Messier 108 (also known as NGC 3556) is a barred spiral galaxy in the constellation Ursa Major, seen almost edge-on from our perspective. It was discovered by Pierre Mechain around 1782, but not officially added to the Messier catalog until the 1950s. William Herschel independently rediscovered this object in 1789, and cataloged it as H V.46.
This fine edge-on galaxy lies about 1° NW of the Owl Nebula, Messier 97. It is easily spotted as a faint (magnitude 10.6) but conspicuous 7' x 1.5' streak. The halo is brighter at its center, which is surrounded by dark and light streaks. In larger telescopes, M 108 appears as a bright bar whose main body is a mix of bright and dark streaks. M 108 is a barred spiral galaxy with somewhat loosely wound arms (morphological classification SBbc), and is inclined 75° to the line of sight.
It is actually surprising how much detail can be seen in this galaxy with small instruments! Although M 108 is a barred spiral, its many obscuring dust lanes and high inclination make it look like an irregular galaxy. It appears to have no central bulge and no pronounced core, just a detail-rich mottled disk, with heavy dust obscuration along the major axis, and a few H II regions and young star clusters exposed against the chaotic background.
Messier 108 is an isolated member of the Ursa Major Cluster cluster of galaxies. It is about 45 million light-years away, and is receding from us at 772 km/sec. The galaxy contais an estimated mass of 125 billion Suns, and includes about 290 globular clusters. M 108 is surrounded by "supershells" of expanding neutral hydrogen gas extending for several kiloparsecs. These may be driven by bursts of star formation activity. The Chandra X-ray Observatory has identified 83 X-ray sources in M 108; the brightest, located at the nucleus, may be an intermediate-mass black hole containing 24 million times the mass of the Sun.