M
- Mach's Principle
- MACHOs
- Magellanic Clouds
- Magellanic Stream
- Magnetar
- Main Asteroid Belt
- Main Sequence Lifetime
- Mars
- Masers
- Mass
- Megaparsec
- Mercury
- Meridian
- Meridian Transit
- MERLIN
- Metals
- Meteor
- Meteor Shower
- Meteor Stream
- Meteorite
- Meteoroid
- Micrometeorite
- Milky Way
- Millisecond Pulsar
- Mirror
- Molecular Cloud
- Molecular Hydrogen
- Molecule
- Moment of Inertia
- Momentum
- Monolithic Collapse
- Moon
- Mopra Telescope
- Morgan-Keenan Luminosity Class
- Morphological Peculiarities
- Morphology Density Relation