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On the Edge of: Clusters of Galaxies
Most galaxies are not alone
in the vast expanse of space, but are connected to one or more other
galaxies by gravity. The
same force that holds you onto the Earth can keep many individual galaxies
bound together. Groups can be small, such as two galaxies
orbiting each other,
or large, like the rich Coma cluster of thousands
of galaxies extending for more than ten million
light years. These are
the largest objects in the known Universe, and they have many properties
that make them great astrophysical laboratories. For example,
clusters change very slowly (it takes almost as long as the
age of the Universe for significant changes to occur in clusters),
thus clusters retain an imprint
of how they were formed. This makes them a good probe of the
history of structure and galaxy formation.
clusters tend to hold onto the gas in their systems, unlike
galaxies, where the gas is forced out through
supernova explosions. In
other words, clusters
are closed systems. By studying the chemical composition of clusters,
it is possible to get a history of
nucleosynthesis in the Universe.
the force of gravity that holds clusters together comes mostly from
dark matter, making clusters an excellent way to study dark matter in the
Universe.
The most visible part of
galaxy clusters, all of the stars
in all of the galaxies that make up the cluster, is a small fraction of the
sum total of what makes up the cluster, and is probably the least
interesting part of the cluster. For example, scientists from the
ASD study the
X-ray emission from galaxy clusters.
The X-rays come from hot (10-100 million degrees) gas trapped by the
gravitational force of the cluster. This gas makes up a much larger part
of the total mass of the cluster
than the stars, but is completely invisible to human eyes!
The scientists at Goddard Space Flight Center are studying
many different things about galaxy clusters, but these can be divided
into two main groups:
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