A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter that consists of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (milky), which refers to the Milky Way galaxy. Galaxies range in size from dwarfs with a few hundred million (108) stars to giants with one hundred trillion (1014) stars, all orbiting their galaxy’s mass center.
Galaxies are classified as elliptical, spiral, or irregular based on their optical form. Supermassive black holes are expected to exist at the core of many galaxies. Sagittarius A*, the Milky Way’s core black hole, has a mass four million times that of the Sun. GN-z11 is the oldest and furthest galaxy discovered as of March 2016. It is viewed as it existed barely 400 million years after the Big Bang and has a comoving distance of 32 billion light-years from Earth.
Data from NASA’s New Horizons space mission was used in 2021 to reduce the prior estimate of 2 trillion galaxies to around 200 billion (21011). This came after a 2016 estimate that there were two trillion (21012) or more galaxies in the visible universe, with an estimated 1104 stars in each (more stars than all the grains of sand on planet Earth).
The majority of galaxies have a diameter of 1,000 to 100,000 parsecs (about 3,000 to 300,000 light years) and are separated by millions of parsecs (or megaparsecs). The Milky Way has a diameter of at least 30,000 parsecs (100,000 ly) and is separated by 780,000 parsecs from the Andromeda Galaxy, its nearest big neighbor (2.5 million ly.)
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