KEY TERMS

• • Galaxy

    An enormous collection of stars, usually numbering in the billions to hundreds of billions. Our sun and its family of planets (our solar system) is just one of perhaps 400 billion stars in the Milky Way Galaxy. Galaxies come in a variety of shapes and sizes. Ours is shaped like a spiral disk with a bright bulge in the center and is so vast that it takes light (the fastest thing we know of) 100,000 years to cross from one side to the other.