The earth's diameter is 12,756.3 km and its revolution period is 1 year. You probably knew the last fact, but most likely not the first. The Web site Absolutely Astronomy can provide this and a large amount of other facts about planets, moons, stars, constellations, messier objects, nebulas, and galaxies. Visitors can browse via the pull-down menus or search by a particular question. The basic...

This Topic in Depth presents the science and research of galaxies. First, the University of Cambridge offers clear, logical descriptions and images of the Milky Way, spiral and elliptical galaxies, clusters, and dark matter of the Universe (1). Users can also find educational tutorials on many other cosmology topics. Next, the Space Telescope Science Institute's "Galaxies Galore, Games and More is...

Recent surprising discoveries have led Hubble space telescope astronomers to rethink theories on a class of galaxies called ultra-luminous infrared galaxies (ULIRG). It was previously believed that only pairs of galaxies were interacting in ULIRGs, but recent photos from Hubble show multiple collisions of four or even five galaxies smashing together. The images, provided at this site, offer a...