The Evolution of Galaxy

52 Scientific American December 1998 T royal Ferret of Comets was busy tracking his prey. On the night of April 15, 1779, Charles Messier watched from his Paris observatory as the Comet of 1779 slowly passed between the Virgo and Coma Berenices constellations on its long journey through the solar system. Messier’s renown in comet spotting had inspired the furry moniker from King Louis XV, but on this night he took his place in astronomy history books for a different reason. He noticed three fuzzy patches that looked like comets yet did not move from night to night; he added them to his list of such impostors so as not to be misled by them during his real work, the search for comets. Later he commented that a small region on the Virgo-Coma border contained 13 of the 109 stationary splotches that he, with the aid of Pierre Mechain, eventually identified—the Messier objects well known to amateur and professional astronomers today. As so often happens in astronomy, Messier found something completely different from what he was seeking. He had discovered the first example of the most massive things in the universe held together by their own gravity: clusters of galaxies. Clusters are assemblages of galaxies in roughly the same way that galaxies are assemblages of stars. On the cosmic organizational chart, they are the vice presidents—only one level below the universe itself. In fact, they are more massive relative to a human being than a human being is relative to a subatomic particle. In many ways, clusters are the closest that astronomers can get to studying the universe from the outside. Because a cluster contains stars and galaxies of every age and type, it represents an average sample of cosmic material—including the dark matter that choreographs the movements of celestial objects yet The Evolution of Galaxy