As it stands, there is no formal definition for what makes a galaxy because, up till recently, astronomers have simply known them when they see them. This will probably be changing soon, as some astronomers have proposed a formal definition which will then be voted upon at a congress of the International Astronomical Union. Until then, we’ll stick with the traditional “Island Universe” definition: A galaxy is a vast structure composed of hundreds of billions of stars and enormous clouds of gas and dust, all bound together by gravity. Classically, galaxies have either an elliptical or a disk shape, with the ellipse shapes ranging from perfect spheres to elongated cigars, while the disks usually show a spiral structure, with variations in the number of spiral arms, and how tightly they are wound. Galaxies have a core region, densely packed with stars and usually containing a supermassive black hole at the very centre, and thinner outer regions. They also have a large halo stretching out into space, composed of gas, and filled with Globular clusters and a few errant stars.
Written by Allen Versfeld
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