Galaxies can also have satellites in the form of dwarf galaxies and globular clusters. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets. Draco also features several interacting galaxies and galaxy clusters. The arms of spiral galaxies rotate around the galactic center. These combine to provide fundamental distance estimates to supernovae in other galaxies. Interspersed among superclusters are large voids of space where few galaxies exist. There is consensus that supermassive black holes exist in the centers of most galaxies. The Andromeda Galaxy has a total of 15 satellite galaxies, including M32 and M110. Astronomers speculate that this is due to interactions with other galaxies. Most galaxies are gravitationally bound to a number of other galaxies. Stars in elliptical galaxies are much older than stars in spiral galaxies. M87 is one of the most massive galaxies in the local Universe. Applications include simulations of plasmas, galaxies and molecules. Observations indicate that there are SMBHs located near the center of most normal galaxies. Lewin was closely involved in ROSAT observations of the nearby galaxies M31 and Messier 81.