Clusters of Galaxies
Clusters of galaxies
are larger than groups, typically containing from about 50 to
10,000
members.
We refer to those clusters near the upper end of this range as
rich clusters, while those near the lower end are termed poor
clusters.
We may further classify clusters according to whether they are regular
(spherical with a central region of higher density) or irregular
(amorphous in
shape
without a condensed central region).
The adjacent image shows an example of a cluster. This is the
Virgo Cluster of galaxies, which is a nearby rich, irregular cluster that
we shall discuss more in the next section.
All of the fuzzy patches that can be seen in this image are galaxies.
Typical Properties of Clusters
Clusters typically have masses from 1014
to
1015 solar masses,
the diameters are typically 3-5 Mpc, the velocity
dispersions are 800-1000 km/s, and the average separation
from other clusters is about 10 Mpc. In clusters the typical mass-to-light
ratio is about 400 in units of solar masses divided by solar luminosities.
Thus, they contain large amounts of
dark matter.
