A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally “milky”, a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (109) stars to giants with one trillion (1012) stars, each orbiting its galaxy’s center of mass.
Most galaxies are organized into groups, clusters, and superclusters. Our own Milky Way has about 400 billion stars and is part of the Local Group—a group of more than 54 galaxies that includes the Andromeda Galaxy and about 30 other smaller galaxies. The largest known galaxy is IC 1101 in the Abell 2029 cluster; it has over 100 trillion stars. There are probably more than 170 billion galaxies in the observable universe.
Formation and evolution
Galaxies form through gravitational instability: regions of higher density collapse under their own gravity to form individual objects. As these objects orbit one another within a common potential well their mutual gravitational attraction slowly dissipates their orbital energy causing them to sink towards the center of mass where they merge together forming larger objects—in this case galaxies. This process can be seen happening today on small scales within our own Milky Way as globular clusters slowly spiral inwards due to dynamical friction losing energy as they pass through areas of dense interstellar gas and dust which act to increase drag on the clusters..