hubble called E0) to almost cigar shaped (called E7). This classification is based on our perspective from Earth and not on the actual shape.

S - Spiral Galaxies As their name implies, spiral galaxies 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, and Sc. In terms of the arms, Sa is the tightest wound while Sc is the most open. In terms of the nucleus, Sa has the largest while Sc has the smallest. The galaxies that appear to have a spiral disc but no visible arms are called S0. Spiral galaxies are the top branch of the fork. They are denoted by the letter S. They range from the letter (a) to (d). (a) is tightly wound, and (d) is loosely wound. Here is an example of a spiral galaxy.

SB - Spiral Bar Spiral bar galaxies are the bottom branch of the fork. They are called Spiral bar galaxies because there appears to be a bar between the galactic bulge and the spiral arm. These also range from a-d. This is a picture of a spiral bar galaxy. Barred spirals show the same spiral structure as normal spirals, and also a prominent bar through the nucleus. The spiral arms emerge from the end of the bar. The sub-classifications are the same as for normal spirals.