The Hercules galaxy cluster – aka Abell 2151 – lies approximately 500 million light-years away in the constellation of Hercules.
Interestingly enough, Abell 2151 is unlike other galactic assemblies in close proximity. For example, as well as being rather irregular in shape, it contains a wide variety of galaxy types, particularly young, star-forming spiral galaxies. In addition, there are no giant elliptical galaxies in sight.
According to astronomers, galaxy pairs on their way to merging into single, larger galaxies can be seen all over this image snapped by the VLT Survey Telescope in Chile. To be sure, the numerous interactions, along with the large number of gas-rich, star-forming spiral galaxies in the cluster, make the members of the Hercules cluster look like the young galaxies of the more distant Universe.
Because of this similarity, astronomers believe Hercules is actually a relatively young cluster. Essentially, it is a vibrant and dynamic swarm of galaxies that will one day mature into one more similar to the older galaxy clusters that are more typical of our galactic neighborhood.
Galaxy clusters are typically formed when smaller groups of galaxies join together due to the pull of their gravity. As these groups get closer to each other, the cluster becomes more compact and more spherical in shape. At the same time, the galaxies themselves move closer together and many start to interact. Even if spiral galaxies are dominant in the initial groups, the galactic collisions eventually distort their spiral structure and strip off gas and dust, quenching most star formation.
As such, most of the galaxies in a mature cluster are elliptical or irregular in shape. One or two large elliptical galaxies, formed from the merger of smaller galaxies and permeated by old stars, usually reside at the centers of these old clusters.
The Hercules galaxy cluster is believed to be a collection of at least three small clusters and groups of galaxies that are currently being assembled into a larger structure. The cluster itself is also merging with other large clusters to form a galaxy supercluster, which is considered one of the largest structures in the Universe.
The stunning image above shows not only the galaxies of the Hercules cluster, but also many faint and fuzzy objects in the background, which are galaxies that are much further away from us. Closer to home, several brilliant Milky Way stars are also visible in the foreground and there are even a few asteroids that have left short trails as they moved slowly across the image during the exposures.