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Virgo Cluster
Unlike the Coma Cluster, the Virgo cluster of
galaxies is an "irregular" rich cluster. This means that Virgo's over 2000
members are scattered asymmetrically, and include a variety of different galaxy
types. At 50 million light years away, Virgo is the closest major cluster and the
center of our Local Supercluster. In fact, due to Virgo's immense mass, our Local
Group of galaxies is affected by its gravity. Virgo's gravity is so strong that
it pulls galaxies and groups of galaxies toward it. This effect is called the
Virgo-Centric flow. Eventually, galaxies pulled toward Virgo will join the
cluster.
Most of the Virgo Cluster's elliptical galaxies are near its center, while the
majority of spiral galaxies are toward the outside. All of Virgo's galaxies are
held together by gravitational force. Galaxies make up only five percent of the
total mass of a typical cluster, which is not enough mass to keep a cluster like
Virgo together. It is known that hot gas, at temperatures of 10-100 million
degrees, makes up the majority of Virgo's mass. This gas is called the
"intracluster medium," and was first discovered by Fritz Zwicky in the 1930's.
However, the gravitational force required to hold the Virgo Cluster together
indicates that some type of undetectable matter, called "dark matter," must exist
as well. It is for this reason, and many others, that it is a well-studied part
of the sky. The Virgo Cluster is so close to us that some galaxies in it have
blue-shifts, meaning that they are moving towards us faster than the cluster is
moving away.
Virgo has one giant, active, elliptical galaxy named M87 (its number in Charles
Messier's catalog). M87 is called a "dominant" galaxy, since it is the most
massive and energetic object in the cluster. Over time, in this case 10 billion
years, gravity pulls dominant galaxies like M87 to the center of the cluster as
other members are absorbed into it by intense gravity. Now the center of mass,
M87 stretches across one million light years of space. Much of the visible light,
and most of the X-ray emission from Virgo originates from M87, which is also one
of the brightest known radio sources in the universe. Because of their brightness
and relatively short distance from Earth, M87 and the entire Virgo cluster are
frequently selected for observation.
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