Galaxies are gravitationally bound systems of stars, stellar remnants, interstellar gas, dust, and dark matter. They range in size from dwarfs with just a few billion (109) stars to giants with one hundred trillion (1014) stars, each orbiting their galaxy's centre of mass. Galaxies are categorized according to their shape as elliptical, spiral, and irregular. It is thought that many galaxies have a black hole at the centre. The central black hole of our galaxy, the Milky Way, is known as Sagittarius A, and has a mass 4 million times greater than the sun.
