The Arches Cluster, an open cluster in Sagittarius constellation discovered in 1995, is the densest cluster of its kind known in the Milky Way. It lies at a distance of 25,000 light years from Earth and only about 100 light years from the centre of the Milky Way.
The cluster is heavily obscured by dust in the visual bands and cannot be seen with the naked eye, but it can be observed in infrared, radio and X-ray bands. It offers insights into star formation in an environment close to the Galactic centre.
The Arches Cluster contains about 135 large, massive, young, extremely hot stars and thousands of less massive members. The age of the cluster is estimated at 2.5 million years. The cluster is believed to have formed in a short, but massive burst of star formation.
The clusters, Arches (upper right), Quintuplet (upper center), and the GC cluster (bottom center), contain massive stars that appear as very bright, point-like X-ray sources when winds from their surfaces collide with those from an orbiting companion. Vast amounts of energy are also released when these stars explode as supernovas, heating the surrounding material. Stellar corpses in this image also emit X-rays as either neutron stars or black holes in binary systems.
It is a bit younger than the nearby Quintuplet Cluster, but considerably denser and larger than its neighbour. The oldest stars in the Arches Cluster have barely evolved from the main sequence, while the Quintuplet Cluster contains a number of much more evolved stars, including hot supergiants, a red supergiant and several luminous blue variables (LBV).
The Arches Cluster is extremely dense, with close to 150 hot young stars packed within a radius of 1 light year. For comparison, if a region with a radius equal to the distance between the Sun and its closest neighbour, Proxima Centauri, had the same density, it would contain more than 100,000 stars. X-ray observations have revealed an envelope of gas around the cluster, heated to 60 million degrees by the cluster’s stars intense stellar winds that are colliding with the interstellar medium.
The cluster has a mass equivalent to more than 10,000 Suns, which makes it 10 times heavier than typical open clusters in our galaxy.
The Arches Cluster was named for the arch-shaped filaments detected in its vicinity at radio wavelengths. These filaments are produced as a result of the cluster approaching and ionizing the surface of a molecular cloud that lies in the background. Similar interactions between a cluster and a molecular cloud are occurring in the Quintuplet Cluster.
Stars
The most massive stars in the cluster are Wolf-Rayet stars, extremely hot, highly luminous stars showing prominent emission lines of highly ionised helium and nitrogen or carbon.
Wolf-Rayet stars are some of the most massive stars known. Some of them have a mass more than 100 times that of the Sun. Their stellar winds are enriched with helium and nitrogen. The Arches Cluster is believed to contain about 5 percent of all known Wolf-Rayet stars in the Milky Way.
The 28 brightest stars in the Arches Cluster are either H-rich WN7-9 stars or O-type supergiants with an age between 2 and 4 million years.
At least 150 of the member stars are some of the brightest stars ever discovered within our galaxy. Astronomers estimate that only one out of every 10 million stars in the Milky Way is as bright as the members of the Arches Cluster. The hot young stars in the cluster have relatively short lives, burning their fuel in only a few million years before they die in supernova explosions.
The Arches Cluster
Constellation: Sagittarius
Right ascension: 17h 45m 50.5s
Declination: –28° 49′ 28″
Distance: 25,000 light years (8,500 parsecs)
Mass: 10,000 solar masses
Age: 2.5 million years
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