M MACHOs Mach's principle macula Maffei galaxies Magellan Magellanic Clouds Magellanic Stream Magellan Project magnetic monopole magnetic star magnetic storm magnetograph magnetometer magnetopause magnetosphere magnetotail magnification magnitude magnitude of an eclipse main sequence main-sequence star major planet Maksutov telescope manganese star mantle mare Maria Mitchell Observatory Mariner Markarian galaxy Mars(1) Mars(2) Mars 96 Mars Global Surveyor Mars Observer Mars Pathfinder Mars Surveyor 1998 mascon maser mass function mass-luminosity relation mass transfer Mathilde Mauna Kea Observatories Maunder diagram Maunder minimum Max-Planck-Institut für Astronomie Max-Planck-Institut für Radioastronomie Maxwell Montes Maxwell Telescope Mayall Telescope McDonald Observatory McMath-Pierce Solar Telescope Facility mean anomaly mean equinox mean motion mean parallax mean solar day mean solar time mean Sun mechanics megaparsec Melpomene Mensa mensa Merak Mercury(1) Mercury(2) mercury star meridian(1) meridian(2) meridian circle meridian passage MERLIN Merope mesosiderite mesosphere Messier Catalogue metagalaxy metal abundance metallic-line star metamorphic rock meteor Meteor Crater meteorite meteoroid meteor shower meteor stream Metis(1) Metis(2) Metonic cycle metric Meudon Observatory Mice Michelson interferometer Michelson stellar interferometer microlensing micrometeorite micrometeoroid micrometer micrometre Microscope Microscopium microwave astronomy microwave background radiation Midcourse Space Experiment midsummer Milankovic cycles Milk Dipper Milky Way Millimeter Array millimetre-wave astronomy Mills Cross Milne universe Mimas Mimosa mineral Minkowski's Footprint minor planet Mintaka minute minute of arc Mir Mira Mirach Miranda Mira star Mirfak mirror Mirzam missing mass mixmaster universe Mizar MK classification MMT mock Sun model molecular cloud molecular line radio astronomy Molonglo Observatory momentum monocentric eyepiece Monoceros monochromator mons month moon Moon Morgan, Keenan and Kellman classification morning star Mount Bigelow Mount Graham International Observatory mounting Mount Lemmon Mount Stromlo and Siding Spring Observatories Mount Wilson Observatory moving cluster MRAO Mrkos, Comet msh MSSL M star MSX M-type asteroid Mu Cephei Mullard Radio Astronomy Observatory Mullard Space Science Laboratory Multiple Mirror Telescope multiple star mural quadrant Murzim Musca MUSES-B MUSES-C