Clusters of Galaxies (2) ...
There are many clusters of galaxies. Members of some of the closest can be seen
with a small telescope in the constellations
Virgo
(left image below)
and
Coma Berenices
(right image below).
The Virgo Rich Cluster
The Virgo Cluster of galaxies lies at a distance of about 18 Mpc,
near the intersection of the constellations
Virgo
and
Coma Berenices.
It is an irregular rich cluster containing
approximately 250 large galaxies and more than 2000 small galaxies that covers
approximately
a 10 x 10 degree patch of our sky (corresponding to a width of
about 3 Mpc). Here is a
listing
of many galaxies in the Virgo Cluster.
Irregular rich clusters tend to have a variety of galaxy types. This is true
of the Virgo Cluster, which
contains more spirals than ellipticals, but ellipticals (mostly dwarf
ellipticals) are increasingly common near the center.
Although spirals are more numerous (65 percent of the 205 brightest galaxies in the
Virgo Cluster are spirals), the four brightest galaxies are giant ellipticals,
with M87 being the largest and brightest of these.
As we shall see,
clusters like Virgo also contain large amounts
of
hot gas between the galaxies, and large amounts of unseen
dark matter.
The Coma Rich Cluster
The nearest regular rich cluster lies about 15 degrees away from the
Virgo
Cluster in the constellation
Coma Berenices. The Coma Cluster is at a
distance of about 90 Mpc and has an angular diameter of about 4 degrees on the
celestial sphere as observed from Earth. This
corresponds to a width of
6 Mpc. It contains approximately
10,000 galaxies, most of which are faint
dwarf ellipticals.
The brightest galaxies are mostly ellipticals and
class S0, with only about 15 percent of
the
brightest galaxies being spirals or irregulars. This predominance of
ellipticals is typical of rich, regular clusters (and in
contrast to rich, irregular
clusters like Virgo that have a broader mix of galaxy types).
