Magellanic Clouds Two nearby small irregular galaxies about 160,000 light years (Large Magellanic Cloud), and 200,000 light years (Small Magellanic Cloud) distant, visible to the naked eye from the southern hemisphere
magnetic field Field which accompanies any electric current or changing electric field, and governs the influence of magnetized objects on one another.
magnetosphere The region of space surrounding a rotating, magnetized object in which the motions of charged particles are controlled by the object's magnetic field..
magnitude The method we use today to compare the apparent brightness (magnitude) of stars began with Hipparchus, a Greek astronomer who lived in the second century BC. Hipparchus called the brightest star in each constellation "first magnitude." Ptolemy, in 140 A.D., refined Hipparchus' system and used a 1 to 6 scale to compare star brightness, with 1 being the brightest and 6 the faintest. [More Info]
main sequence A well-defined band on an H-R diagram on which most stars tend to be found, running from the top left of the diagram to the bottom right.
main-sequence turnoff Special point on an H-R diagram for a cluster. If all the stars in a particular 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.
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.
matter-dominated universe A universe in which the density of matter exceeds the density of radiation. The present-day universe is matter-dominated.
matter-antimatter annihilation A highly efficient process in which equal amounts of matter and anti-matter collide and destroy each other, producing a burst of energy, primarily in the form of gamma rays.
microwave background radiation See cosmic microwave background radiation.
Milky Way Galaxy The specific galaxy to which the Sun belongs, so named because most of its visible stars appear overhead on a clear, dark night as a milky band of light extending across the sky. [More Info: Field Guide]
millisecond pulsar A pulsar whose period indicates that the neutron star is rotating nearly 1000 times each second.
minute of arc See "arc minute."
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 150 light years 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.
momentum A measure of the state of motion of a body; the momentum of a body is the product of its mass and velocity. In the absence of a force, momentum is conserved.