Title:[0993] M81, NGC 3031, Type Sb spiral galaxy Caption:M81, NGC 3031, type Sb spiral galaxy in the constellation Ursa Major. Kitt Peak National Observatory 4-meter Mayall telescope photograph. Copyright: Credit:National Optical Astronomy Observatories

Title:[1175] Spiral galaxy in Ursa Major Caption:M81, NGC 3031, type Sb spiral galaxy in the constellation Ursa Major. Kitt Peak National Observatory 4-meter Mayall telescope photograph. Copyright: Credit:National Optical Astronomy Observatories

Title:[0022] Optical image of M82 (NGC 3034) Caption:This optical photograph of M82 shows a complex structure of stars and dust. M82, a nearby edge-on galaxy (inclination ~10 degrees), lies at a distance of ~3 Mpc (10 million light years) and has been a popular target for study since it was one of the closest examples of what were believed to be exploding galaxies. This characterization of M82 was supported by its appearance in the light of Ha. In Ha light, the galaxy exhibits extensive filaments extending 10,000 light years above and below the plane of the galaxy. The velocity field of these filamentary structures indicates radial velocities of ~ 100 km/sec which for material ejected perpendicular to the plane of the galaxy can be converted to space velocities of nearly 1,000 km/sec. These high velocity filaments would seem to support the "explosion" scenario. However, subsequent observations at other wavelengths have caused a reinterpretation of the mechanism responsible for M82's complex properties. Radio observations of the distribution of neutral hydrogen show that M82 has strongly interacted with its large neighbor galaxy M81. The result of this interaction is a cloud of material from the spiral galaxy M81, which may be passing through M82 at a speed of ~ 100 km/sec. The observed 100km/sec motions in the filaments are then reinterpreted as resulting from light from the center of M82 scattered by this screen of slow-moving dust through which M82 is drifting. Copyright:(c) Smithsonian Astrophysical Observatory Credit:Courtesy of Lick Observatory