Groups and Clusters of Galaxies

Click icon for an index of galaxy groups and clusters containing galaxies of Messier's catalog

Our icon shows a region near the center of the Virgo Cluster of galaxies, near the large lenticulars M84 and M86.

Only some few galaxies occur as isolated "island universes" in space, which float lonely through an otherwise empty neighborhood in the universe. More typically, galaxies form groups of several individuals, in a wide range from few or few dozens of galaxies to large clusters of up to several thousands. The galaxies of these groups are in mutual gravitational interaction which may have significant influence on their appearance.

Messier's galaxies are no exceptions, but in virtually all cases members of groups and clusters of galaxies; moreover, Messier had even discovered the nearest big cluster of galaxies, the Virgo Cluster, although at his time, the nature of galaxies was not recognized; thus he wrote of a concentration of nebulae, of which he had cataloged 16. This huge agglomeration contains several dozens of large and thousands of small galaxies.

Our own galaxy, the Milky Way, is also a member of a smaller group of galaxies, the Local Group, which contains three large and over 30 small galaxies. Together with most nearby galaxy groups (and field galaxies), the Local Group is part of the so-called Local or Virgo Supercluster, which is dominated by the big Virgo Cluster.

Big clusters like Virgo have a tendency to attract and finally incorporate the small groups and individual galaxies in their immediate neighborhood as time goes by. Besides accumulating mass, the cluster also grows in volume because of the following process: The incoming galaxies are accelerated by the cluster's gravity, and fall in with high velocities. Having reached the cluster, they transfer their kinetic energy during encounters to member galaxies, and thus "heat" the cluster. Like a gas, the heated cluster expands to a larger volume.

It is not yet clear if our Local Group will at one time be "eaten" by the Virgo cluster.

Groups and Clusters of Galaxies

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