Many are thought to have supermassive black holes at their centers. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Supermassive black holes are a common feature at the centres of galaxies. 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.
Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass.
The word is derived from the Greek galaxias ( γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.Ī galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity.
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