M Abbreviation for the ➤ Messier catalog of galaxies, nebulae and star clusters.

Mab A small inner satellite of Uranus, discovered in 2003. It is about 25 km (16 miles) across. Mab orbits Uranus at the same distance as one of Uranus's rings and may be the source of the material for that ring.

Magellan A US spacecraft placed in orbit around ➤ Venus to map the surface by means of ➤ synthetic aperture radar. It was launched from the Space Shuttle Atlantis on May 4, 1989. The use of radar was essential because Venus is perpetually covered by opaque cloud. Magellan arrived at Venus on August 10, 1990, and completed its first phase of operations in May 1991, having mapped 84 percent of the surface. The next phase of observation involved filling in gaps and making more detailed observations. Magellan burned up in the venusian atmosphere in 1994.

Magellanic Clouds Two small, irregular galaxies, which are satellites of our own ➤ Galaxy. They are visible as hazy patches in the southern sky. The Large Magellanic Cloud (LMC) is in the constellation Dorado and is about 170 000 light years away. The Small Magellanic Cloud (SMC), in Tucana, is about 210 000 light years distant.

Magellanic Stream A long streamer of neutral hydrogen gas apparently spanning the 200 000 light years between the ➤ Magellanic Clouds and our own ➤ Galaxy. It forms an arc 150° long in the southern sky.