Clusters of galaxies, in turn, are grouped in larger structures called superclusters. On even larger scales, galaxies are arranged in huge networks. The networks consist of interconnected strings or filaments of galaxies surrounding relatively empty regions known as voids. One of the largest structures ever mapped is a network of galaxies known as the Great Wall. Shapes of galaxies.
Astronomers classify most galaxies by shape as either spiral galaxies or elliptical galaxies.
A spiral galaxy is shaped like a disk with a bulge in the center. The disk resembles a pinwheel, with bright spiral arms that coil out from the central bulge. The Milky Way is a spiral galaxy. The Milky Way, for example, makes a complete revolution once every 250 million years or so. New stars are constantly forming out of gas and dust in spiral galaxies.