British astronomer and writer David Darling has created an impressive, online encyclopedia devoted to topics in astronomy, astrobiology, spaceflight, space physics, and much more. From the Abell cluster to the Zwicky Catalogue, the site provides clear, straightforward explanations of a variety of concepts, along with brief biographies and much other material, including relevant images.
Go to: http://www.daviddarling.info/encyclopedia/ETEmain.html
