| Magellanic Clouds | Two neighboring galaxies visible to the naked eye from southern latitudes. |
| magnetic pole | One of two points on a magnet (or the Earth) at which the greatest density of lines of force emerge. A compass needle aligns itself along the local lines of force on the Earth and points more or less toward the magnetic poles of the Earth. |
| magnetosphere | The region around a planet in which its intrinsic magnetic field dominates the interplanetary field carried by the solar wind; hence, the region within which charged particles can be trapped by the planetary magnetic field. |
| magnitude | A measure of the amount of light flux received from a star or other luminous object. |
| main sequence | A sequence of stars on the Hertzsprung-Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right. |
| major axis (of ellipse) | The maximum diameter of an ellipse. |
| mantle (of Earth) | The greatest part of the Earth's interior, lying between the crust and the core. |
| mare (pl. maria) | Latin for "sea"; name applied to the dark, relatively smooth features that cover 17 percent of the Moon. |
| mass | A measure of the total amount of material in a body; defined either by the inertial properties of the body or by its gravitational influence on other bodies. |
| mass-light ratio | The ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity. The mass-light ratio gives a rough indication of the types of stars contained within a galaxy and whether or not substantial quantities of dark matter are present. |
| mass-luminosity relation | An empirical relation between the masses and luminosities of many (principally main-sequence) stars. |
| Maunder Minimum | The interval from 1645 to 1715 when solar activity was very low. |
| mean solar day | Average length of the apparent solar day. |
| merger (of galaxies) | When galaxies (of roughly comparable size) collide and form one combined structure. |
| meridian (celestial) | The great circle on the celestial sphere that passes through an observer's zenith and the north (or south) celestial pole. |
| meridian (terresteial) | The great circle on the surface of the Earth that passes through a particular place and the North and South Poles of the Earth. |
| Messier catalog | A catalog of nonstellar objects compiled by Charles Messier in 1787 (includes nebulae, star clusters, and galaxies). |
| metals | in general, any element or compound whose electron structure makes it a good conductor of electricity. In astronomy, all elements beyond hydrogen and helium. |
| meteor | The luminous phenomenon observed when a small piece of solid matter enters the Earth's atmosphere and burns up; popularly called a "shooting star." |
| meteorite | A portion of a meteoroid that survives passage through the atmosphere and strikes the ground. |
| meteoroid | A particle or chunk of typically rocky or metallic material in space before any encounter with the Earth. |
| meteor shower | Many meteors appearing to radiate from a common point in the sky caused by the collision of the Earth with a swarm of solid particles, typically from a comet. |
| Milky Way | Our own galaxy. The band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Galaxy. |
| minor planet | See asteroid. |