Galaxies And Clusters

Our galaxy, the Milky Way, is made up of 200 billion or more stars, their associated gas and dust clouds, and the planetary systems that orbit many of them. The Milky Way is visible to the naked eye as a pale band across the sky and can be resolved into countless points of light by any optical instrument. Within the galaxy, many stars are found in clusters. Loose "open" ones— the aftermath of star formation—are found in the plane of the galaxy, while dense, globular clusters orbit above and below the galaxy, and have much more ancient origins. Our own galaxy is just one of at least 200 billion. Many are spirals like the Milky Way, but there are other types, equally numerous. Ellipticals are ancient and sometimes huge balls of stars, while irregulars are chaotic havens of starbirth on a massive scale. Most galaxies are dwarfs, too faint to see except when they are on our cosmic doorstep. All these galaxies are typically found in clusters, but it is only recently that astronomers have begun to piece together the relationship between the different types.

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