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A galaxy is a system of stars, stellar remnants,
interstellar gas, dust, dark matter, bound together by
gravity. The word is derived from the Greek galaxias
(γαλαξίαΟ), literally 'milky', a reference to the Milky Way
galaxy that contains the Solar System.
Galaxies, averaging an estimated 100 million stars,
range in size from dwarfs with less than a hundred million stars, to the
largest galaxies known β supergiants with one hundred trillion stars,
each orbiting its galaxy's center of mass. Most of the mass in a typical
galaxy is in the form of dark matter, with only a few
percent of that mass visible in the form of stars and nebulae.
Supermassive black holes are a common feature at the
centres of galaxies.
Galaxies are categorized according to their visual morphology as
elliptical, spiral, or
irregular. Many are thought to have supermassive black
holes at their centers. The Milky Way's central black hole, known as
Sagittarius A*, has a mass four million times greater than the Sun. As
of March 2016, GN-z11 is the oldest and most distant galaxy observed. It
has a comoving distance of
32 billion light-years from Earth, and is seen as it
existed just 400 million years after the Big Bang.
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