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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
 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.
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