A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter, an important but poorly understood component. 1 2 The word galaxy is derived from the Greek galaxias (γαλαξίας), literally milky , a reference to the Milky Way. Examples of galaxies range from dwarfs with as few as ten million (107) stars 3 to giants with one hundred trillion (1014) stars, 4 each orbiting their galaxy s own center of mass.
Galaxies contain varying numbers of planets, star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Many galaxies are believed to have supermassive black holes (SMBH) at their center. These are thought to be the primary driver of active galactic nuclei found at the core of such galaxies. The Milky Way s presumed SMBH, known as Sagittarius A*, has a mass four million times that of our sun. 5