Most galaxies are found in groups or clusters which contain a few to thousands
of galaxies bound together by their mutual gravity.
These clusters contain not only galaxies, but also contain the material which lies
between the galaxies. The space between the galaxies in a cluster is filled
with hot gas as well as invisible dark matter which has not yet been identified.
According to studies of how the galaxies in a cluster move
(which is influenced by the total mass in the cluster),
this dark matter could have about five times the mass of all the galaxies and
hot gas in a cluster combined.
Galaxy clusters are classified by their appearance as being either regular or
irregular clusters. They are also classified as rich or poor clusters depending
on the number of galaxies they contain.
The Coma Cluster of Galaxies (
O. Lopez-Cruz (INAOEP) et al., AURA, NOAO, NSF
Regular clusters are spherically symmetric, with the galaxies concentrated
toward the center of the cluster. They usually contain at least 1,000 galaxies
which are brighter than an absolute magnitude of -16.
Most of galaxies in regular clusters are
An example of a rich, regular cluster is the Coma Cluster (shown to the left),
which contains thousands of galaxies.
Almost every object in this image is a galaxy.
Irregular clusters do not have a well-defined center, but are often made up of
loose groups of small clusters. They contain all types of galaxies:
spirals, irregulars and ellipticals.
Irregular clusters can contain just a few to over 1,000 galaxies.
The Local Group of galaxies, the cluster which contains our Milky Way galaxy,
is an example of a poor, irregular cluster. Our Local Group contains about
30 galaxies: 3 spirals, 13 irregulars and 15 ellipticals.
The Fornax Irregular Cluster
A Galaxy Cluster showing Gravitational Lensing
Andrew Fruchter (STScI) et al., WFPC2, HST, NASA
Gravitational Lensing by Clusters
Einstein's general theory of relativity shows that a very large amount of mass
can bend the path of light and warp spacetime. This effect is seen in many
rich, massive clusters of galaxies. The huge mass of large clusters acts as
a gravitational lens, bending light which enters the cluster from objects lying
far behind it. This bent light is then focused by gravity to create one
or more images of the light source. The image might be distorted, or even
magnified by the cluster's gravitational lens, depending on the position of the
light emitting source relative to the cluster and to the observer. Galaxy
clusters usually create these images in the form of distended arcs.
Cluster gravitational lenses allow us to observe objects that are much too far
or too faint to be seen directly, helping us view the very distant, early universe.
The Spitzer Space Telescope
will investigate the extent to which cooling flows of gas, gravitationally
attracted to the cluster center, produces low-mass stars and brown dwarfs.
Spitzer will also use the gravitational lensing effect of galaxy clusters
to image distant galaxies that would otherwise be too faint to