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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.
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