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type for red stars, such as Betelgeuse, Antares, and Proxima Centauri.
A great globular cluster in the constellation Hercules.
The Andromeda galaxy, the largest member of the local group. It is a giant spiral galaxy that lies 2.4 million light-years away.
An elliptical galaxy that orbits the Andromeda galaxy.
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.
The Orion Nebula, a star-forming region in the constellation Orion.
The Pleiades, a beautiful open star cluster in the constellation Taurus. It is 410 light-years away.
The Whirlpool Galaxy, a stunning spiral in the constellation canes venatici.
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.
A giant elliptical galaxy in the Virgo cluster.
The Sombrero galaxy, in which galactic rotation was first detected. It lies in the constellation Virgo.
The magnitude derived from observations at an infrared wavelength of 5 microns.
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.
M-type stars with ZrO bands.
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
(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
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.
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).
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.
Refers to scales typically encountered in the everyday world and larger; roughly the opposite of microscopic.
|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.
(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.
(a) A strand
of gas spanning 300,000 light-years that the Milky Way has ripped out
of the Magellanic Clouds.
(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.
Element with atomic number twelve. it is the fifth most common metal
in the Universe and was produced by high-mass stars that exploded.
see Synchrotron Radiation
Radiation emitted by a rotating magnet.
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
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.
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.
(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
|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.
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.
Stars (usually of spectral type a) with strong integrated magnetic fields ranging up to 30,000 gauss.
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.
MHD The study of how magnetic fields interact with conducting fluids (e.g. plasmas or liquid metals).
The mass equivalent of the binding energy of a nucleus. see Binding Energy
see Bohr Magneton
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.
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.
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.
(a) A logarithmic brightness scale for astronomical
|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.
|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.
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.
The lobe of maximum sensitivity in a radio telescope.
|Main Lines (of an OH source)|
The transitions that emit radiation at 1665 and 1667 mhz.
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.
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.
A reflector whose primary mirror is spheroidal instead of parabolic. The light initially passes through a large concave lens to remove the spherical aberration.
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.
A correction introduced into star counts distributed by apparent magnitude.
(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.
Stars with an anomalously high mn-fe ratio, which show deviations from the odd-even effect for phosphorus, gallium, and yttrium.
A mathematical concept used to describe the geometry of spacetime.
The decimal part of a common logarithm.
The difficulty of calculating the interactions - e.g., the newtonian gravitational interactions - of three or more objects.
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
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.
An area on the moon that appears darker and smoother than its surroundings. Lunar maria are scattered basaltic flows. (plural is Maria)
A series of US spaceprobes launched to explore the planets of the Solar System, particularly Mercury, Venus and Mars.
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.
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.
Fourth major planet out from the Sun.
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.
(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.
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.
|Mass Absorption Coefficient|
A measure of the fraction of radiation absorbed at a certain wavelength per unit mass.
The mass equivalent of the binding energy of a nucleus. See binding
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.
The fractional amount (by mass) of a given element or nuclide in a given composition.
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.
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
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
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.
see Atomic Mass Number.
|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.
|Mass-Radius Relation (Chandrasekhar)|
For any given mass less than the Chandrasekhar limit, there is a unique radius for a totally degenerate star.
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
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.
|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.
Spherical distributions of dark matter surrounding galaxies. see Dark Matter
|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.
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.
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.
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.
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.
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)
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.
Characteristic by virtue of which matter, like energy, displays the qualities of waves as well as of particles. See wave-particle duality.
|Maximum Entropy Method|
MEM: An image reconstruction methodology which defines a measure of information content and seeks to maximize it.
The cgs unit of magnetic flux through 1 cm2 normal to a field of 1 gauss. 1 Mx corresponds to 10-8 Wb.
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.
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.
An expression for the statistical distribution of velocities among the
molecules of a gas at a given temperature.
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.
Equations governing the varying electric and magnetic fields in a medium.
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.
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.
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.
Stars of spectral type M with emission lines in their spectra.
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
The semi-major axis of an elliptic orbit.
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.
|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.
|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
|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
The mean time before decay of a large number of similar particles. It is equal to 1.44 times the half-life.
|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.
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.
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).
|Mean Profile||The relatively stable curve obtained by synchronously averaging together many pulses of a pulsar together. (also called Integrated Profile or Pulse Window|
|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).
|Mean Solar Second|
1/86,400 of a Mean Solar May (cf. Ephemeris Second ).
|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.
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.
The study, in physics, of the influence of forces.
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.
A prefix meaning 106.
Millions of floating-point operations per second. A computer benchmark.
A unit of distance equal to a million parsecs, or 3.2616 million light-years.
Molecular bands of the N2+ radical near 8000 Å.
the phenomenon in which a metal cooled through its superconducting transition temperature in the presence of a magnetic field completely expels the field.
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.
Maximum Entropy Method: An image reconstruction methodology which defines a measure of information content and seeks to maximize it.
A radioactive transuranic element of the actinoid
series, not found naturally on Earth. Several short-lived isotopes
have been synthesized.
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.
Mercury-Cadmium-Telluride (HgCdTe) -- A semiconductor alloy useful as an infrared photoelectric detector. Also known as CMT (Cad-Mer-Tel)
(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.
The formation of a galaxy from the collision of two or more separate galaxies.
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.
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.
Any particle made of two quarks. Examples are the pion (pi-meson) and
The part of Earth's atmosphere immediately above the stratosphere, where the temperature drops from about 270 K to 180 K.
Smallest bundle of a force field; microscopic conveyer of a force.
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
A synonym for the Universe.
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.
|Metal-Enhanced Star Formation|
A hypothesis according to which stars form preferentially from regions of above-average Z in a chemically inhomogeneous interstellar medium.
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.
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.
An object's abundance of metals. In practice, this usually means the abundance of iron, which is easy to measure.
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.
Having a low metallicity with respect to Solar.
Having a high metallicity with respect to olar.
Stars having metal-to-hydrogen ratios greater than those of the Hyades.
A state which is not stable, but which lives long enough to have significance, is called metastable.
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
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
(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).
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
A small particle orbiting the Sun in the vicinity of Earth.
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).
Gaseous hydrogen compound, one of the alkanes, in which every carbon atom is surrounded by four hydrogen atoms.
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.
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.
The metric gives the spacetime interval de between two neighboring events.
the mathematical object that describes the deviation of Pythagoras's theorem in a curved space.
One million (106) electron volts.
|MHD|| Magnetohydrodynamics -- (a) The study of the collective
motions of charged particles in a magnetic field. (sometimes called hydromagnetics)|
(b) The study of how magnetic fields interact with conducting fluids (e.g. plasmas or liquid metals).
A prefix meaning 10-6.
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.
Device used in conjunction with a telescope in order to measure extremely small angular distances.
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.
A unit of length equal to 10-4 cm, or 104 Angstroms.
A device for measuring the variations in density in a photographic emulsion.
A very large silicon integrated circuit with essentially all the functions of a computer on a single chip.
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
The 2.7 degree Kelvin radiation that pervades the Universe and is believed to be the afterglow of the Big Bang.
|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.
|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.
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.
Munich Image Data Analysis System -- A suite of programs and a software environment developed at the European Southern Observatory for astronomy applications.
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).
A theory of the diffraction of light by small spherical particles.
The mile employed in this book is the statute mile, equal to 5,280 feet.
Our own galaxy, the second largest in the local group.
A prefix meaning 10-3.
A thousand thousand (106).
An antenna array consisting of two antennas oriented at right angles to each other. It produces a single narrow pencil beam.
|Milne Cosmological Model|
A Friedmann model of the Universe in which matter does not exist. Only radiation is present in a Milne Universe.
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.
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.
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.
|Minimum Resolvable Angle|
In radians, 1.22 divided by the aperture of the telescope.
|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.
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.
form of the metric that is valid in an inertial frame; underlying geometry of special relativity.
A four-dimensional spacetime with a flat (i.e., Euclidean) geometry,
which is used in the special theory of relativity.
space and time considered together, with special importance attached to the progress of a light flash, and to the light-cone and the `interval'.
One of the three stars in Orion's belt, and the star along whose line of sight interstellar gas was first spectroscopically detected.
|Minute of Arc|
A unit of angle equal to 1/60 of a degree.
Millions of Instructions Per Second -- A computer benchmark.
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.
Cyclic variables with cycles 100-500 days, and of spectral types K, M, S and C. (Also called long-period variables.)
The innermost satellite of Uranus, discovered by Kuiper in 1948. P = 1d10h, diameter about 500 km.
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.
Metal Insulator Semiconductor.
Alternate term for dark matter.
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
|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.
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.
A semiempirical theory used to describe convection phenomena in stars.
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)
A double star in Ursa Major.
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.
Refer to the Messier Catalogue.
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.
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.
|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.
A cloud of interstellar gas and dust that consists mostly of molecular hydrogen.
A molecule consisting of two hydrogen atoms (H2) and the
most common molecule in space.
The smallest units of a chemical compound. A molecule is composed of two or more atoms, linked by interactions of their electrons.
A transition element used in alloy steels, lamp
bulbs and catalysts.
|Moment of Inertia|
The product of the mass of a body and the square of its radius of gyration.
A filamentary loop nebula about 1 kpc distant, the remnant of a supernova that occurred about 300,000 years ago.
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.
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
obtained. Low-quantum-energy gamma
rays emitted by atoms bound in crystals may have values of the order
The property that all paths of points of a body simply rotating about an axis shall return into themselves.
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.
Either continuously increasing or decreasing.
|Monte Carlo Method|
A trial-and-error technique used on computers to solve complex problems.
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.
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.
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.
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.
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.
An alternative word for spicule.
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.
Stars sharing the M and S characteristics. They thus exhibit bands of both TiO and SrO.
The location, in California, of the 100-inch diameter telescope used by Edwin Hubble and others.
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.
Having a spectral type of M, that is, red like Betelgeuse and Antares.
Modulation Transfer Function
Spectral bands of the C2 radical.
A generalization of the hole found in a doughnut to higher-dimensional versions.
Also, Multi-Handled Doughnut. A generalization of a doughnut shape (a torus) that has more than one hole.
in very intense radiation fields atoms or molecules can absorb several photons simultaneously in a multiphoton process.
|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.
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.
Combining many signals into one or a small number of signals.
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.
Hypothetical enlargement of the cosmos in which our Universe is but one of an enormous number of separate and distinct Universes.
A second-generation lepton. It is essentially a more massive electron.
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.
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.
A type II carbonaceous chondrite that fell near Murray, Kentucky, in 1950.