M	(a) Spectral type for red stars, such as Betelgeuse, Antares, and Proxima Centauri. [C95] (b) Designation of objects in the Messier Catalog of nebulae, star clusters, and galaxies, published in the Eighteenth Century. [f88]
M13	A great globular cluster in the constellation Hercules. [C95]
M31	The Andromeda galaxy, the largest member of the local group. It is a giant spiral galaxy that lies 2.4 million light-years away. [C95]
M32	An elliptical galaxy that orbits the Andromeda galaxy. [C95]
M33	The Pinwheel Galaxy, the third largest member of the Local Group, after Andromeda and the Milky Way. It is a spiral galaxy that lies 2.6 million light-years away. [C95]
M42	The Orion Nebula, a star-forming region in the constellation Orion. [C95]
M45	The Pleiades, a beautiful open star cluster in the constellation Taurus. It is 410 light-years away. [C95]
M51	The Whirlpool Galaxy, a stunning spiral in the constellation canes venatici. [C95]
M81	A giant spiral galaxy 11 million light-years away in the constellation Ursa Major. It rules the M81 group, the second nearest galaxy group to the Local Group. [C95]
M87	A giant elliptical galaxy in the Virgo cluster. [C95]
M104	The Sombrero galaxy, in which galactic rotation was first detected. It lies in the constellation Virgo. [C95]
M magnitude	The magnitude derived from observations at an infrared wavelength of 5 microns. [H76]
M Star	Stars of spectral type M are cool red stars with surface temperatures of less than 3600 K whose spectra are dominated by molecular bands, especially those of TiO. Examples are Betelgeuse and antares. M dwarfs are the most numerous type in our galaxy. [H76]
MS Stars	M-type stars with ZrO bands. [H76]
Mach Number	(a) The ratio of the speed of a moving object (e.g. a high-speed aircraft) to the speed of sound in the air or other medium through which the object is traveling. An aircraft passes through the `sound barrier' as the Mach number exceeds one; at this speed the air resistance increases sharply. [DC99] (b) In shock flow: The ratio of the speed of a shock wave to the sound speed in the same medium. [H76] (c) The Mach Number is used frequently in aerodynamics. It is the ratio of the speed of an object compared with the speed of sound in the same medium. The number owes its origin to the Austrian scientist E. Mach (1838-1916) who first used it in 1887. It is also employed in ballistics and in heat-transfer work. [JM92]
Mach's Principle	(a) The precept that the inertia of objects results not from their relationship to Newtonian absolute space, but to the rest of the mass and energy distributed throughout the Universe. though unproved and perhaps unprovable, mach's principle inspired einstein, who sought with partial success to incorporate it into the general theory of relativity. [F88] (b) A pre-relativity statement to the effect that the local inertial frame is determined by some average of the motion of all the matter in the Universe. in essence, mach's principle says that space, which is the arena in which matter interacts, is itself an aspect of that matter. [H76] (c) The hypothesis that the inertia of bodies - that is, their resistance to acceleration by applied forces - is determined not by any absolute properties of space but by the effects of distant matter in the Universe. equivalently, mach's principle proposes that the distinction between accelerated and nonaccelerated frames of reference is determined by the effects of distant matter. [LB90] (d) The hypothesis that the local inertial frame and the inertia of any body is determined by the distribution of all the matter in the Universe. [SILK90]
MACHO	Massive Compact Halo Object -- These are black holes, neutron stars and brown dwarfs, none of which are luminous and all of which are postulated to exist in the halos of galaxies. They are a form of dark matter. [C97]
Maclaurin Series	a power series expansion of f(x) of the form f(x) = f(0) + f'(0) x + [f"(0)/2!] x2 + . . . + [f(n) (0)/n!] xn + . . .(= taylor series at x = 0). [H76]
Maclaurin Spheroid	A form which a homogeneous self-gravitating mass can take when in a state of uniform rotation. its eccentricity varies from zero (when it is not rotating) to 1 in the limit of infinite angular momentum. [H76]
Macroscopic	Refers to scales typically encountered in the everyday world and larger; roughly the opposite of microscopic. [G99]
Maffei 1 and 2	Two galaxies discovered on infrared plates in 1968 and identified in 1970 as members (probably temporary) of the local group. the two galaxies lie in the Zone of Avoidance. Maffei 2 has since been classified as a medium-sized, average luminosity SBc II spiral at a distance of 5 ± 2 Mpc - too far away to belong to the local group - but Maffei 1 (a giant elliptical) is probably only 1 Mpc distant, marginally within the local group. [H76]
Magellanic Clouds	(a) Two relatively small, nebulous stellar systems visible only in the southern hemisphere; the larger is, however, the brightest "nebular" object in the sky. both are members of the local group of galaxies, and in fact seem to be associated, though detached, parts of the milky way system. [A84] (b) The Large and Small Magellanic Clouds, the two nearest and largest of the galaxies that orbit the milky way. The Magellanic Clouds lie in the southern sky and cannot be seen from the United States. [C95] (c) Two galaxies that lie close to the milky way galaxy. They are visible in the southern skies of Earth. [F88] (d) Two small irregular (or possibly barred spiral) galaxies (satellites of the Milky Way galaxy) about 50-60 kpc (LMC, in Dorado) and 60-70 kpc (SMC, in Toucana) distant, visible to the naked eye from the southern hemisphere. both clouds contain mainly population i stars. The LMC contains numerous ob stars and at least 10 stars that are an order of magnitude brighter (mv = - 9) than any supergiants known in our galaxy. It also contains several times our galaxy's concentration of interstellar matter. [H76]
Magellanic Stream	(a) A strand of gas spanning 300,000 light-years that the Milky Way has ripped out of the Magellanic Clouds. [C95] (b) A name given to the long HI filament that extends from the region between the Magellanic Clouds down to the south galactic pole and which appears to make a 180° arc of a great circle across the sky. [H76] (c) A name given to the common envelope of neutral hydrogen in which the LMC and SMC are embedded. [H76]
Magic Number	(a) In nuclear physics, one of several numbers of neutrons or protons that characterize especially stable atomic nuclei. They are 2, 8, 20, 28, 50 and 82. [DC99] (b) Magic numbers are the names given to numbers which signify the number of electrons in atoms of unusual stability or the number of protons and/or neutrons in very stable nuclei. For atoms the numbers are 2, 10, 18, 36, 54, 86, which are the atomic numbers of the noble gases helium, neon, argon, krypton, xenon and radon. For nuclei the numbers are 2, 8, 20, 28, 50, 82 and 126. The term magic number was first used in connection with nuclei in 1949. [JM92]
Magnesium	(a) Element with atomic number twelve. it is the fifth most common metal in the Universe and was produced by high-mass stars that exploded. [C95] (b) A light metallic element. It has the electronic configuration of neon with two additional outer 3s electrons. The element accounts for 2.09% of the Earth's crust and is eighth in order of abundance. Magnesium metal is industrially important as a major component in lightweight alloys (with aluminum and zinc). The metal surfaces develop an impervious oxide film which protects them from progressive deterioration. Symbol: Mg; m.p. 649°C; b.p. 1090°C; r.d. 1.738 (20°C); p.n. 12; r.a.m. 24.3050. [DC99]
Magnetic Bremsstrahlung	see Synchrotron Radiation [H76]
Magnetic-Dipole Radiation	Radiation emitted by a rotating magnet. [H76]
Magnetic Fields
Magnetic Moment	(a) A measure of the extent to which a physical system (e.g. an atom, or nucleus, or particle) behaves like a tiny magnet. It is generally measured in units of magnetons, i.e. e / 2mc. [CD99] (b) Symbol: m A measure of the strength of a magnet or current-carrying coil. It relates to the turning effect (moment) on it when in a given field. It is defined as the torque T observed in a unit field at 90° to the magnetic axis: m = T/B The units are ampere meters-squared (A m2). For a coil with N turns and area A carrying a current I: m = NIA In this case m is often called the electromagnetic moment of the coil. [DC99]
Magnetic Moment	(a) The intrinsic spins of the electrons in an atom or ion, together with the motion of the electrons round the nucleus, give rise to a magnetic field around the atom. The magnitude of this field is determined by the magnetic moment of the atom or ion. [D89] (b) The ratio of the torque experienced by a magnet aligned perpendicular to a magnetic field to the strength of the magnetic field. [H76]
Magnetic Monopole	(a) A hypothetical particle that carries an isolated north or south magnetic pole. This is in contrast to magnets which are north-south pole pairs. If magnetic monopoles exist, they must be very massive. [CD99] (b) A massive particle with but one magnetic pole, the production of which is indicated in some theories of the early Universe. [F88] (c) An elementary particle postulated but not discovered, equivalent to an isolated N- or S-pole. The monopole was postulated by analogy with the proton to provide symmetry between electricity and magnetism. [DC99] (d) Hypothetical particle that carries an isolated north or south magnetic pole. All known magnets are dipoles. [D89] (e) A magnet with an isolated north (or south) pole, rather than a pair of equal-strength north and south poles, as in conventional magnets. Magnetic monopoles have never been observed, but they are predicted to exist by grand unified theories. [G97] (f) An hypothesized particle that would have either a magnetic north pole or a magnetic south pole but not both. all magnetic particles and magnets ever observed have both poles. magnetic monopoles are predicted by grand unified theories of physics. that grand unified theories predict the existence of large numbers of magnetic monopoles, when none have been discovered, is called the monopole problem. see Grand Unified Theories [LB90]
Magnetic Monopole Problem	A problem, discovered by John Preskill in 1979, concerning the compatibility of grand unified theories with standard cosmology. Preskill showed that if standard cosmology were combined with grand unified theories, far too many magnetic monopoles would have been produced in the early Universe. [G97]
Magnetic Pressure	The pressure exerted by a magnetic field on the material that contains the field. in gaussian units it is given by pm = b2 / 8, where b is the magnetic field strength. [H76]
Magnetic Stars	Stars (usually of spectral type a) with strong integrated magnetic fields ranging up to 30,000 gauss. [H76]
Magnetic Susceptibility	when a magnetic field is applied to a material, magnetisation is induced. The ratio of the magnetisation induced to the applied magnetic field is the magnetic susceptibility. [D89]
Magnetohydrodynamics	MHD The study of how magnetic fields interact with conducting fluids (e.g. plasmas or liquid metals).
Mass Defect	The mass equivalent of the binding energy of a nucleus. see Binding Energy [DC99]
Magneton	see Bohr Magneton [H76]
Magnetopause	The region in earth's ionosphere where the magnetosphere meets the Solar Wind. essentially, it is the place where earth's magnetic field stops; the region above the magnetopause is no longer part of earth's atmosphere, but is part of interplanetary space. [H76]
Magnetosphere	(a) The extent of a planet's magnetic field. the earth's magnetosphere is shaped roughly like a teardrop, with the point opposite the sun; this is due to the effect of the Solar Wind. [A84] (b)The region of space surrounding a rotating, magnetized sphere. specifically, the outer region of earth's ionosphere, starting at about 1000 km above earth's surface, and extending to about 60,000 km (or considerably farther, on the side away from the sun). [H76] (c) The region about a star or planet in which the magnetic field of the body itself dominates the gas dynamics of the system. The Earth's magnetosphere is bounded by a shock wave associated with the flow of the Solar Wind past the magnetic cavity associated with the Earth's magnetic field. In the case of compact stars embedded in the winds from companion stars, magnetospheres similar to that of the Earth are set up. In the case of pulsars, the magnetosphere is bounded by the `light cylinder' at which the corotation velocity of the magnetic field approaches the velocity of light. [D89]
Magnification	The effect of an optical system on the apparent angular size of an object. An increase in angular size occurs if the magnification factor is greater than 1. If the factor is less than 1 then demagnification occurs. [McL97]
Magnitude	(a) A logarithmic brightness scale for astronomical objects. [McL97] (b) The measured brightness of a celestial body. Dim objects have magnitudes of high numbers, bright objects have magnitudes of low or even negative numbers. Seen from earth, stars of (apparent) magnitude 6 or higher cannot be detected with the naked eye. The Full Moon has a magnitude of -11, and the Sun one of -26.8. In order to standardize measurements of the brightness of more distant objects, the system of absolute magnitude is used. A measure of the radiation at all wavelengths emitted by a star is known as the bolometric magnitude. [A84] (c) The measure of a star's brightness. apparent magnitude measures a star's apparent brightness - that is, how bright a star looks from Earth. absolute magnitude measures a star's intrinsic brightness - that is, how much light the star actually emits. [C95] (d) The brightness of a star or planet, expressed on a scale in which lower numbers mean greater brightness. apparent magnitude indicates the brightness of objects as we see them from earth, regardless of their distance. Absolute magnitude is defined as the apparent magnitude a star would have if viewed from a distance of ten parsecs, or 32.6 light-years. each step in magnitude equals a difference of 2.5 times in brightness: the brightest stars in the sky are apparent magnitude 1; the dimmest, 6. The magnitudes of extremely bright objects are expressed in negative values - e.g., the apparent magnitude of the Sun is about -26. [F88] (e) An arbitrary number, measured on a logarithmic scale, used to indicate the brightness of an object. Two stars differing by 5 mag differ in luminosity by 100. 1 magnitude is the fifth root of 100, or about 2.512. The brighter the star, the lower the numerical value of the magnitude. see also Pogson's Ratio [H76] (f) A measure. on a logarithmic scale, used to indicate the brightness of a celestial object. A 1-magnitude difference in brightness between two stars corresponds to a difference in luminosity by 100.4 or 2.51; 5 magnitudes corresponds to factor of 100 in luminosity. [SILK90]
Magnitude of a Lunar Eclipse	The fraction of the lunar diameter obscured by the shadow of the Earth at the greatest phase of a lunar eclipse (see Eclipse, Lunar), measured along the common diameter. [s92]
Magnitude of a Solar Eclipse	The fraction of the Solar diameter obscured by the moon at the greatest phase of a Solar eclipse (see Eclipse, Solar), measured along the common diameter. [s92]
Maia Sequence	a hypothetical sequence of blue variable stars named for the B-type star Maia (20 tau) in the Pleiades. Maia has the lowest rotational velocity of any B star in the Pleiades. [H76]
Main Beam	The lobe of maximum sensitivity in a radio telescope. [H76]
Main Lines (of an OH source)	The transitions that emit radiation at 1665 and 1667 mhz. [H76]
Main Sequence	(a) Band that runs from top left to bottom right on the hertzsprung-russell diagram representing the majority of stars. Stars off the main sequence are in some way uncharacteristic and include red giants, blue dwarfs, Cepheids and novae. [A84] (b)The curving path in the Hertzsprung-Russell diagram along which most stars lie. [F88] (c) The principal sequence of stars on the HR diagram, containing more than 90% of the stars we observe, that runs diagonally from the upper left (high temperature, high luminosity) to the lower right (low temperature, low luminosity). A star appears on the main sequence after it has started to burn hydrogen in its core, and is estimated to stay on the main sequence until it has used up about 12% of its hydrogen (for a 1 m star, about 1010 yr). The observed upper limit of the main sequence is 60 m; stars above this limit are unstable to nuclear driven pulsations in the early stages of stellar evolution. The calculated lower limit of the main sequence is 0.085 m; stars below this limit cannot achieve nuclear chain reactions. H to He fusion represents more than 80% of the maximum possible nuclear energy available to a star. [H76] (d) The pattern of stars in a luminosity temperature diagram, or H-R diagram. there is a correlation between intrinsic luminosity and temperature for stars powered by hydrogen-burning nuclear reactions, and such stars are said to be main sequence stars, or to lie along the main sequence of the H-R diagram. see H-R Diagram [LB90] (e) The principal sequence of stars on the graph of luminosity versus effective temperature (the Hertzsprung-Russell diagram) for a group of stars. More than 90 percent of the stars we observe lie on the main sequence, where they remain throughout their hydrogen-burning phase. The observed upper mass limit of the main sequence is about 60 Solar masses, more massive stars being unstable, and the calculated lower limit is 0.085 Solar mass, less massive stars being incapable of igniting their hydrogen. [SILK90] (f) A star, like the Sun, that fuses hydrogen into helium at its core. Ninety percent of all stars are main-sequence stars; examples are Sirius, Vega, Altair, and Alpha Centauri A, B, and C. [C95]
Main-Sequence Turn-Off	The point on the HR diagram of a star cluster where main-sequence stars are beginning to leave the main sequence. The main-sequence turnoff measures age: all other things being equal, the older a star cluster, the fainter the main-sequence turnoff. [C95]
Maksutov Telescope	A reflector whose primary mirror is spheroidal instead of parabolic. The light initially passes through a large concave lens to remove the spherical aberration. [H76]
Malmquist Bias	The systematic distortion in a standard candle's effective range due to failure in detecting the fainter examples of the standard candle at large distances. [C97]
Malmquist Correction	A correction introduced into star counts distributed by apparent magnitude. [H76]
MAMA	(a) Multi-Anode Microchannel Analyzer . A detection system used with microchannel plates to detect events. Used as an imaging system in the ultraviolet. see Microchannel Plates. [McL97] (b) Machine a Mesurer pour l'Astronomie (France). [LLM96]
Manganese
Manganese Stars	Stars with an anomalously high mn-fe ratio, which show deviations from the odd-even effect for phosphorus, gallium, and yttrium. [H76]
Manifold	(a) A mathematical concept used to describe the geometry of spacetime. [H76] (b) A curved space described by the mathematical discipline of differential geometry. [D89]
Mantissa	The decimal part of a common logarithm. [H76]
Many-Body Problem	The difficulty of calculating the interactions - e.g., the newtonian gravitational interactions - of three or more objects. [F88]
Many-Worlds	Everett-Wheeler interpretation of quantum mechanics: The view of quantum mechanics holding that a physical system simultaneously exists in all of its possible states prior to and after a measurement of the system. (compare with the copenhagen interpretation of quantum mechanics.) In the many-worlds interpretation, each of these simultaneous existences is part of a separate Universe. Every time we make a measurement of a physical system and find it to be in a particular one of its possible states, our Universe branches off to one of the Universes in which the system is in that particular state at that moment. The system, however, continues to exist in its other possible states, in parallel Universes. see Copenhagen Interpretation of Quantum Mechanics; Quantum Mechanics [LB90]
Marching Subpulse	The weaker component of a pulsar pulse when its period is more than half that of the main pulse, so that the subpulse occurs at progressively later intervals between successive main pulses. [H76]
Mare	An area on the moon that appears darker and smoother than its surroundings. Lunar maria are scattered basaltic flows. (plural is Maria)[H76]
Mariner Spaceprobes	A series of US spaceprobes launched to explore the planets of the Solar System, particularly Mercury, Venus and Mars. [A84]
Markarian Galaxy	A galaxy in Markarian's list of galaxies with abnormally strong ultraviolet continua. they have broad emission lines arising in a bright, semi-stellar nucleus. Markarian 231 is the most luminous galaxy known if it is at its Hubble distance. [H76]
Markovian process	a random process in which the probability of performing a transition to a certain state at a given time depends solely on the state in which the system is found at this time. [D89]
Mars	(a) Fourth major planet out from the Sun. [A84] (b) Fourth planet from Sun. Mass 6.45 × 1026 g (0.11 Earth's); radius (1974) 3394 km. Oblateness 0.0092. mean density 4.0 g cm-3. Mean distance from sun 1.5237 au; e = 0.0934; i = 1°.85. Sidereal period 687 days; synodic period 779.9 days. Mean orbital velocity 24.2 km s-1. Surface temperature 248 k. rotational period 24h37m22s.6. Obliquity 23°59'. Surface gravity 0.38 that of Earth: escape velocity 5.1 km s-1. Albedo 0.16. Atmosphere more than 90% co2, traces of o2, co, h2o. Atmospheric pressure from mariner 7 3.5 millibars. The core is probably liquid ni-fe. two tiny satellites (phobos and deimos), both of which are locked in synchronous rotation with mars. [H76]
Mascons	Abbreviated form of mass concentrations: apparent regions on the lunar surface where gravity is somehow stronger. The effect is presumed to be due to localized areas of denser rock strata. [A84] (b) gravity anomalies found on the moon. As of 1971, 12 mascons were known. [H76]
Maser	(microwave amplification by stimulated emission of radiation). A device that utilizes the natural oscillations of an atom or molecule to amplify electromagnetic radiation. Molecules are pumped into a metastable upper state by incident radiation of broad frequency via upper states that connect to the metastable state. They are then stimulated downward by radiation of a certain frequency connecting the metastable and ground states. When a bound electron in the metastable state is hit by a photon of the right frequency, the electron can return to a lower state by emitting a photon of exactly the same frequency as the incident photon; and it will emit it in exactly the same direction in which the incident photon is scattered. This means that the photons move off precisely in phase. If each hits another electron in the same state, there will be four photons in phase, etc. [H76]
Mass	(a) The quantitative property of an object due to the matter it contains. (Weight, in contrast, describes a force with which a body is attracted towards a gravitational focus.) Units of mass are grams and kilograms. [A84] (b) Measure of the amount of matter in an object. Inertial mass indicates the object's resistance to changes in its state of motion. Gravitational mass indicates its response to the gravitational force. In the general theory of relativity, gravitational and inertial mass are revealed to be aspects of the same quantity. [F88] (c)The measure of the inertia of an object, determined by observing the acceleration when a known force is applied. The gravitational force acting on an object is found to be proportional to its mass, as is the gravitational force that it exerts on other objects. [G97]
Mass Absorption Coefficient	A measure of the fraction of radiation absorbed at a certain wavelength per unit mass. [H76]
Mass Defect	(a) The mass equivalent of the binding energy of a nucleus. See binding energy. [DC99] (b) The deficiency in the mass of an atomic nucleus with respect to the sum of the individual masses of its constituent particles. It represents the amount of mass converted into the binding energy of the nucleus; and when the nucleus is formed, this binding energy is released. The Sun is losing more than 4 million tons of mass every second by the conversion of hydrogen into helium. [H76]
Mass Discrepancy	In the study of clusters of galaxies, the difference between the mass of a cluster obtained by using the virial theorem and the mass (usually much smaller) inferred from the total luminosities of the member galaxies. Typically MVT / ML > 10. [H76]
Mass Fraction	The fractional amount (by mass) of a given element or nuclide in a given composition. [H76]
Mass Function	A numerical relation between the masses of the two components of a spectroscopic binary when the spectral lines of only one component can be seen: f (MpMs) = (Ms3 sin3 i) / (Mp + sM)2, where Mp = mass of primary, Ms = mass of secondary, and i = inclination of the orbit. [H76]
Mass-Luminosity-Radius Relation	All nondegenerate stars with the same mass and the same chemical composition will have the same radius and the same luminosity. see Vogt-Russell Theorem [H76]
Mass-Luminosity Ratio	(a) The ratio of the mass of a system, expressed in Solar masses, to its visual luminosity, expressed in Solar luminosities. The Milky Way has a mass-luminosity ratio in its inner regions of 10, indicating that the typical star is a dwarf of mass about half that of the sun. A rich cluster of galaxies such as the Coma cluster has a mass-luminosity ratio of about 200, indicating the presence of a considerable amount of dark matter. [SILK90] (b) Plot, on a logarithmic scale, of bolometric absolute magnitude versus mass for main-sequence stars of given chemical composition, derived by Eddington in 1924: (L / L) = (M / M), where in general 3.5. [H76]
Mass Models	Models that attempt to infer the distribution of mass in an astronomical system by comparing the observed properties of the system (such as the distribution of light) with those properties predicted by various theoretical distributions of mass. [LB90]
Mass Number	see Atomic Mass Number. [C95]
Mass of the Galaxy	The mass has been assessed at various galactocentric distances: At R = 9 kpc, M = 1.0 × 1011 Solar masses. At R = 35 kpc, M = 4.0 × 1011 Solar masses. And, at R = 230 kpc, M = 13.0 × 1011 Solar masses. [BFM2002]
Mass-Radius Relation (Chandrasekhar)	For any given mass less than the Chandrasekhar limit, there is a unique radius for a totally degenerate star. [H76]
Mass-Shell	In quantum mechanics, a particle's energy and momentum are essentially independent of each other. A particle is said to be "on mass-shell" when its energy and momentum satisfy the formula from special relativity: E2 = p2c2 + m02 c4 which is necessary for it to exist as a real observable particle. Otherwise, the particle is "virtual". [CD99]
Mass-to-Light Ratio	(a) The amount of mass in an object divided by its luminosity, both measured in Solar units. The Sun has a mass-to-light ratio of one, because it has one Solar mass and one Solar luminosity. Stars brighter than the Sun, such as upper main-sequence stars, giants, and supergiants, have low mass-to-light ratios, because most have somewhat more mass than the Sun but much more luminosity. Stars fainter than the Sun, such as red, orange, and white dwarfs, have high mass-to-light ratios, because most have smaller masses than the Sun but much smaller luminosities. Dark matter, by definition, has a high mass-to-light ratio: it has much mass but radiates little or no light. [C95] (b) The ratio of total mass in a physical system to the amount of light produced by that system. Often, mass that does not produce light or radiation of any kind can nevertheless be detected by its gravitational effects. Systems with a large amount of such dark matter would have a high mass-to-light ratio. [LB90]
Massive Black Hole	Utilized in a theoretical model for quasars and active galactic nuclei, according to which the energy source is due to infall (and resultant heating) of gas and stars onto a supermassive central black hole. [SILK90]
Massive Halos	Spherical distributions of dark matter surrounding galaxies. see Dark Matter [LB90]
Massless Black Hole	In string theory, a particular kind of black hole that may have large mass initially, but that becomes ever lighter as a piece of the Calabi-Yau portion of space shrinks. When the portion of space has shrunk down to a point, the initially massive black hole has no remaining mass - it is massless. In this state, it no longer manifests such usual black hole properties as an event horizon. [G99]
Master Equation	an equation describing the evolution of the probability of a state at a given time as the balance between transitions leading to this state, and transitions removing the system from this state. [D89]
Materialism	Belief that material objects and their interactions constitute the complete reality of all phenomena, including such seemingly insubstantial phenomena as thoughts and dreams. Compare spiritualism. [F88]
Matrix	A rectangular array of numbers or algebraic quantities representing a system of entities related in a systematic manner. Matrices do not obey the commutative laws of multiplication. An m × n matrix has m rows and n columns. Matrices may often be abbreviated to A = [aij]. By convention, the first subscript (i) gives the number of the row; the second (j) gives the number of the column. [H76]
Matter Era	The era following the radiation era. The matter era started when the temperature of the primeval fireball had dropped to 3000 K, at which time the recombination of hydrogen became possible. [H76]
Matter Fields	the fields whose quanta describe the elementary particles making up the material content of the Universe (as opposed to the gravitons and their supersymmetric partners).[D89]
Matter-to-Antimatter Ratio	The ratio of mass in particles to mass in antiparticles. For every type of particle, there is an antiparticle counterpart. The positron, for example, is the antiparticle of the electron and is identical to the electron except for having opposite electrical charge. The abundances of particles and antiparticles do not have to be equal. It appears that our Universe is made up almost entirely of particles, rather than antiparticles, although there is no fundamental difference between the two kinds of matter. [LB90]
Matter Waves	Characteristic by virtue of which matter, like energy, displays the qualities of waves as well as of particles. See wave-particle duality. [F88]
Maximum Entropy Method	MEM: An image reconstruction methodology which defines a measure of information content and seeks to maximize it. [McL97]
Maxwell	The cgs unit of magnetic flux through 1 cm2 normal to a field of 1 gauss. 1 Mx corresponds to 10-8 Wb. [H76]
Maxwell-Boltzmann Distribution	The distribution function that any species of particle will have if it is in thermodynamic equilibrium. This distribution function describes both the equilibrium in velocity space or kinetic energy, and the equilibrium in potential energy. [H76]
Maxwell-Boltzmann Statistics	The statistical rules for studying systems of identical particles in classical physics. It was tacitly assumed that particles, although identical, could be distinguished in principle. It can be shown that, for low concentrations of particles, especially at high temperatures, the classical statistics gives results similar to the more exact Fermi-Dirac and Bose-Einstein statistics. It was the failure of classical statistics to predict results in agreement with experiment in certain cases which led to the development of quantum theory. See Maxwell distribution, ultraviolet catastrophe. [DC99]
Maxwell Distribution	(a) An expression for the statistical distribution of velocities among the molecules of a gas at a given temperature. [H76] (b) The laws for the distribution of speeds or kinetic energies among the molecules of a gas in equilibrium. The number of molecules per unit range of speed at speed v is given by Nv = A v2 exp(-mv2 / 2kT) where m is the mass of a molecule, T is the kelvin temperature, k is the Boltzmann constant, and A is a constant. The number of molecules per unit range of kinetic energy at kinetic energy E is given by NE = [(BE) exp(-E / kT)]1/2 where B is a constant. This distribution is very closely obeyed by gases at ordinary pressures. For large concentrations of particles more exact laws are needed, particularly for the valence electrons in a solid. see Bose-Einstein Statistics; Fermi-Dirac Statistics; Maxwell-Boltzmann Statistics [DC99]
Maxwell's Equations	(a) Equations governing the varying electric and magnetic fields in a medium. [H76] (b) The equations of electromagnetism, formulated by the Scottish physicist James Maxwell in the nineteenth century. [LB90] (c) The equations that describe the variations in space and time of the electromagnetic field (after James Clerk Maxwell).[D89]
Maxwell's Theory	Theory uniting electricity and magnetism, based on the concept of the electromagnetic field, devised by Maxwell in the 1880s; shows that visible light is an example of an electromagnetic wave. [G99]
MCAO	Multi-Conjugate Active Optics -- An active optics techniquefor increasing the resolution-emhance field of view in a terrestrial telescope, by compensating fo atmospheric turbulence simultaneously along several adjacent lines of sight. [BFM2002]
MCMC	Markov Chain Monte Carlo -- Iterative simulations such as Markov Chain Monte Carlo make it possible to fit complex and more realistic Bayesian models to large and/or incomplete datasets.
Me Stars	Stars of spectral type M with emission lines in their spectra. [H76]
Mean Anomaly	In undisturbed elliptic motion, the product of the mean motion of an orbiting body and the interval of time since the body passed pericenter. Thus the mean anomaly is the angle from pericenter of a hypothetical body moving with a constant angular speed that is equal to the mean motion. see True Anomaly; Eccentric Anomaly [S92]
Mean Distance	The semi-major axis of an elliptic orbit. [S92]
Mean Elements	Elements of an adopted reference orbit (see Elements, Orbital ) that approximates the actual, perturbed orbit. Mean elements may serve as the basis for calculating perturbations. [S92]
Mean Equator and Mean Equinox	The celestial reference system determined by ignoring small variations of short period in the motions of the celestial equator. Thus the mean equator and equinox are affected only by precession. Positions in star catalogs are normally referred to the mean catalog equator and equinox (see Catalog Equinox) of a standard epoch. [S92]
Mean Free Path	(a) Symbol: 1. The average distance traveled by the particles of a fluid between collisions. It is given by = 1 / 2 n where r is the particle radius and n the density of particles. 2. The average distance traveled by electrons between collisions with the lattice in conduction. [DC99] (b) Mean length of the path of a particle between collisions. [H76] (c) The mean distance traversed by a particle before undergoing a significant deflection or collision. [SILK90]
Mean Free Time	For gas atoms or molecules in a container, or electrons and impurity atoms in a semiconductor, the average time between particle collisions. see also Mean Free Path [DC99]
Mean Life	The mean time before decay of a large number of similar particles. It is equal to 1.44 times the half-life. [H76]
Mean Molecular Weight	Total atomic (or molecular) weight divided by the total number of particles. For instance, the mean molecular weight of a plasma of pure ionized 4He would be 4 (the atomic mass number) divided by 3, the total number of particles (1 nucleus plus 2 electrons). So µ would equal 4/3. [H76]
Mean Motion	In undisturbed elliptic motion, the constant angular speed required for a body to complete one revolution in an orbit of a specified semi-major axis. [S92]
Mean Place	The coordinates, referred to the mean equator and equinox of a standard epoch, of an object on the celestial sphere centered at the Sun. A mean place is determined by removing from the directly observed position the effects of refraction, geocentric and stellar parallax, and stellar aberration (see Aberration, Stellar), and by referring the coordinates to the mean equator and equinox of a standard epoch. In compiling star catalogs it has been the practice not to remove the secular part of stellar aberration (see Aberration, Secular). Prior to 1984, it was additionally the practice not to remove the elliptic part of annual aberration (see Aberration, Annual; Aberration, E-terms of). [S92]
Mean Profile	The relatively stable curve obtained by synchronously averaging together many pulses of a pulsar together. (also called Integrated Profile or Pulse Window [H76]
Mean Solar Day	The mean length of time (24h00m00s) between two successive culminations of the Sun (i.e., the mean period from apparent noon to apparent noon). [H76]
Mean Solar Second	1/86,400 of a Mean Solar May (cf. Ephemeris Second ). [H76]
Mean Solar Time	A measure of time based conceptually on the diurnal motion of the fictitious mean sun, under the assumption that the Earth's rate of rotation is constant. [S92]
Mean Sun	A fictitious body that moves eastward in a circular orbit along the celestial equator, making a complete circuit with respect to the vernal equinox in a tropical year. [H76]
Mechanics	The study, in physics, of the influence of forces. [F88]
Median	Literally the middle value in a sequence of values arranged in increasing size order. A useful mathematical estimator of the true value from a set of values when one of these values is contaminated, i.e. known to be much larger than the average. [McL97]
mega-	A prefix meaning 106. [H76]
Megaflops	Millions of floating-point operations per second. A computer benchmark. [McL97]
Megaparsec	A unit of distance equal to a million parsecs, or 3.2616 million light-years. [C95]
Meinel Bands	Molecular bands of the N2+ radical near 8000 Å. [H76]
Meissner Effect	the phenomenon in which a metal cooled through its superconducting transition temperature in the presence of a magnetic field completely expels the field. [D89]
Meitnerium	A radioactive metallic element not found naturally on Earth. Only a few atoms of the element have ever been detected; it can be made by bombarding a bismuth target with iron nuclei. The isotope 266Mt has a half -life of about 3.4 × 10-3s. Symbol: Mt; p.n. 109. [DC99]
MEM	Maximum Entropy Method: An image reconstruction methodology which defines a measure of information content and seeks to maximize it. [McL97]
Mendelevium	A radioactive transuranic element of the actinoid series, not found naturally on Earth. Several short-lived isotopes have been synthesized. Symbol: Md; p.n. 101; most stable isotope 258Md (half-life 57 minutes). [DC99]
Meniscus Mirror	A very thin mirror with a high curvature. A method of constructing very large mirrors which assumes from the outset that the mirror is too thin to hold its shape against gravity and will require an active control system. [McL97]
Mer-Cad-Tel	Mercury-Cadmium-Telluride (HgCdTe) -- A semiconductor alloy useful as an infrared photoelectric detector. Also known as CMT (Cad-Mer-Tel) [McL97]
Mercury	(a) A transition metal that is liquid at room temperatures (bromine is the only other element with this property). The vapor is very poisonous. Mercury is used in thermometers, special amalgams for dentistry, scientific apparatus, and in mercury cells. Symbol: Hg; m.p. -38.87°C; b.p. 356.58°C; r.d. 13.546 (20°C); p.n. 80; r.a.m. 200.59. [DC99] (b) Innermost planet of the Solar System. Mass (from Mariner 10) 3.15 × 1026 g (0.054 Earth's); equatorial radius 2446 km; mean density 5.44 g cm-3. Vesc 4.2 km s-1; surface gravity 360 cm s-2. Mean distance from Sun 0.387 AU; e = 0.206, i = 7°.0. Orbital period 88 days; Vorb 47.9 km s-1. Rotation period 58.646 days, exactly 2/3 of its orbital period. Oblateness < 0.001. Synodic period 116 days. Albedo 0.06. Maximum elongation 28°. Transits of the Sun occur either 7 or 13 years apart - last transit 1973 November 10. Advance of perihelion 476" per century; relativistic advance 42".6 per century. Subsolar point temperature (from infrared observations) 408 ± 8 K; from Mariner 10, 700 K. Optical spectrum is similar to lunar maria and uplands. Surface is probably covered with lunar-like soil of low-density silicates (the core must be rich in iron to account for the high density and the magnetic field). Mariner 10 has established the existence of an extremely thin (< 2 × 10-9 millibars) atmo- sphere of He and Ar, and also the presence of a weak magnetic field (200-300 gammas), apparently inclined < 10° to the pole and offset 0.47 RM. [H76]
Merger	The formation of a galaxy from the collision of two or more separate galaxies. [C95]
Meridian	(a) Theoretical north-south line on the Earth's surface, or an extension of that line onto the night sky, connecting the observer's zenith with the celestial pole and the horizon. The meridian is used to state directional bearings. Devices and structures - such as meridian arcs - marking the meridian were once common in observatories. [A84] (b)The great circle passing through the zenith of the observer and the north and south points on his horizon. [H76] (c) A great circle passing through the celestial poles and through the zenith of any location on Earth. For planetary observations a meridian is half the great circle passing through the planet's poles and through any location on the planet. [S92]
Meridional Flow	Flow between the poles, or between the equator and the poles. A positive value indicates flow away from the equator: a negative value, flow toward the equator. [H76]
Meson	(a) Any particle made of two quarks. Examples are the pion (pi-meson) and the kaon. [C97] (b)The generic name for any strongly interacting particle with integer spin in units of (e.g. the pion and kaon). [CD99] (c) A strongly interacting particle consisting of a quark and an antiquark. [G97] (d) A nuclear particle (see Boson) with a mass intermediate between that of a proton and an electron, which is believed to be responsible for the strong nuclear force. Mesons have a spin of 0. In contrast to the case of baryons or leptons, meson number is not conserved: like photons, mesons can be created or destroyed in arbitrary numbers. Their charge can be positive, negative, or zero. [H76] (e) A particle consisting of a quark and an antiquark. [D89]
Mesosphere	The part of Earth's atmosphere immediately above the stratosphere, where the temperature drops from about 270 K to 180 K. [H76]
Messenger Particle	Smallest bundle of a force field; microscopic conveyer of a force. [G99]
Messier Catalogue	(a) List of the locations in the sky of more than 100 galaxies and nebulae, compiled by Charles Messier between 1760 and 1784. Some designations he originated are still used in identification; M1 is the Crab Nebula (in Taurus). [A84] (b) One of the earliest catalogues of nebulous-appearing astronomical objects, compiled in 1781 by the French astronomer Charles Messier. Messier's catalogue included many objects that were later realized to be galaxies. [LB90]
Metagalaxy	A synonym for the Universe. [H76]
Metal	(a) To an astronomer, a metal is any element heavier than hydrogen and helium; thus, not only are iron and copper metals, but so are elements like oxygen and neon. [C95] (b) As used in stellar spectroscopy, any element heavier than helium. [H76]
Metal-Enhanced Star Formation	A hypothesis according to which stars form preferentially from regions of above-average Z in a chemically inhomogeneous interstellar medium. [H76]
Metal-Rich Stars	A small subgroup of A-type stars in which the lines of MgII are very strong. Also called metal-strong stars or CN-strong stars or super-metal-rich Mg-strong stars. [JJ95]
Metal-Weak Stars	Weak-line stars. [JJ95]
Metallic Hydrogen	A hypothetical form of hydrogen in which the molecules have been forced by extremely high pressures to assume the lattice structure typical of metals. It is estimated that as much as 40% of Jupiter's mass (but not more than 3% of Saturn's) may be in the form of metallic hydrogen. [H76]
Metallicity	An object's abundance of metals. In practice, this usually means the abundance of iron, which is easy to measure. [C95]
Metallicity Gradient	The progressive change in metallicity from the center of a galaxy to its edge. A galaxy exhibiting a metallicity gradient is more metal-rich at its center than at its edges. [C95]
Metal-Poor	Having a low metallicity with respect to Solar. [C95]
Metal-Rich	Having a high metallicity with respect to olar. [C95]
Metal-Rich Stars	Stars having metal-to-hydrogen ratios greater than those of the Hyades. [H76]
Metastable	A state which is not stable, but which lives long enough to have significance, is called metastable. [G97]
Metastable State	(a) A condition of a system or body in which it appears to be in stable equilibrium but, if disturbed, can settle into a lower energy state. For example, supercooled water is liquid at below 0°C (at standard pressure). When a small crystal of ice or dust (for example) is introduced rapid freezing occurs. [DC99] (b) An excited state in an atom, of relatively long duration, from which the atom, under laboratory conditions, cannot pass directly to the ground state by emitting radiation. Under laboratory conditions, the mean time between collisions is so much shorter than the lifetime of the metastable state that an atom is virtually certain to collide with another atom and lose its energy by collision before it has had a chance to radiate. In the extremely rarefied interstellar medium, however, such highly improbable transitions do occur, and the spectral lines emitted by such transitions are called forbidden lines. [H76]
Meteor	(a) Fragment or particle that enters the Earth's atmosphere and is then destroyed through friction, becoming visible as this occurs as a momentary streak of light. At certain times of the year, meteors apparently emanating from a single area of the sky (a radiant) form meteor showers. They are thought to originate within the Solar System. see also Meteorite [A84] (b) A "shooting star" - the streak of light in the sky produced by the transit of a meteoroid through the Earth's atmosphere; also the glowing meteoroid itself. The term "fireball" is sometimes used for a meteor approaching the brightness of Venus; the term "bolide" for one approaching the brightness of the full Moon. [H76]
Meteor Shower	(a) A profusion of meteors that fall within a period of a few hours and that appear to radiate from a common point in the sky. Shower meteors are usually low-density material, have high eccentricities, and are known to be associated with comets (e.g., the orbit of the Leonids is identical with that of comet Tempel-Tuttle 1866 I). [H76]
Meteorite	(a) Object that enters the Earth's atmosphere and is too large to be totally destroyed by friction before it hits the surface. Meteorites may in some way be connected with asteroids. see also Meteor [A84] (b) A solid-body portion of a meteor that has reached Earth's surface. Meteorites are divided into three main classes: aerolites (stony meteorites), siderites (iron meteorites), and siderolites (stony iron meteorites). Most meteorites are high-density objects related to asteroids. They range in size from subplanetary to microscopic. Several hundred tons of meteoritic material are estimated to fall on Earth daily. [H76] (c) A solid portion of a meteoroid that has reached the earth's surface. There are three types of meteorites - stony, iron, and stony-iron meteorites - and they range in size from microscopic to the size of an asteroid. Several hundred tons of meteoric material are estimated to fall on the earth each day. [SILK90]
Meteoroid	A small particle orbiting the Sun in the vicinity of Earth. [H76]
Meter	(a) The SI unit of length. The meter is the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the Krypton-86 atom (11th CGPM, 1960, Resolution 6). [H76] (b) The meter was defined at the 17th CGPM (1984) as the length of the path travelled by light in vacuum during a time interval of 1/(299 792 458) second. This definition does not change the size of the unit but it was introduced to take into account recent developments in measurement techniques whereby length and time can be reproduced with very high accuracy, in the case of the second to an accuracy better than one in 1013. An outcome of the new definition is that the speed of light has become the second physical constant to be fixed by convention, the other being the permeability of free space. The metre had its origin in August 1793 when the Republican Government in France decreed the unit of length would be 10-7 of the earth's quadrant passing through Paris and that the unit would be called the metre. A survey of the arc was made from Dunkerque to Rodez by Mechain and from Rodez to Barcelona by Delambre and this was completed in five years whereupon three platinum standards and several iron copies of the metre were made. Subsequent examination showed that the length of the earth's quadrant had been wrongly surveyed but instead of altering the length of the metre to maintain the 10-7 ratio, the metre was redefined as the distance between two marks on a bar. In the nineteenth century scientists throughout the world gradually adopted the metric system for their measurements. This led to the standardization of the metre which was ratified at the Metric Conference in Paris in 1875. The present international prototype metre, which was established in 1889, was a direct consequence of this conference. During the century many of the non-metric countries passed the necessary legislation to define the metre in terms of their own units of length. Thus the United States Congress defined the metre in terms of the US yard and in 1866 the British Parliament, through the Weights and Measures Act 1866, permitted the metric system to be used for contracts but not for trade. Scientists, however, were not satisfied at having to use an arbitrary standard as their fundamental unit. As early as 1828 Babinet suggested the wavelength of light as a natural unit of length but over 130 years had to elapse before the wavelength unit was adopted. In the intervening years two serious attempts were made to establish an optical unit of length. One was by Michelson in 1895 when he suggested the red line of cadmium; the other took place in 1907 when spectroscopists defined their unit - the ångström - in terms of this line which made the ångström equal to 1.0000002 × 10-10 m, whereas it was meant to represent a distance of 10-10 m exactly. This discrepancy was overcome when the metre was redefined in terms of the wavelength of the orange line of krypton at the 11th CGPM (1960). In the same year a proposal was before the British Parliament to define the imperial standard yard in terms of the metre so that 1 yard = 0.9144 metre exactly. Pressure of parliamentary business delayed the passing of the necessary act until 1963, since which time the metre has been the basic unit of length in both the metric and imperial system of units. [JM92]
Methane	Gaseous hydrogen compound, one of the alkanes, in which every carbon atom is surrounded by four hydrogen atoms. [A84]
Methanol	CH3OH -- An organic molecule (also called methyl alcohol) discovered in interstellar space in 1970. More rotational lines have been observed astronomically for it than for any other molecule. [H76]
Methylamine	CH3NH2 -- A molecule discovered in interstellar apace in 1974, in Sgr B2, at a frequency of 87.77 GHz. Methylamine can react with formic acid to produce glycine, the simplest amino acid. [H76]
Metric	(a) The metric gives the spacetime interval de between two neighboring events. [H76] (b) A mathematical description of the geometrical properties of the Universe. [LB90] (a) A mathematical variable that describes the geometry of space-time; it is a tensor made up of ten functions or `components'. [D89]
Metric Tensor	the mathematical object that describes the deviation of Pythagoras's theorem in a curved space. [D89]
MeV	One million (106) electron volts. [F88]
MHD	Magnetohydrodynamics -- (a) The study of the collective motions of charged particles in a magnetic field. (sometimes called hydromagnetics)[H76] (b) The study of how magnetic fields interact with conducting fluids (e.g. plasmas or liquid metals).
micro-	A prefix meaning 10-6. [H76]
Microchannel Plate	A compact electrostatic high-voltage electron multiplier with a very large number of narrow pores or channels. A photoelectron generated at the entrance face (photocathode) stimulates a cascade of secondary electrons down the nearest channel to produce a huge cloud of charge at the output face. The output pulse can be used in many different ways to record the event. If it impacts a phosphor screen then light emission can be detected with a CCD. Direct electrical detection can be obtained using a Multi Anode Microchannel Analyzer. [McL97]
Micrometer	Device used in conjunction with a telescope in order to measure extremely small angular distances. [A84]
Micrometry	The measurement of the apparent sizes and separations of astronomical objects by use of knife blades or crosshairs in the eyepiece of a telescope. If the distance of an object is known, its size can be determined through micrometry. [F88]
Micron	A unit of length equal to 10-4 cm, or 104 Angstroms. [H76]
Microphotometer	A device for measuring the variations in density in a photographic emulsion. [H76]
Microprocessor	A very large silicon integrated circuit with essentially all the functions of a computer on a single chip. [McL97]
Microwave	(a) An electromagnetic wave (in the radio region just beyond the infrared) with a wavelength of from about 1 mm to 30 cm (about 109-1011 Hz). [H76] (b) An electromagnetic wave with a wavelength between one millimeter and 30 centimeters, or sometimes one meter. [G97]
Microwave Background	The 2.7 degree Kelvin radiation that pervades the Universe and is believed to be the afterglow of the Big Bang. [C95]
Microwave Background Anisotropy Experiment	An experiment designed to measure the intensity of the cosmic microwave background radiation in different directions. A fundamental prediction of the cosmological origin of this radiation is that the earth's motion relative to the distant regions of the Universe should be detectable. The effect amounts to an increase of about 10-3 K in brightness in the direction we are traveling and a similar decrease in the opposite direction. [SILK90]
Microwave Background Radiation	thermal radiation with a temperature of about 3 K that is apparently uniformly distributed in the Universe; the radiation, discovered by A.A. Penzias and R.W. Wilson in 1964, is believed to be a redshifted remnant of the hot radiation that was in thermal equilibrium with matter during the first hundred thousand years after the big bang. [D89]
Microwave Radiation	(a) Radiation in the electromagnetic spectrum between infrared and radio waves. This range has wavelengths of between about 20 cm and about 1 mm. Radiation of this type was detected as background radiation. [A84] (b) Radio radiation with wavelengths of about 10-4 to 1 meter, equal to 109 to 1013 hertz. [F88]
MIDAS	Munich Image Data Analysis System -- A suite of programs and a software environment developed at the European Southern Observatory for astronomy applications. [McL97]
Mie Scattering	Scattering of light (without regard to wavelength) by larger particles, such as those of dust or fog in Earth's atmosphere (see also Rayleigh Scattering). [H76]
Mie Theory	A theory of the diffraction of light by small spherical particles. [H76]
Mile	The mile employed in this book is the statute mile, equal to 5,280 feet. [F88]
Milky Way	(a) Our own galaxy, the second largest in the local group. [A84] (b) A softly glowing band of light that bisects the skies of Earth, produced by light from stars and nebulae in the galactic disk. [F88]
milli-	A prefix meaning 10-3. [H76]
Million	A thousand thousand (106). [F88]
Mills Cross	An antenna array consisting of two antennas oriented at right angles to each other. It produces a single narrow pencil beam. [H76]
Milne Cosmological Model	A Friedmann model of the Universe in which matter does not exist. Only radiation is present in a Milne Universe.
Milne-Eddington Approximation	A first approximation in the analysis of stellar spectra, in which a line is assumed to be formed in such a way that the ratio of the line absorption coefficient to the continuous absorption coefficient is constant with depth. It is used primarily in analyses of the lines of ionized metals, for which cases it is often an accurate approximation. [H76]
Mimas	The second innermost satellite of Saturn, discovered by Herschel in 1789. P = 0d.94, R 250 km. Albedo 0.49. It is the perturbations of Mimas and Janus that produce the divisions in Saturn's rings. [H76]
Minimal Surface	A mathematical term referring to surfaces that satisfy a minimization procedure. Soap bubbles, for example, minimize their energy by forming shapes with the minimum possible area. The world surface of a string is likewise a minimal surface. [P88]
Minimum Resolvable Angle	In radians, 1.22 divided by the aperture of the telescope. [H76]
Mini Black Holes	In a chaotic early Universe, black holes may form at eras as early as the Planck time. The characteristic size of these mini black holes is 10-6 gram, the minimum mass of a collapsing inhomogeneity at that time. Larger mini black holes may form at later eras. Since conventional theories of stellar evolution show that only very massive stars can form black holes, the possible formation of mini black holes is a unique characteristic of the very early Universe. [SILK90]
Mini-Quasar	Sandage's term for the blue nucleus of an N galaxy. According to Sandage, N galaxies can be understood as ordinary giant elliptical galaxies with "mini-quasars" embedded in them. [H76]
Minkowski Metric	form of the metric that is valid in an inertial frame; underlying geometry of special relativity. [D89]
Minkowski Space	(a) A four-dimensional spacetime with a flat (i.e., Euclidean) geometry, which is used in the special theory of relativity. [H76] (b) Space that is empty of matter or energy. [LB90]
Minkowski Space-Time	space and time considered together, with special importance attached to the progress of a light flash, and to the light-cone and the `interval'. [D89]
Minor Planet	see Asteroid [H76]
Mintaka	One of the three stars in Orion's belt, and the star along whose line of sight interstellar gas was first spectroscopically detected. [C95]
Minute of Arc	A unit of angle equal to 1/60 of a degree. [LB90]
MIPS	Millions of Instructions Per Second -- A computer benchmark. [McL97]
MIR	Mid Infra-Red
Mira	(a) A red giant in the constellation Cetus that varies in brightness as it pulsates. When brightest, Mira is visible to the naked eye; when dimmest, Mira can be viewed only with optical aid. Mira is the prototype of all pulsating red giants, which are called Miras in its honor. [C95] (b) A M6e-M9e III irregular long-period intrinsic variable about 70 pc distant. Mean period 331 days. It can be more than 5 mag brighter at maximum than at minimum. It was named Mira ("wonderful") in 1596 by Fabricius, who made the first recorded observations of its brightness fluctuations. Mira is a double star with a faint B companion which is itself variable. [H76]
Mira Variables	Cyclic variables with cycles 100-500 days, and of spectral types K, M, S and C. (Also called long-period variables.) [JJ95]
Miranda	The innermost satellite of Uranus, discovered by Kuiper in 1948. P = 1d10h, diameter about 500 km. [H76]
Mirror Symmetry	In the context of string theory, a symmetry showing that two different Calabi-Yau shapes, known as a mirror pair, give rise to identical physics when chosen for the curled-up dimensions of string theory. [G99]
MIS	Metal Insulator Semiconductor. [LLM96]
Missing Matter	Alternate term for dark matter. [F88]
Missing Mass	The cosmic mass that some scientists hypothesize so that the Universe will have the critical density of matter, with an exact balance between gravitational energy and kinetic energy of expansion. Such mass is called missing because it represents about 10 times as much mass as has actually been detected. see Closed Universe; Critical Mass Density; Dark Matter [LB90]
Missing Mass Problem	Poses the question: why does the Universe seem to have much more mass in it than can be seen with a telescope? Dynamical and theoretical constraints place the proportion of missing mass to be somewhere between 90-99 per cent of the total mass of the Universe. [C97]
Mixer	The critical element of a radio detection system which allows the incoming wave to be combined with the reference frequency from the local oscillator. Usually a diode. [McL97]
Mixing-Length Theory	A semiempirical theory used to describe convection phenomena in stars. [H76]
Mixmaster Model	(a) A non-Friedmannian cosmological model that begins with a highly anisotropic infant Universe and shows how anisotropies are reduced in time. (see also Friedmann Models) [LB90] (b) A cosmological model in which the early Universe was homogeneous but highly anisotropic - the Universe expanded at different rates in two directions while collapsing in a third direction and then reversed itself. Any given amount of matter would alternately go through pancakelike and cigar-like configurations that gradually increase in volume as the Universe expands. [SILK90]
Mizar	A double star in Ursa Major. [A84]
MKK System	A classification of stellar spectra according to luminosity, devised by Morgan, Keenan and Kellman (see Luminosity Class). The MKK system uses two parameters (Spectral Type and Luminosity Class) to describe a system with three variables: temperature, luminosity, and abundance. [H76]
M Numbers	Refer to the Messier Catalogue. [A84]
MMT	Multi-Mirror Telescope. [LLM96]
Moiré Fringes	The pattern obtained when two regular sets of lines or points overlap. The effect can be seen through folds in netting drapes. Moiré patterns can be used as models of interference patterns. Another application is in comparing two diffraction gratings by superimposing them and observing the moir° pattern produced. [DC99]
Mole	The SI unit of the amount of substance, defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12 [14th CGPM 1971, Resolution 3]. 1 mole, which is equal to gram multiplied by the molecular weight, contains 6.02 × 1023 molecules (see Avogadro's Number). In general, 1 mole of any gas occupies a volume of 22.4 liters. [H76]
Molecular Cloud Complex	A region of extensive emission of molecular line radiation by dense, cold interstellar gas. There are often several distinct intensity peaks, each representing individual clumps or clouds of gas and dust in a region that characteristically extends for 50 light-years and is often associated with T-Tauri stars - young, pre-main-sequence stars - and also hot massive stars and the ionized gas around them. [SILK90]
Molecular Clouds	A cloud of interstellar gas and dust that consists mostly of molecular hydrogen. [C95]
Molecular Hydrogen	(a) A molecule consisting of two hydrogen atoms (H2) and the most common molecule in space. [C95] (b) A molecule of hydrogen, discovered in interstellar space in 1970. H2 is a very hard molecule to detect. None of its transitions lie in the visible part of the spectrum. Second, being a symmetric homonuclear molecule, it does not have an electric-dipole rotation-vibration spectrum, and detection must be based on the weak quadrupole spectrum. Third, ultraviolet radiation is a very efficient dissociator of H2, so any H2 that survived would presumably be located inside very dense interstellar clouds. So far observations have borne out this supposition. Measurements of the region within about 1 kpc of the Sun suggest that H2 is about twice as abundant as atomic H. [H76]
Molecules	The smallest units of a chemical compound. A molecule is composed of two or more atoms, linked by interactions of their electrons. [F88]
Molybdenum	A transition element used in alloy steels, lamp bulbs and catalysts. Symbol: Mo; m.p. 2620°C; b.p. 4610°C; r.d. 10.22 (20°C); p.n. 42; r.a.m. 95.94. [DC99]
Moment of Inertia	The product of the mass of a body and the square of its radius of gyration. [H76]
Monoceros Loop	A filamentary loop nebula about 1 kpc distant, the remnant of a supernova that occurred about 300,000 years ago. [H76]
R Monocerotis	An A-F pec variable star that illuminates the variable cometary nebula NGC 2261. Its temperature is about 810 K, and it is a source of CO emission. [H76]
Monochromatic Radiation	Electromagnetic radiation of an extremely narrow range of wavelengths. (The word means `of one color'.) It is impossible to produce completely monochromatic radiation, although the output of some lasers is not far off. The `lines' in line spectra produced even in the most ideal circumstances have some width in wavelength terms. The half-width is the measure used. It is the range of wavelengths defined in the figure, and contains almost 90% of the energy emitted. The half-width of a sharp line in an optical spectrum is typically 10-6 to 10-7 of the wavelength. Using lasers, half-widths of the order 10-12 can be obtained. Low-quantum-energy gamma rays emitted by atoms bound in crystals may have values of the order 10-13 . Simple quantum theory leads one to expect perfectly sharp lines in a line spectrum - the energies of the levels concerned appear to be exactly defined, so that = hc / E. However, because of the uncertainty principle, no energy level or transition can be defined exactly; this means that any line is naturally broadened rather than being sharp. A second broadening influence is the Doppler effect, which is relevant as the radiating particles are always in motion. Thirdly, collisions between emitting particles will broaden the emitted line. Compare Polychromatic Radiation [DC99]
Monodromy	The property that all paths of points of a body simply rotating about an axis shall return into themselves. [H76]
Monopole	A hypothetical quantum object being a single, isolated magnetic pole. Normally, magnetic poles, the sources of a magnetic field, occur in pairs as north and south poles. [P88]
Monotonic	Either continuously increasing or decreasing. [SILK90]
Monte Carlo Method	A trial-and-error technique used on computers to solve complex problems. [H76]
Month	The period of one complete synodic or sidereal revolution of the Moon around the Earth; also a calendrical unit that approximates the period of revolution. [S92]
Moon	Natural satellite of Earth. Mass 7.35 × 1025 g = 0.0123 Earth's; mean radius 1738 km; mean density 3.34 g cm-3; mean distance from Earth 384,404.377 ± 0.001 km (1.28 lt-sec); Vesc 2.38 km s-1; surface gravity 162.2 cm s-2 = 0.165 Earth's. Sidereal period 27d7h43m11s, e = 0.0549, inclination of orbital plane to ecliptic 5°8'43". Obliquity 6°41'. Synodic period 29d12h44m2s.9. Vorb = 1.02 km s-1. Albedo 0.07. The Moon's center of mass is displaced about 2 km in the direction of Earth. Studies of lunar rocks have shown that melting and separation must have begun at least 4.5 × 109 years ago, so the crust of the Moon was beginning to form a very short time after the Solar System itself. Thickness of crust, 60 km; of mantle, 1000 km. Temperature of core, 1500 K. It would have taken only 107 years to slow the Moon's rotation into its present lock with its orbital period. The Moon's orbit is always concave toward the Sun. [H76]
Moonrise, Moonset	The times at which the apparent upper limb of the Moon is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°34' + s - , where s is the Moon's semidiameter, is the horizontal parallax, and 34' is the adopted value of horizontal refraction. [S92]
MOS	Metal Oxide Semiconductor -- A construction used to fabricate microelectronic components. including CCDs, which consists of three layers, namely a metal conductor. an insulating layer usually made from an oxide of silicon, and a semiconductor such as silicon. [McL97]
Mössbauer Effect	(a) After an atomic nucleus has emitted a gamma-ray photon (during radioactive decay), the absorption of the momentum of the atom by the whole of its crystal lattice because it is so firmly bound that it cannot recoil. The same effect occurs with absorption of gamma rays. [DC99] (b) Recoil-free gamma-ray resonance absorption. [H76]
Mottle	An alternative word for spicule. [H76]
Moving Cluster	A group of stars dynamically associated so that they have a common motion with respect to the local standard of rest. Examples are the Hyades and the Ursa Major group. [H76]
MS Stars	Stars sharing the M and S characteristics. They thus exhibit bands of both TiO and SrO. [JJ95]
Mt. Wilson	The location, in California, of the 100-inch diameter telescope used by Edwin Hubble and others. [LB90]
M-Theory	Theory emerging from the second superstring revolution that unites the previous five superstring theories within a single overarching framework. M-theory appears to be a theory involving eleven spacetime dimensions, although many of its detailed properties have yet to be understood. [G99]
M-Type	Having a spectral type of M, that is, red like Betelgeuse and Antares. [C95]
MTF	Modulation Transfer Function [LLM96]
Mulliken Bands	Spectral bands of the C2 radical. [H76]
Multi-Dimensional Hole	A generalization of the hole found in a doughnut to higher-dimensional versions. [G99]
Multi-Doughnut	Also, Multi-Handled Doughnut. A generalization of a doughnut shape (a torus) that has more than one hole. [G99]
Multiphoton Process	in very intense radiation fields atoms or molecules can absorb several photons simultaneously in a multiphoton process. [D89]
Multiples and Numbers	In April 1795 the French Revolutionary Government introduced prefixes to represent the multiples and submultiples of the basic metric units. Those given below, with their recognized modern abbreviations, are still in use. 103, kilo (k); 102, hecto (h); 101, deca (da); 10-1, deci (d); 10-2, centi (c); 10-3, milli (m). In the past 150 years the following additional prefixes have been added to the list: 1024, yotta (Y); 1021, zetta (Z); 1018, exa (E); 1015, peta (P); 1012, tera (T); 109, giga (G); 106, mega (M); 104, myria (my); 10-6, micro (µ); 10-9, nano (n); 10-12, pico (p); 10-15 femto (f); 10-18, atto (a); 10-21, zepto (z); 10-24, yocto (y). The names for the prefixes of 10±21 and 10±24 were approved by the CIPM in 1990. The names are derived from septo (7 × 3 = 21) and octo (8 × 3 = 24) but to avoid confusion with s, the abbreviation for second, the initial letter of septo was changed to z. The 9th CGPM (1948) approved a nomenclature for large numbers based on multiples of a million in which the name (N)illion is used to indicate values of 106N. Thus, when N = 1, the resulting number 106 is called a million; with N = 2 the number 1012 is a billion; with N = 3 it is 1018, a trillion; N = 4, 1024, a quadrillion. A different practice, however, is followed by financiers and many of the public on both sides of the Atlantic in which the nomenclature is based on 10(3Y+3), thus when Y = 1, the number (106) is still a million, but 109 is called a billion, 1012 a trillion and 1015 a quadrillion. This system is not recommended by IUPAP[53]. The prefix micro was at one time used to indicate 10-12 but it was superseded by pico during the 1939-1945 war. The 13th CGPM (1968) recommended that powers of 10 should be expressed in units of 10±3n where n is an integer; thus the Planck constant should be written as 662.6 × 10-36 J S whereas it had previously been given as 6.626 × 10-34 J S. In the 1860s G. J. Stoney proposed a nomenclature in which powers of ten were indicated by an appended cardinal number when the exponent of 10 was positive and a prefixed ordinal when the exponent was negative; thus 1010 metre would be called a metre ten and 10-10 metre a tenth metre. During the 1970s a system somewhat similar to Stoney's began to be used in computer printouts. In this a number such as 47 × 103 would be shown as 47E + 3, whereas 47 × 10-9 would appear as 47E - 9. The advantage of the system is that the indices denoting the power of 10 are printed on the same line as the numbers to which they refer. Numbers which have neither to be raised nor reduced by a power of 10 are printed without the letter E. Thus 47 would be written as 47 or alternatively as 47E + 0. [JM92]
Multiplet	A group of spectral lines arising from transitions having a common lower energy level. The group of lines have the same values of L and S but different values of J. [H76]
Multiplex	Combining many signals into one or a small number of signals. [McL97]
Multi-Pinned-Phase	Also multi-phase-pinned. The method of driving all CCD phases (gates), including the integrating phase, into inversion and thereby greatly reducing the dark current. The penalty is a slightly poorer well-depth for charge storage. The advantage is a much higher operating temperature. [McL97]
Multiverse	Hypothetical enlargement of the cosmos in which our Universe is but one of an enormous number of separate and distinct Universes. [G99]
Muon	(a) A second-generation lepton. It is essentially a more massive electron. [CD99] (b) Short-lived elementary particle with negative electrical charge. Muons are leptons. They resemble electrons, but are 207 times more massive. [F88] (c) An elementary particle, formerly called a mu-meson but now classified with the leptons because it seems to be identical with the electron except for its much greater mass (207 times that of an electron). The muon family includes the muons and their neutrinos (and their antiparticles). Muons may have a positive or a negative charge. [H76] (d) Elementary particles produced when cosmic rays enter the upper atmosphere. [McL97] (e) A charged lepton. The analogue of electron in the second generation of particles. [D89]
Mural Arc	Sixteenth- to nineteenth-century astronomical apparatus comprising a carefully oriented wall on which a calibrated device was fixed, by which the altitudes of celestial objects could be measured. [A84]
Murchison Meteorite	A type II carbonaceous chondrite which fell in 1969 near Murchison, Australia, and which was found to contain at least 17 amino acids. Left-handed and right-handed forms were present in roughly equal quantities. [H76]
Murray Meteorite	A type II carbonaceous chondrite that fell near Murray, Kentucky, in 1950. [H76]
2MASS	Two-Micron All-Sky Survey. [LLM96]