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 Glossary: Life in Universe

    Glossary of Terms - M to R
    Index: A to F | G to L | M to R | S to Z

    Ma - (n.)

    Mega anna, one million (1,000,000 or 1 x 106) years.

    Magellanic clouds - (n.)

    The two irregular galaxies that are the nearest neighbors of the Milky Way; they are visible to the unaided eye in the southern hemisphere.

    magnetic braking - (n.)

    The slowing of the spin of a young star (such as the early sun) by magnetic forces exerted on the surrounding ionized gas.

    magnetic dynamo - (n.)

    A rotating internal zone inside the sun or a planet, thought to carry the electrical currents that create the solar or planetary magnetic field.

    magnetosphere - (n.)

    A region, surrounding a star or planet, that is permeated by the magnetic field of that body.

    magnitude - (n.)

    A measure of the brightness of a star. It is based on a system established by Hipparchus, in which stars were ranked according to how bright they appeared to the unaided eye. In the modern system, a difference of five magnitudes corresponds exactly to a brightness ratio of 100, so that a star of a given magnitude has a brightness that is 1001/5 = 2.51 times that of a star one magnitude fainter.

    main group elements - (n.)

    The elements in the A groups of the periodic table that is customary in the United States and in Groups 1, 2, and 13 to 18 in the periodic table recommended by IUPAC.

    main sequence - (n.)

    principal sequence of stars on the graph of luminosity versus effective temperature (H-R diagram), encompassing more than 90% of observable stars. These stars are converting hydrogen to helium by nuclear reactions in their cores The lower mass limit for the Main Sequence is 0.085 Mo and the upper limit is about 60 Mo.

    main-sequence fitting - (n.)

    A distance-determination technique in which an H-R diagram for a cluster of stars is compared with a standard H-R diagram to establish the absolute magnitude scale for the cluster H-R diagram.

    main-sequence turn-off - (n.)

    In an H-R diagram for a cluster of stars, the point where the main sequence turns off toward the upper right. The main-sequence turn-off, showing which stars in the cluster have evolved to become red giants, is an indicator of the age of the cluster.

    major axis - (n.)

    The longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci. Also, the length of that line.

    mare - (n.)

    One of the smooth areas on the moon or on some of the other planets.

    mascon - (n.)

    A concentration of mass under the surface of the moon, discovered from its gravitational effect on spacecraft orbiting the moon.

    maser - (n.)

    An acronym for "microwave amplification by stimulated emission of radiation," a device by which certain energy levels are more populated than normal, resulting in an especially dense emission of radio radiation at a certain frequency when the system drops to a lower energy level. A laser operating in the microwave region of the electromagnetic spectrum.

    Index: A to F | G to L | M to R | S to Z

    mass - (n.)

    A measure of the inherent amount of matter in a body. That property which resists change of position by applied forces.

    mass-energy equation - (n.)

    Einstein's equation E = mc2, in which E is energy, m is mass, and c is the speed of light.

    mass-luminosity relation - (n.)

    A well-defined relation between the mass and luminosity for main sequence stars.

    mass number A - (n.)

    Nucleon number, the sum of the numbers of protons and of neutrons in the nucleus of an atom.

    mass spectrometry - (n.)

    Instrumental method of identifying the chemical constituents of a substance by means of the separation of gaseous ions according to their differing mass and charge.

    matter - (n.)

    Stuff of which all materials are made; anything that has mass. As opposed to pure energy.

    matter era - (n.)

    collective name for the most recent three epochs in the history of the Universe (atom, galaxy, stellar), covering all of time after the Radiation Era.

    Maunder minimum - (n.)

    virtual disappearance of sunspots in the period 1645 to 1715.

    MCSA - (n.)

    short for MCSA/SD, a digital, energy-efficient, real-time, multichannel spectrum analyzer and signal detector; hardware device that continuously accepts a significant portion of the electromagnetic spectrum and divides the spectral energy into a set of many contiguous frequency bins or output channels. Operational functions included are frequency analysis and signal detection, averaging, threshold testing, etc..

    mega - (n.)

    –106 (as in megahertz, MHz); one million.

    megaparsec (Mpc) - (n.)

    A million parsecs.

    melting point - (n.)

    The temperature at which a substance changes from the solid to the liquid state.

    metal - (n.)

    (a) For stellar abundances, any element higher in atomic number than 2, that is, heavier than helium, (b) For a planet or solid, matter that is a good conductor of heat and electricity.

    meteor - (n.)

    Solar system matter observable when it falls through Earth's atmosphere and is heated by friction to temporary incandescence; a "shooting star."

    Index: A to F | G to L | M to R | S to Z

    meteorite - (n.)

    An interplanetary chunk of rock after it impacts on a planet or moon, especially on the earth.

    meter - (n.)

    The SI unit of length, slightly longer than 39 inches.

    methane - (n.)

    The colorless gaseous hydrocarbon CH4.

    MHz - (n.)

    one million Hertz; megahertz ; a million cycles per second.

    micrometeorite - (n.)

    a particle from space that is small enough to be slowed down when it reaches the Earth's atmosphere without being burnt up. Approximately 50 micrometeorites per square meter hit the top of Earth’s atmosphere each day.

    micrometer (μ) - (n.)

    A unit of length, one-millionth (10-6 )of a meter.

    microwave - (n.)

    electromagnetic wave roughly in the range 0.01-1 m in wavelength (ordinary broadcasting utilizes waves in the 200-600 m range; the "short waves" used in long-distance communications are rarely shorter than 10 m).

    Milky Way - (n.)

    The band of light across the sky from the stars and gas in the plane of the Milky Way Galaxy.

    minor axis - (n.)

    The shortest diameter of an ellipse; the line from one side of an ellipse to the other that passes midway between the foci and is perpendicular to the major axis. Also, the length of that line.

    Mo - (n.)

    solar mass.

    molar mass - (n.)

    The formula weight expressed in grams.

    molar volume - (n.)

    The volume occupied by 1 mole of a substance under specified conditions.

    mole - (n.)

    The formula weight in grams of an element or compound; or a quantity of chemical substance that contains 6.02 x 1023 formula units of the substance.

    molecule - (n.)

    A collection of atoms bound together that is the smallest collection that exhibits a certain set of chemical properties, mollusk : Any of an animal phylum (Mollusca) characterized by a large muscular foot and a mantle that secretes spicules or shells, such as a snail, clam, or squid.

    Index: A to F | G to L | M to R | S to Z

    momentum - (n.)

    A measure of the tendency that a moving body has to keep moving. The momentum in a given direction (the "linear momentum") is equal to the mass of the body times its component of velocity in that direction. ~ See Also: angular momentum.

    nanometer (nm) - (n.)

    A unit of length, one billionth (10-9) of a meter.

    NASA - (n.)

    National Aeronautics and Space Administration of the United States of America.

    natural philosophy - (n.)

    Philosophical speculation about nature.

    nebula - (n.)

    rarefied cloud of gas or dust observed in interstellar space.

    nebular hypothesis - (n.)

    a general theory that describes how stars and their associated solar systems are formed from the condensation of clouds of dust and gas in space

    neutrino - (n.)

    Spinning, neutral elementary particle. A neutrino has no rest mass and always travels at the speed of light.

    neutron - (n.)

    A massive, neutral elementary particle, one of the fundamental constituents of an atom. Neutrons and protons are similar in mass, about 1830 times more massive than electrons.

    neutron star - (n.)

    A star that has collapsed to the point where it is supported against gravity by neutron degeneracy.

    new moon - (n.)

    The phase of the moon when the side of the moon facing the earth is the side that is not illuminated by sunlight.

    Newtonian telescope - (n.)

    A type of reflecting telescope where the beam reflected by the primary mirror is reflected by a flat secondary mirror so that the focus falls to the side of the telescope tube.

    New General Catalogue (NGC) - (n.)

    The common name for "A New General Catalogue of Nebulae and Clusters of Stars," put together by J. L. E. Dreyer in 1888.

    noble gas - (n.)

    one of a group of rare but extremely stable gases with low reaction rates (helium, neon, argon, krypton, xenon, radon).

    node - (n.)

    A point of intersection between two great circles. Eclipses of the Sun and Moon occur when these bodies are simultaneously near the nodes of their paths in the sky.

    Index: A to F | G to L | M to R | S to Z

    nonmetals - (n.)

    The group of elements to the right of the heavy, stepped, diagonal line in the periodic table.

    nonthermal radiation - (n.)

    Radiation that cannot be characterized by a single number (the temperature). Normally, we derive this number from Planck's law, so that radiation that does not follow Planck's law is called nonthermal.

    nova - (n.)

    A star that suddenly increases in brightness. Most novae are thought to result in binary systems when matter from the giant component falls on the white dwarf component.

    nuclear fission - (n.)

    The splitting of an atomic nucleus into two large fragments.

    nuclear fusion - (n.)

    Combination of two small atomic nuclei to produce one larger nucleus.

    nuclear reactor - (n.)

    A plant that produces energy by nuclear fission.

    nuclear winter - (n.)

    A period of dark, cold weather that may be caused by the dust and smoke entering the atmosphere from the explosion of nuclear bombs.

    nucleon number - (n.)

    The total number of protons and neutrons in an atom; the mass number A.

    nucleons - (n.)

    Protons and neutrons in an atomic nucleus.

    nucleosynthesis - (n.)

    The formation of the elements.

    nucleus - (n.)

    (a.) Of an atom, the core of an atom, which has a positive charge, contains most of the mass, and takes up only a small part of the volume; (b) of a comet, the chunks of matter, taking up a volume no more than a few kilometers across, at the center of the head of a comet; (c) of a galaxy, the innermost regions of a spiral galaxy; it does not show spiral structure and is visible from the sky as a bulge in the otherwise flat disk of the galaxy.

    O and B association - (n.)

    A group of O and B giant Young stars close together in space. The members of an O and B association were formed at roughly the same time.

    objective - (n.)

    The principal lens or mirror of an optical system.

    oblate - (n.)

    Having an equatorial diameter greater than the polar diameter.

    Index: A to F | G to L | M to R | S to Z

    obliquity - (n.)

    The angle by which the spin axis of a planet to the plane of its ecliptic differs from 90°.

    occultation - (n.)

    The hiding of one astronomical body by another, such as the occultation of a star by the moon.

    Olbers's paradox - (n.)

    Tine observation that the sky is dark at night contrasted to a simple argument that shows that the sky should be uniformly bright, due to there being a star in every line of sight in an infinite and uniform universe. The paradox is resolved by the redshift of the expanding universe.

    Oort cloud - (n.)

    a vast collection of bodies made up of rock and ice that orbit the Sun at a distance starting in the region beyond the orbit of Pluto and extending out to nearly 1.5 light-years or 50,000 A. U. Unlike the bodies of the Solar System, which orbit the Sun in roughly the same plane, the Oort cloud objects form a vast sphere around the Sun. It is estimated that billions of objects exist in this region, and there is evidence that this is the place where most comets originate.

    opacity - (n.)

    The lack of complete transparency of a gas.

    open cluster - (n.)

    A galactic cluster, an asymmetric type of star cluster.

    open universe - (n.)

    The version of big bang cosmology in which the universe will expand forever. An open universe has infinite volume.

    opposition - (n.)

    The passage of a planet through the point most directly opposite the sun on the other side of the earth.

    optical activity - (n.)

    the property of some crystals, gases, liquids, and solutions to rotate plane-polarized light to the left or right. It occurs because the molecules that make up the substance through which the light beam is shone are asymmetric, i.e., they have no plane of symmetry. Asymmetric molecules are mirror images of each other that cannot be superimposed. This asymmetric property is also referred to as handedness. Examples are the L- and D-forms of amino acids.

    optical brightener - (n.)

    A compound that absorbs the invisible ultraviolet component of sunlight and re-emits it as visible light at the blue end of the spectrum.

    optical depth - (n.)

    The number of factors of the transcendental number (2.71828. . . ) that radiation is dimmed in passage through a gas.

    optical double - (n.)

    A pair of stars that appear extremely close together in the sky even though they are at different distances from us and are not physically linked.

    orbital - (n.)

    A region of space in an atom occupied by one or two electrons.

    ozone - (n.)

    O3 a gaseous molecule, made of three oxygen atoms, which forms a layer in the upper atmosphere that shields the Earth against excessive ultraviolet radiation.

    Index: A to F | G to L | M to R | S to Z

    ozone layer - (n.)

    layer of Earth's atmosphere at about 20 to 30 miles, marked by a high ozone (O3) content.

    polycyclic aromatic hydrocarbon (PAH) - (n.)

    Any of various organic compounds composed of a few to many six-membered rings of carbon atoms linked by an alternating sequence of single and double bonds and to which hydrogen atoms are attached.

    PAHs - (n.)

    Polycyclic aromatic hydrocarbons, i.e., hydrocarbons in which the carbon atoms are arranged in rings which are themselves linked together.

    paradigm - (n.)

    A well-established example or model, an archetype.

    parallax - (n.)

    (a) When used by itself, the word "parallax" refers to trigonometric parallax, half the angle through which a star appears to be displaced when the earth moves from one side of the sun to the other, that is, through 2 A.U. The parallax of a star is inversely proportional to the distance to the star from the sun. (b) Some of the other ways of measuring distances, usually in those cases referred to with an adjective, as in spectroscopic parallax.

    parsec (pc) - (n.)

    parallax second, the distance at which I AU subtends an angle of 1second of arc: 1 parsec = 1.9x1013 mi. = 3.26 light years.

    peculiar velocity - (n.)

    The velocity of a star with respect to the local standard of rest.

    penumbra - (n.)

    (a.) For an eclipse, the part of the shadow from which the sun or other radiating body is only partially occulted; (b) of a sunspot, the outer region of the sunspot, not as dark as the central umbra.

    perfect cosmological principle - (n.)

    The assumption that on a large scale the universe is homogeneous and isotropic in space and unchanging in time.

    periastron - (n.)

    The near point of the orbit of a body to the star around which it is orbiting.

    perihelion - (n.)

    The near point to the sun of the orbit of a body orbiting the sun.

    periodic table - (n.)

    A systematic arrangement of the elements in columns and rows; elements in a given column have similar properties.

    photomultiplier - (n.)

    An electronic device that through a series of internal stages multiplies the small current that is given off when light is incident on the device so that a relatively large current results.

    photon - (n.)

    A packet of energy that can be thought of as a particle of light travelling at the speed of light. A photon of energy E is equivalent to an electromagnetic wave of wavelength λ=hc/E, where h is Planck's constant and c is the speed of light in a vacuum.

    Index: A to F | G to L | M to R | S to Z

    photosphere - (n.)

    The visible surface layer of the sun and stars; the layer from which continuous radiation escapes and where absorption lines form.

    photovoltaic cell - (n.)

    A solar cell; a cell that converts sunlight directly to electrical energy.

    physical change - (n.)

    A change in physical state or form.

    physical properties - (n.)

    The qualities of a substance that can be demonstrated without changing the composition of the substance.

    plage - (n.)

    The part of a solar active region that appears bright when viewed in H0.

    Planck constant - (n.)

    the numerical factor h relating the frequency v of a photon to its energy e in the expression e = hn. the Planck constant has the value h = 6.62620 x 10-27 erg sec.

    Planck function (also known as the Planck law) - (n.)

    The mathematical expression describing the continuous thermal spectrum of a glowing object. For a given temperature, the Planck function specifies the intensity of radiation as a function of either frequency or wavelength.

    planet - (n.)

    A celestial body of substantial size (more than about 1000 km across), basically non-radiating and of insufficient mass for nuclear reactions ever to begin, ordinarily in orbit around a star.

    planetary nebula - (n.)

    A cloud of glowing, ionized gas, usually taking the form of a hollow sphere or shell, ejected by a star in the late stages of its evolution.

    planetesimal - (n.)

    A small (diameter up to several hundred kilometers) solar-system body of the type that first condensed from the solar nebula. Planetesimals are thought to have been the principal bodies that combined to form the planets.

    plasma - (n.)

    A state of matter similar to a gas but composed of isolated electrons and nuclei rather than discrete whole atoms or molecules.

    plate tectonics - (n.)

    the theory that the Earth's continental and oceanic crust and outermost portion of the mantle is fractured into large plates that move relative to each other. Convective currents in the mantle provide the driving force for this motion. The plate motion is responsible for global mountain building, earthquake activity, and volcanism, all of which are most pronounced along plate boundaries ~ See Also: continental drift.

    polar axis - (n.)

    The axis of an equatorial telescope mounting that is parallel to the earth's axis of rotation.

    polarization - (n.)

    The arrangement of electromagnetic waves so that all the planes in which the waves are oscillating are parallel to each other.

    Index: A to F | G to L | M to R | S to Z

    population I - (n.)

    The class of stars with relatively high abundances of heavy elements. These stars are generally found in the disk and spiral arms of spiral galaxies, and are relatively young. The term Population I is also commonly applied to other components of galaxies associated with the star formation, such as the interstellar material.

    population II - (n.)

    The class of stars with relatively low abundances of heavy elements. These stars are generally found in a spheroidal distribution about the galactic center and throughout the halo, and are relatively old.

    Population III - (n.)

    Hypothetical class of 1st generation stars composed solely of H and He, with mass 100-1000 x solar and luminosity 1-30 million times greater. As usual in astronomy, the numbering is backward: these are the first stars.

    positron - (n.)

    A subatomic particle with the same mass as the electron, but with a positive electrical charge; the antiparticle of the electron.

    potential energy - (n.)

    Energy that is stored, and which may be converted into kinetic energy under certain circumstances. In astronomy, the most common form of potential energy is gravitational potential energy.

    prebiotic - (n.)

    relating to the chemical or environmental precursors of the origin of life.

    precession - (n.)

    The slowly changing position of stars in the sky with respect to earth-based coordinates, resulting from the slowly varying orientation (the precession) of the earth's axis. The apparent precession of the stars is the result of the actual precession of the earth's axis.

    primary cosmic rays - (n.)

    The cosmic rays arriving at the top of the earth's atmosphere.

    primary mirror - (n.)

    The principal light-gathering mirror in a reflecting telescope.

    prime focus - (n.)

    The location at which the main lens or mirror of a telescope focuses an image without being reflected or refocused by another mirror or other optical element.

    primordial background radiation - (n.)

    Radiation detected in millimeter and submillimeter wavelength regions that is coming from all directions in space and interpreted to be the remnant of the big bang. Also known as 3° background radiation, background radiation, or the remnant of the primeval fireball.

    prograde motion - (n.)

    Orbital or spin motion in the forward or "normal" direction; in the solar system, this is counterclockwise as viewed looking down from above the north pole.

    Project Cyclops - (n.)

    a 10-week design study sponsored by NASA, Stanford University, and the American Institute for Engineering Education, of possible means for detecting extraterrestrial civilizations.

    prolate - (n.)

    Having the diameter along the axis of rotation longer than the equatorial diameter.

    Index: A to F | G to L | M to R | S to Z

    prominence - (n.)

    Solar gas protruding over the limb, visible to the naked eye only at eclipses but also observed outside of eclipses by its emission line spectrum. Prominences are at approximately the same temperature as the chromosphere.

    proper motion - (n.)

    Motion across the sky with respect to a framework of galaxies or fixed stars, usually measured in seconds of arc per century.

    proto- - (n.)

    A prefix from the Greek for "before." When used in conjunction with the name of a celestial body, means the state of the body just before it is considered to have formed.

    proton - (n.)

    A positively charged elementary particle of mass equal to that of a neutron and almost two thousand times that of an electron; a component, with neutrons, of all atomic nuclei : The nucleus of the hydrogen atom : A hydrogen ion, H*, arising either through the removal of the peripheral electron from a hydrogen atom or, together with a hydroxyl ion, from the dissociation of a water molecule. ~ See Also: Electron, Hydroxyl ion, Isotopes, Neutron, Nucleus.

    protostar - (n.)

    A star in the process of formation, specifically one that has entered the slow gravitational contraction phase.

    pulsar - (n.)

    A rapidly rotating object, now known to be a neutron star, an extremely dense collapsed star where the electrons have been forced into the protons. The object is thus made up mainly of neutrons and a few kilometers in diameter. Pulsars emit magnetic field constrained beams of radio, visible, x-ray and gamma radiation, which we perceive as pulses as these objects whirl, sometimes at hundreds of revolutions a second.

    q0 - (n.)

    The deceleration parameter, a cosmological parameter that describes the rate at which the expansion of the universe is slowing up.

    QSO - (n.)

    Quasi-stellar object. ~ See Also: quasar.

    quadrature - (n.)

    The configuration where a superior planet or the moon is 90 degrees away from the sun, as seen from the earth.

    quantum - (n.)

    The amount of energy associated with a photon, equal to hv, where h is the Planck constant, and v is the frequency. The quantum is the smallest amount of energy that can exist at a given frequency.

    quantum mechanics - (n.)

    The branch of 20th century physics that describes atoms and radiation; the theory involves bundles of energy known as quanta.

    quark - (n.)

    One of the subatomic particles from which many modern theoreticians believe such elementary particles as protons and neutrons are composed. The various kinds of quarks have positive or negative charges of 1/3 or 2/3.

    quasi-stellar object - (n.)

    Any of a class of extragalactic objects also known as quasars, characterized by emission lines with very large redshifts. The quasi-stellar objects are thought to lie at great distances, in which case they existed only at earlier times in the history of the universe; they may be cores of young galaxies.

    quiet sun - (n.)

    The collection of solar phenomena that do not vary with the solar activity cycle.

    Index: A to F | G to L | M to R | S to Z

    radar - (n.)

    The acronym for radio detection and ranging; an active rather than passive radio technique in which radio signals are transmitted and their reflections received and studied.

    radial velocity - (n.)

    The velocity of an object along a line (the radius) joining the object and the observer; the component of velocity toward or away from the observer.

    radian - (n.)

    The unit of angular measure, defined as the ratio of a length of arc intercepted by two radii to the length of the radius. P radians = 180°.

    radiant - (n.)

    The point in the sky from which all the meteors in a meteor shower appear to be coming.

    radiation - (n.)

    Electromagnetic radiation.

    radiation era - (n.)

    collective name for the first three epochs of the history of the Universe (chaos, hadron epoch, lepton epoch), lasting overall about 100 sec and dominated by radiation rather than matter.

    radiation pressure - (n.)

    Pressure created by light hitting a surface.

    radiative transport - (n.)

    The transport of energy, inside of a star or in other situations, by radiation.

    radioactive - (n.)

    Having the property of spontaneously changing into another isotope or element.

    radioactive dating - (n.)

    A technique for estimating the age of material, such as rock, based on the known initial isotopic composition and the known rate of radioactive decay for unstable isotopes originally present.

    radioactive decay - (n.)

    Disintegration of an unstable atomic nucleus by spontaneous emission of radiation.

    radioactivity - (n.)

    Spontaneous emission of alpha, beta, or gamma rays by the disintegration of the nuclei of atoms.

    radio galaxy - (n.)

    Any of a class of galaxies whose luminosity is greatest in radio wavelengths. Radio galaxies are usually large elliptical galaxies, with synchrotron radiation emitted from one or more pairs of lobes located on opposite sides of the visible galaxy.

    radionuclide - (n.)

    radioactive nuclear species or nuclide.

    Index: A to F | G to L | M to R | S to Z

    radio telescope - (n.)

    An antenna or set of antennas, often together with a focusing reflecting dish, that is used to detect radio radiation from space.

    radio waves - (n.)

    Electromagnetic radiation with wavelengths longer than about one millimeter.

    rare earth elements - (n.)

    series of elements usually taken to include elements with atomic numbers 58 to 71 (lanthanum) and sometimes yttrium and scandium.

    ray - (n.)

    (a) A light ray, a wave of electromagnetic radiation; (b) on the surface of a moon or planet, a streak of material that is relatively light in shade, presumable representing material ejected when a crater was formed.

    recombination - (n.)

    The addition of an electron to an ion, usually resulting in radiation (recombination lines) when the electron subsequently jumps down to lower energy states.

    recurrent nova - (n.)

    A star known to flare up in nova outbursts more than once. A recurrent nova appears to be a binary system containing a white dwarf and a mass-losing star, in which the white dwarf sporadically flares up when material falls onto it from the companion.

    reddening - (n.)

    The phenomenon by which the extinction of blue light by interstellar matter is greater than the extinction of red light so that the redder part of the continuous spectrum is enhanced.

    red giant - (n.)

    A post-main-sequence stage of the lifetime of a star; the star becomes relatively bright and relatively cool.

    redshift - (n.)

    The shift of a spectrum, usually of spectral lines in particular, to longer wavelengths.

    reducing agent - (n.)

    A substance that causes reduction and is itself oxidized.

    reducing atmosphere - (n.)

    atmosphere comprised of substances that readily provide electrons.

    reduction - (n.)

    The gain of one or more electrons or hydrogen atoms (electrons + protons) by an atom or molecule. ~ See Also: Electron, Oxidation, Proton : In cell biology, chromosome reduction refers to the halving of the chromosome number that occurs at meiosis. ~ See Also: Diploid, Haploid, Meiosis.

    reflecting telescope - (n.)

    A type of telescope that uses a mirror or mirrors to form the primary image.

    reflection nebula - (n.)

    An interstellar cloud containing dust that shines by light reflected from a nearby star.

    Index: A to F | G to L | M to R | S to Z

    refracting telescope - (n.)

    A telescope that uses lenses to bring light to a focus.

    refraction - (n.)

    The bending of electromagnetic radiation as it passes from one medium to another or between parts of a medium that has varying properties. The index of refraction of a substance is the ratio of the speed of light in a vacuum to that in the substance.

    refractory - (n.)

    The property of being able to exist in solid form under conditions of very high temperature. Refractory elements are characterized by a high temperature of vaporization; they are the first to condense into solid form when a gas cools, as in the solar nebula.

    regolith - (n.)

    The mantle of unconsolidated fragmental material that covers a land surface; i.e., soil and fractured rock.

    relativistic - (n.)

    Having a velocity that is such a large fraction of the speed of light that the special theory of relativity must be applied.

    relativity - (n.)

    Either of the theories of relativity worked out by Albert Einstein. The special theory of relativity (1905) is a theory of relative motion. The general theory of relativity (1916) is a theory of gravitation.

    research - (n.)

    A study or investigation to find new information.

    resolution - (n.)

    The ability of an optical system to distinguish fine detail.

    rest wavelength - (n.)

    The wavelength of a spectral line as measured in a laboratory, when there is no relative motion between source and observer.

    retrograde motion - (n.)

    Orbital or spin motion in the opposite direction from prograde motion; in the solar system, retrograde motions are clockwise as seen from above what we call the North Pole of the Earth.

    revolution - (n.)

    The orbiting of one body around another.

    right ascension (RA) - (n.)

    The east-west coordinate in the equatorial coordinate system. The right ascension is measured in units of hours, minutes, and seconds to the east from a fixed direction in the sky, which itself is defined as the line of intersection of the ecliptic and the celestial equator.

    rille - (n.)

    A type of winding, sinuous valley commonly found on the moon.

    Roche limit - (n.)

    The point near a massive body such as a planet or star, inside of which the tidal forces acting on an orbiting body exceed the gravitational force holding that body together. The location of the Roche limit depends on the size of the orbiting body.

    Index: A to F | G to L | M to R | S to Z

    Roentgen - (n.)

    unit of X or gamma radiation dosage: The amount of such radiation sufficient to produce ions carrying 1 electrostatic unit of charge in I ~11733 of air.

    Rosetta stone - (n.)

    A slab of black basalt stone found in 1799 that bears an inscription in hieroglyphics, demotic characters, and Greek, celebrated for having given the first clue to the decipherment of Egyptian hieroglyphics; from this, any breakthrough discovery of great magnitude.

    rotation - (n.)

    Spin on an axis.

    RR Lyrae variable - (n.)

    A member of a class of pulsating variable stars named after the prototype star, RR Lyrae. These stars are blue-white giants with pulsation periods of less than one day, and are Population II objects found primarily in globular clusters.

    Russell-Vogt theorem - (n.)

    The theorem that the evolution of a star is completely determined by its mass and chemical composition.

    Index: A to F | G to L | M to R | S to Z


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