A galaxy is a massive, gravitationally bound system that consists of stars, stellar objects (such as brown dwarfs and neutron stars), nebulae, an interstellar medium of gas and dust, black holes, and an unknown component of dark matter. Examples of galaxies range from dwarfs with as few as ten million stars to giants with a hundred trillion stars or more, each orbiting through their galaxy. There are probably more than 170 billion galaxies in the observable universe.
