MACHOs: Massive Compact Halo Objects – like dim stars, brown dwarfs, or black holes – that could account for some of the dark matter. The name was chosen to contrast with WIMPs.
Magellanic Clouds: Two small irregular galaxies, satellites of a Milky Way Galaxy, visible in the southern sky.
magnetic-field lines: Directions mapping out the direction of the force between magnetic poles; the packing of the lines shows the strength of the force.
magnetic lines of force: See magnetic field lines.
magnetic monopole: A single magnetic charge of only one polarity; may or may not exist.
magnetosphere: A region of magnetic field around a planet.
magnification: An apparent increase in angular size.
magnitude: A factor of 5√100 = 2.511886 . . . in brightness. See absolute magnitude and apparent magnitude. An order of magnitude is a power of ten.
magnitude scale: The scale of apparent magnitudes and absolute magnitudes used by astronomers, in which each factor of 100 is brightness corresponds to a difference of 5 magnitudes.
main sequence: A band on a Hertzsprung-Russell diagram in which stars fall during the main, hydrogen-burning phase of their lifetimes.
major axis: The longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci. Also, the length of that line.
mantle: The shell of rock separating the cores of a differentiated planet from its thin surface crust.
mare (Plural: maria): One of the smooth areas of the Moon or on some of the other planets.
mascon: A concentration of mass under the surface of the Moon, discovered from its gravitational effect on spacecraft orbiting the Moon.
maser: An acronym for “microwave amplification by simulated emission of radiation,” a device by which certain energy levels are more populated than normal, resulting in an especially intense emission of radio radiation at a certain frequency when the system drops to a lower energy level.
mass: A measure of the inherent amount of matter in a body.
mass-luminosity relation: A well-defined relation between the mass and luminosity for main-sequence stars.
mass number: The total number of protons and neutrons in a nucleus; also known as atomic weight.
Maunder minimum: The period 1645-1715, when there were very few sunspots, and no periodicity, visible.
mean solar day: A solar day for the “mean Sun,” which moves at a constant rate during the year.
merging: The interaction of two galaxies in space with a single galaxy as the result; the merging of two spiral galaxies to from an elliptical galaxy, or of two galaxies interacting with a ring galaxy resulting, as examples.
meridian: The great circle on the celestial sphere that passes through the celestial poles and the observer’s zenith.
mesosphere: A middle layer on the Earth’[s atmosphere, where the temperature again rises above the stratosphere’s decline.
Messier numbers: Numbers of fuzzy-looking objects in the 18th-century list of Charles Messier.
metal: (a) For stellar abundances, any element higher in atomic number than 2, that is, more massive than helium. (b) In general, neutral matter that is a good conductor of electricity.
meteor: A track of light in the sky from rock or dust burning up as it galls through the Earth’s atmosphere.
meteor shower: The occurrence at yearly intervals of meteors at a higher-than-average rate as the Earth goes through a comet’s orbit and the dust left behind by that comet.
meteorite: An interplanetary chunk of rock after it impacts on a planet or Moon, especially on the Earth
meteoroid: An interplanetary chunk of rock smaller than an asteroid.
micrometeorite: A tiny meteorite. The micrometeorites that hit the Earth’s surface are sufficiently slowed down that they can reach the ground without being vaporized.
mid-Atlantic ridge: The spreading of the sea floor in the middle of the Atlantic Ocean as upwelling material forces the plates to move apart.
midnight sun: The Sun seen around the clock from locations sufficiently far north or south at the suitable season.
Milky Way: The band of light across the sky from the stars and gas in the plane of the Milky Way Galaxy.
Milky Way Galaxy: The collection of gas, dust and perhaps 400 billion stars in which we live; the Milky Way is our view of the plane of the Milky Way Galaxy.
mini black hole: A black hole the size of a pinhead (and thus the mass of an asteroid) or less, thought to be left over from the big bang; Stephen Hawking deduced that such mini black holes should seem to emit radiation at a rate that allows a temperature to be assigned to it.
minor axis: The shortest diameter of an ellipse; the line from one side of an ellipse to the other that passes midway between the foci and is perpendicular to the major axis. Also, the length of that line.
minor planets: Asteroids.
Mira variable: A long-period variable star similar to Mira (omicron Ceti).
missing-mass problem: The discrepancy between the mass visible and the mass derived from calculating the gravity acting on members of clusters of galaxies.
molecule: Bound atoms that make the smallest collection that exhibits a certain set of chemical properties.
momentum: A measure of the tendency that a moving body has to keep moving. The momentum in a given direction (the “linear momentum”) is equal to the mass of the body times its speed in that direction. See also angular momentum.
mountain ranges: Sets of mountains on the Earth, Moon, etc.
multiverse: The set of parallel universes that may exist, with our observable universe as only one part.