galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, and interstellar medium of gas and dust, and, it is hypothesized, an important but poorly understood component called dark matter. The word galaxy is derived from the Greek galaxia, literally “milky”, a reference to the Milky Way. Examples of galaxies range from dwarfs with as few as ten million (107) stars to giants with a hundred trillion (1014) stars, each orbiting their galaxy’s own center of mass.