Open star clusters like the one seen here are not just perfect subjects for pretty pictures. Most stars form within clusters and these clusters can be used by astronomers as laboratories to study how stars evolve and die. The cluster captured here by the Wide Field Imager (WFI) at ESO’s La Silla Observatory is known as IC 4651, and the stars born within it now display a wide variety of characteristics.
The 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 Bailey, who pioneered the establishment of observatories in the high dry sites of the Andes, and it was catalogued in 1896 by the Danish–Irish astronomer John Louis Emil Dreyer.
The Milky Way is known to contain over a thousand of these open clusters, with more thought to exist, and many have been studied in great depth. Observations of star clusters like these have furthered our knowledge of the formation and evolution of the Milky Way and the individual 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 . 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 interact 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 structure begins to change. Eventually, the remaining mass in the cluster becomes small enough that even the stars can escape. Recent observations of IC 4651 showed that the cluster contains a mass of 630 times the mass of the Sun  and yet it is thought that it initially contained at least 8300 stars, with a total mass 5300 times that of the Sun.
As this cluster is relatively old, a part of this lost mass will be due to the most massive stars in the cluster having already reached the ends of their lives and exploded as supernovae. However, the majority of the stars that have been lost will not have died, but merely moved on. They will have been stripped from the cluster as it passed by a giant gas cloud or had a close encounter with a neighbouring cluster, or even simply drifted away.
A fraction of these lost stars may still be gravitationally bound to the cluster and surround it at a great distance. The remaining lost stars will have migrated away from the cluster to join others, or have settled elsewhere in the busy Milky Way. The Sun was probably once part of a cluster like IC 4651, until it and all its siblings were gradually separated and spread across the Milky Way.
This image was taken using the Wide Field Imager. This camera is permanently mounted at the MPG/ESO 2.2-metre telescope at the La Silla Observatory. It consists of several CCD detectors with a total of 67 million pixels and can observe an area as large as the full Moon. The instrument allows observations from visible light to the near infrared, with more than 40 filters available. For this image, only three of these filters were used.
 Although many of the stars captured here belong to IC 4651, most of the very brightest in the picture actually lie between us and the cluster and most of the faintest ones are more distant.
 This quantity is in fact much larger than the numbers quoted by previous studies which surveyed smaller regions, leaving out many of the cluster’s stars that lie further from its core.
ESO is the foremost intergovernmental astronomy organisation in Europe and the world's most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become "the world’s biggest eye on the sky".
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This is a translation of ESO Press Release eso1534.
Richard Hook | ESO-Media-Newsletter
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