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Groups & Clusters of Galaxies
clusters are the largest gravitationally bound objects in the universe. They
have three major components:
T he hot gas envelopes the galaxies and fills the space between galaxies. It
contains more mass than all the galaxies in the cluster. Although the galaxies
and hot gas clouds are very massive, scientists have determined that about 10
times more mass is needed to hold the cluster together. Something, namely dark
matter must exist to provide the additional gravity.
- Hundreds of galaxies containing stars, gas and dust;
- Vast clouds of hot (30 - 100 million degrees
Celsius) gas that is invisible to optical telescopes;
- Dark matter, a
mysterious form of matter that has so far escaped direct detection with any
type of telescope, but makes its presence felt through its gravitational pull
on the galaxies and hot gas.
Astronomers think that galaxy clusters form as clumps of dark matter and their
associated galaxies are pulled together by gravity to form groups of dozens of
galaxies, which in turn merge to form clusters of hundreds, even thousands of
Comparison of optical image from La Palma & B.McNamara (left) and
X-ray image from Chandra (right) of the Hydra A cluster of galaxies. This
cluster is so large that it takes light millions of years to cross it.
The gas in galaxy clusters is heated as the cluster is formed. This heating can
be a violent process as gas clouds enveloping groups of galaxies collide and
merge to become a cluster over billions of years.