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Three Types of Galaxies

Elliptical galaxies are collections of stars that remain pretty close together because of the gravitational attraction between them. There is not a certain known rotation axis, so the stars in the galaxy show a wide range of orbital paths. The motions of stars are not entirely random; there is a degree of organization in the direction of these motions, and this determines the overall shape of the galaxy. Elliptical galaxies have a plain appearance, and some elliptical galaxies are nearly circular in shape, and some elliptical galaxies are extremely stretched out or flattened. The ones that are spherical are classed as E0 galaxies, and those that are flattest are E7, the types in between have the numbers E1 to E6. Elliptical galaxies show a larger range in brightness, some can be up to 10 times brighter than the brightest spiral galaxies, and the dullest ellipticals can be 1000 times less luminous than the dullest spiral. An elliptical galaxy is given the label En, where the galaxy shape determines the number n. Most galaxies are elliptical. This type of galaxy dependends on the orientation of the galaxy from our viewpoint; in other words, it describes how the galaxy appears to us, it does not tell us a lot about the true physical properties. Unless a galaxy is perfectly spherical, it will have a different appearance according to the direction from which we observe it.
Another type of galaxies is called spiral galaxies. It has wide-ranging spiral arms like a hurricane, and a densely populated bar of old stars near the center. All that is clear in visible light. Each spiral galaxy is classified with a label, which gives some indication of its appearance. All spiral galaxy labels begin with the prefix “S”, and usually is followed by a lower case letter, which describes the “style” or overall appearance of the particular object. This letter may be either 0, a, b or c, and is determined by

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