Telescopic observations of galaxies show that they have a wide range of different shapes first classified by the American astronomer Edwin Hubble in 1924 into four basic types, including spiral galaxies, barred spiral galaxies, elliptical galaxies, and irregular galaxies. Hubble's classification has since been modified and elaborated upon, but astronomers continue to use the same basic scheme.
Spiral galaxies are characterized by a flattened disk shape that exhibits a central bulge and spiral-
shaped arms that emerge from this central bulge and extend in variously curved forms to distant reaches of the galaxy. These are surrounded by a galactic halo made of a ball of old faint stars forming a sphere around the other parts of the galaxy. The Milky Way, which contains billions of star and planetary systems, including Earth, is a spiral galaxy. One of the major differences between different types of spiral galaxies is how tightly wrapped the spiral arms are as they circle the bulge in the core of the galaxy. Generally, the larger the central bulge of the galaxy, the more tightly wrapped the arms become. Galaxies with small central bulges in their cores tend to have loosely wrapped spiral arms and more lumpy or knotty distributions of matter within their arms. In the Hubble classification, forms of spiral galaxies are abbreviated by the letter S, with small letters a-d denoting progressively more open spiral forms, such that Sc galaxies are more open than Sa galaxies.
Most spiral galaxies have galactic disks rich in gas and dust, and halos comprised largely of old dim stars. The spiral arms have many younger stars and newly forming star systems and are the densest parts of these galaxies, providing material for the birth of new star systems.
Barred spirals are a special class of spiral galaxies in which a concentrated "bar" of stellar and interstellar matter passes through the central bulge of the galaxy, and the spiral arms extend from the ends of this bar. Most of these have unusual shapes, resembling giant Z or S shapes, with spiral trails of luminous matter extending around the letter. Barred spiral galaxies are designated by the Hubble classification as SB galaxies, with the small letters a-c denoting how open the spiral arms are around the bar.
Elliptical galaxies are circular to highly elliptical concentrations of stellar and interstellar matter whose density increases toward the center of the galaxies. The size of elliptical galaxies and the number of stars contained within elliptical galaxies vary widely. Some are small and known as dwarf ellipticals, being only a kiloparsec across and containing on the order of a million stars. others are huge, many times the size of the Milky Way Galaxy, and contain trillions of stars in giant elliptical galaxies that can be several megaparsecs across. Elliptical galaxies typically exhibit little internal structure, with no spiral arms and no central bulge. They are designated in the Hubble classification by the letter E with the numbers 1-7 indicating a progression from least- to most-elongated varieties.
Elliptical galaxies also differ from spiral galaxies in that they contain little gas and dust, and they seem not to have any young stars or places where star formation is in the process of occurring (when the light
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