Magellanic Clouds Two small irregular galaxies that are gravitationally bound to the Milky Way Galaxy.

magnetic field Field which accompanies any changing electric field, and governs the influence of magnetized objects on one another.

magnetosphere A zone of charged particles trapped by a planet's magnetic field, lying above the atmosphere.

magnitude scale A system of ranking stars by apparent brightness, developed by the Greek astronomer Hipparchus. Originally, the brightest stars in the sky were categorized as being of first magnitude, while the faintest stars visible to the naked eye were classified as sixth magnitude. The scheme has since been extended to cover stars and galaxies too faint to be seen by the unaided eye. Increasing magnitude means fainter stars, and a difference of 5 magnitudes corresponds to a factor of 100 in apparent brightness.

main sequence Well-defined band on the Hertzsprung—Russell diagram, on which most stars are found, running from the top left of the diagram to the bottom right.

main-sequence turnoff Special point on the Hertzsprung—Russell diagram for a cluster, indicative of the cluster's age. If all the stars in the cluster are plotted, the lower mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence toward the red giant branch. The point where stars are just beginning to evolve off is the main-sequence turnoff.

mantle Layer of the Earth just interior to the crust.

mare Relatively dark-colored and smooth region on the surface of the Moon.

mass A measure of the total amount of matter contained within an object.

mass-luminosity relation The dependence of the luminosity of a main-sequence star on its mass. The luminosity increases roughly as the mass raised to the third power.

mass-radius relation The dependence of the radius of a main sequence star on its mass. The radius rises roughly in proportion to the mass.

mass-transfer binaries See semi-detached binary.

matter-dominated universe A universe in which the density of matter exceeds the density of radiation. The present-day universe is matter-dominated.

mesosphere Region of Earth's atmosphere lying between the stratosphere and the ionosphere, 50-80 km above Earth's surface.

meteor Bright streak in the sky, often referred to as a "shooting star," resulting from a small piece of interplanetary debris entering Earth's atmosphere and heating air molecules, which emit light as they return to their ground states.

meteor shower Event during which many meteors can be seen each hour, caused by the yearly passage of the Earth through the debris spread along the orbit of a comet.

meteorite Any part of a meteoroid that survives passage through the atmosphere and lands on the surface of Earth.

meteoroid Chunk of interplanetary debris prior to encountering Earth's atmosphere.

meteoroid swarm Pebble-sized cometary fragments dislodged from the main body, moving in nearly the same orbit as the parent comet.

micrometeoroids Relatively small chunks of interplanetary debris ranging from dust particle size to pebble-sized fragments.

Milky Way Galaxy The spiral galaxy in which the Sun resides. The disk of our Galaxy is visible in the night sky as the faint band of light known as the Milky Way.

millisecond pulsar A pulsar whose period indicates that the neutron star is rotating nearly 1000 times each second. The most likely explanation for these rapid rotators is that the neutron star has been spun up by drawing in matter from a companion star.

molecular cloud A cold, dense interstellar cloud which contains a high fraction of molecules. It is widely believed that the relatively high density of dust particles in these clouds plays an important role in the formation and protection of the molecules.

molecular cloud complex Collection of molecular clouds that spans as much as 50 parsecs and may contain enough material to make millions of Sun-sized stars.

molecule A tightly bound collection of atoms held together by the electromagnetic fields of the atoms. Molecules, like atoms, emit and absorb photons at specific wavelengths.

moon A small body in orbit about a planet.