Clasification of Galaxies
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In 1926, an astronomer named Edwin Hubble decided to classify the galaxies, grouping them according to some logical scheme. He could have classified them according to color, because galaxies are different colors. He could have classified them according to size, calling small galaxies “dwarf galaxies” and calling large galaxies “massive galaxies”. After considering different schemes, he decided to arrange or group them by shapes. He would classify them according to the way they looked. In science, the study of something according to its form or structure is called “morphology”. Most galaxies are elliptical. Some elliptical galaxies are nearly circular in shape. Some elliptical galaxies are extremely stretched out, flattened,or elongated. To deal with this variation, Hubble divided the “E” classification into 8 sub-groups, which he called “E0”, “E1”, “E2”, “E3”, “E4”, “E5”, “E6”, and “E7”.
E0 galaxies are nearly circular in shape. E1 galaxies are stretched out a little. E2 galaxies are more elongated, E3 galaxies even more elongated or flattened, all the way up to E7 galaxies, which are extremely elongated or stretched out.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”. A third of spiral galaxies,