Free Astronomy Magazine November-December 2015

SPACE CHRONICLES structure begins to change. Even- tually, the remaining mass in the cluster becomes small enough that even the stars can escape. Recent observations of IC 4651 show- ed that the cluster contains a mass of 630 times the mass of the Sun (this quantity is in fact much larger than the numbers quoted by pre- vious studies which surveyed smal- ler regions, leaving out many of the cluster’s stars that lie further from its core) 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 superno- vae. 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 T his wide-field view of the sky around the cluster IC 4651 was created from photographic mate- rial forming part of the Digitized Sky Survey 2. The cluster itself is the clump of faint stars in the cen- tral part of the picture. The bright star at the left is Alpha Arae, one of the brightest stars in the constellation of Ara (The Altar). [ESO/Digitized Sky Survey 2] a neighbouring cluster, or even sim- ply drifted away. A fraction of these lost stars may still be gravitationally bound to the clus- ter and surround it at a great dis- tance. The remaining lost stars will have migrated away from the clus- ter to join others, or have settled else- where 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. The image of the previous page was taken using the Wide Field Imager. This camera is permanently mount- ed at the MPG/ESO 2.2-me- tre 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 instru- ment allows observations from visible light to the near infrared, with more than 40 filters available. For this im- age, only three of these fil- ters were used. T his video starts with a view of the southern Milky Way and takes us on a jour- ney towards the open star cluster IC 4651, in the constel- lation of Ara (The Altar). The MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile captured the final rich and colourful close-up view. [ESO/Digitized Sky Survey 2/N. Risinger (skysurvey.org)] n