Globular clusters in spiral galaxies are mostly found in the galactic halo. Globular clusters tend to be older, more massive, denser, and contain fewer heavy elements than open clusters, which are generally found in the disks of spiral galaxies. Globular clusters are fairly common; there are over 150 known in the Milky Way, with possibly many more still to be found. The Andromeda Galaxy, comparable in size to the Milky Way, may have as many as 500. Some giant elliptical galaxies (particularly those at the centers of galaxy clusters), such as M87, have as many as 13,000 globular clusters.