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.

Galaxies do not usually occur as isolated

“island universes” in space which which float lonely through an otherwise empty

universe, but normally 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

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.