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A galaxy
is a massive,
gravitationally bound system consisting of stars, an interstellar medium
of gas and dust, and dark matter. The name is from the Greek root galaxias, meaning "milky," a reference to the Milky Way galaxy. Typical
galaxies range from dwarfs with as few as ten million stars up to giants
with one trillion stars, all orbiting a common center of mass. Galaxies
can also contain many multiple star systems, star clusters, and various
interstellar clouds. There are probably more than
100 billion galaxies in the observable universe. Most galaxies are 1,000
to 100,000 parsecs in diameter and are usually separated by distances on
the order of millions of parsecs (a unit of length, equal to just over
30 trillion kilometres, or about 3.3 light years) (or megaparsecs).
Intergalactic space (the space between galaxies) is filled with a
tenuous gas of an average density less than one atom per cubic meter.
The majority of galaxies are organized into a hierarchy of associations
called clusters, which, in turn, can form larger groups called
superclusters. These larger structures are generally arranged into
sheets and filaments, which surround immense voids in the universe. |
