Free Astronomy Magazine November-December 2015

32 SPACE CHRONICLES Sibling stars by ESO T he loose speckling of stars in this new ESO image is the open star cluster IC 4651, located within the Milky Way, in the constellation of Ara (The Altar), about 3000 light- years away. The cluster is around 1.7 billion years old — making it middle- aged by open cluster standards. IC 4651 was discovered by Solon Bai- ley, who pioneered the establish- ment of observatories in the high dry sites of the Andes, and it was cata- logued in 1896 by the Danish–Irish astronomer John Louis Emil Dreyer. T his rich view of a tapestry of colourful stars was captured by the Wide Field Imager (WFI) camera, on the MPG/ESO 2.2- metre telescope at ESO’s La Silla Observatory in Chile. It shows a open cluster of stars known as IC 4651, a stellar grouping that lies in the constellation of Ara (The Altar). [ESO] The Milky Way is known to contain over a thousand of these open clus- ters, with more thought to exist, and many have been studied in great depth. Observations of star clusters like these have furthered our knowl- edge of the formation and evolu- tion of the Milky Way and the indi- vidual stars within it. They also allow astronomers to test their models of how stars evolve. The stars in IC 4651 all formed around the same time out of the same cloud of gas (although many of the stars captured here belong to IC 4651, most of the very brightest in the pic- ture actually lie between us and the cluster and most of the faintest ones are more distant). These sibling stars are only bound together very loosely by their attraction to one another and also by the gas between them. As the stars within the cluster inter- act with other clusters and clouds of gas in the galaxy around them, and as the gas between the stars is either used up to form new stars or blown away from the cluster, the cluster’s