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Classifying Galaxies
In the beginning of the 20th century, astronomers recognized areas of space that appeared "fuzzy" to the eye. Called nebulae, most scientists of the time thought they were within our own galaxy. In 1924, Edwin Hubble was the first to recognize nebulae as other galaxies, separated by huge distances from other galaxies, including our own. He was the first to classify them according to size and shape, and his system is still in use today. The Sloan Digital Sky Survey, a project to map the universe, contains more than 138 billion galaxies so far.
Spiral Galaxies
Viewing a spiral galaxy at different angles changed the way it appears. Viewed as if lying flat (two-dimensional or face-on), the spirals are clearly seen. Viewed from the side (edge-on), the central bulge is clear. Some have tightly wound arms, while in others the arms are loose. Some appear to have a line or bar running through them; others do not. By observing these differences, Hubble developed a classification system for spiral galaxies.
Hubble's Spiral Galaxy Classification System
Hubble assigned a letter to identify different galaxy types. An "S" designated a spiral galaxy. If they had a bar running through them, they were "SB" types. Hubble next looked at the tightness of the spirals. "Type a" galaxies have large center bulges, very bright surfaces and tightly wound arms. "Type b" have smaller bulges, less brightness and looser arms, and so on down to "type d." By combining these assignations together, one could tell what type and characteristics a galaxy had. For example, a type SBb is a spiral galaxy with a bar running through it and more loosely wound arms.
Examples of Spiral Galaxies
Since spiral galaxies are the most common of the known galaxies, a complete list would be quite long. Some examples include M101 (Sc), also called the Pinwheel galaxy, ESO269-57 (Sa) in the southern constellation Centaurus, M31 (Sb), commonly known as Andromeda and found in the Andromeda constellation, and M83 (SBa) in the Hydra constellation.
Other Galaxies and Hubble Classifications
Hubble didn't classify just spiral galaxies. He assigned an "E" to elliptical galaxies, followed by a number from zero (circles) to seven (long and thin). Lenticular galaxies, which look like armless spirals, carry the same "S" classification as spiral galaxies but are followed by a number (S0 for normal lenticular or SB0 for barred lenticular). Irregular galaxies, which comprise about 3 percent of known galaxies, have few common features. Astronomers believe they are the result of collisions or near misses of other galaxies. Their classification is "Irr." Hubble used a tuning fork diagram to show how galaxy shapes and characteristics differ. The end of the fork contains the elliptical galaxies from the roundest (E0) to the most flat (E7), and the tines contain the spiral galaxies, with the normal on one tine and the barred on the other.