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 Climate Change · Part One
 Climate Change · Part Two
 Introduction to Astronomy
 Life in the Universe

 Glossary: Climate Change
 Glossary: Astronomy
 Glossary: Life in Universe

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

    gain stability - (n.)

    crudely defined by AG/G = g(t)', the smaller this quantity over the relevant time interval, the less the gain instability or the greater the gain stability. Gain (amplification) of analog signaling systems always varies somewhat with time; g(t) contains a variety of "noise" terms of zero mean and various secular terms; the latter dominate unless proper precautions are in force.

    galactic cluster - (n.)

    An asymmetric type of collection of stars that shared a common origin.

    galaxy - (n.)

    spelled with a lower-case g, galaxy means any of millions of stellar systems once called "island universes" or extragalactic nebulae. Depending on their form, galaxies may be called spirals, barred spirals, ellipticals, or irregulars. Spelled with a capital G, Galaxy refers to that particular stellar system which includes our Sun and all the stars visible to the naked eye. The Milky Way is our view of the Galaxy.

    galaxy epoch - (n.)

    fifth epoch in the history of the Universe, lasting on the order of 1010 yr, during which matter largely coagulated into galactic masses.

    Galilean satellites - (n.)

    The four brightest satellites of Jupiter, discovered by Galileo.

    gamma ray - (n.)

    high-energy electromagnetic particle or photon, especially as emitted by a nucleus in its transition from one energy level to another. Radiation whose wavelength is less than one Angstrom is usually considered to be gamma-ray radiation.

    gas - (n.)

    The state of matter in which the substance maintains neither shape nor volume.

    gegenschein - (n.)

    The diffuse glowing spot, seen on the ecliptic opposite the sun's direction, created by sunlight reflected off of interplanetary dust. ~ See Also: zodiacal light.

    geothermal energy - (n.)

    Energy derived from the heat of Earth's interior.

    giant planets - (n.)

    Jupiter, Saturn, Uranus, and Neptune.

    giant star - (n.)

    A type of star brighter than main sequence stars of the same spectral type.

    gibbous moon - (n.)

    The phases between half moon and full moon

    giga - (n.)

    109 (as in gigahertz, GHz); one billion (U.S.A.).

    globular cluster - (n.)

    A large, spherical cluster of stars located in the halo of the galaxy.These clusters, containing up to several hundred thousand members, are thought to be among the oldest objects in the galaxy.

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

    gram - (n.)

    A unit of mass equal to the quantity' of mass contained in one cubic centimeter of water.

    grating - (n.)

    A surface ruled with closely spaced lines that, through diffraction, breaks up light into its spectrum.

    gravitation - (n.)

    One of the four fundamental forces of nature, the force by which two masses attract each other.

    gravitational wave - (n.)

    propagating field predicted by general relativity to occur as a result of any large-scale change in the distribution of matter (as in the collapse of a star).

    great circle - (n.)

    The intersection of a plane that passes through the center of a sphere with the surface of that sphere; the largest possible circle that can be drawn on the surface of a sphere.

    greenhouse effect - (n.)

    A warming of the Earth's surface and lower layers of the atmosphere caused by interaction of solar radiation with atmospheric gases (mainly carbon dioxide, methane, and water vapor) and its conversion to heat because it is transparent to incoming visible radiation but opaque to the infrared radiation that is emitted by the surface of the planet.

    greenhouse gas - (n.)

    Gases such as carbon dioxide, methane, and water vapor that produce a greenhouse effect.

    Gregorian calendar - (n.)

    The calendar in current use, with normal years that are 365 days long, with leap years every fourth year except for years that are divisible by 100 but not by 400.

    ground state - (n.)

    The state of an atom in which all electrons are in the lowest possible energy levels.

    group - (n.)

    A vertical column of the periodic table; a family of elements; also, A characteristic part of a molecule.

    H0 - (n.)

    The Hubble constant.

    H I region - (n.)

    An interstellar region of neutral hydrogen.

    H II region - (n.)

    An interstellar region of ionized hydrogen.

    Ha - (n.)

    The first line of the Balmer series of hydrogen, at 6563 angstroms.

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

    hadron epoch - (n.)

    second epoch in the history of the Universe, lasting on the order of a second; named for the heavy elementary particles (protons, neutrons, mesons) that were the most abundant form of matter at the time.

    half life - (n.)

    The period of time needed for half of the radioactive isotopes in a sample to decay to daughter atoms.

    halo - (n.)

    (a) The extended outer portions far above and below the plane of a galaxy such as the Milky Way. The halo is thought to contain a large fraction of the total mass of the galaxy, mostly in the form of dim stars and interstellar gas, (b) the extensive cloud of gas surrounding the head of a comet.

    head - (n.)

    Of a comet, the nucleus and coma together.

    heat - (n.)

    A measure of a quantity of energy; of how much energy a sample contains.

    heat capacity (of a substance) - (n.)

    The amount of heat needed to change the temperature of the substance by 1 °C.

    heat of vaporization (of a substance) - (n.)

    The amount of heat involved in the evaporation or condensation of 1 g of the substance.

    heliocentric - (n.)

    Sun-centered; using the sun rather than the earth as the point to which we refer. A heliocentric measurement, for example, omits the effect of the Doppler shift caused by the earth's orbital motion.

    helium - (n.)

    atom consisting of two protons and two electrons.

    helium flash - (n.)

    A rapid burst of nuclear reactions in the degenerate core of a moderate-mass star in the hydrogen shell-burning phase. The flash occurs when the core temperature reaches a sufficiently high temperature to trigger the triple-alpha reaction.

    Hertz (Hz) - (n.)

    measure of frequency with units of sec-1, formerly called cycles per second; an oscillating system that completes a cycle a second has a frequency of 1 Hz.

    Hertzsprung- Russell diagram - (n.)

    A diagram on which stars are represented according to their absolute magnitudes (on the vertical axis) and spectral types (on the horizontal axis).Because the physical properties of stars are interrelated, stars do not fall randomly on such a diagram, but instead lie in well-defined regions according to their state of evolution. Very similar diagrams can be constructed that show luminosity instead of absolute magnitude, and temperature or color index in place of spectral type.

    Hertzsprung gap - (n.)

    A region above the main sequence in a Hertzsprung-Russell diagram through which stars evolve rapidly and thus in which few stars are found.

    heterocyclic compound - (n.)

    A cyclic compound in which one or more atoms in the ring is (are) not carbon.

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

    heterotrophic hypothesis - (n.)

    The concept introduced by A.I. Oparin and J.B.S. Bernal that the earliest forms of life were heterotrophs that used nonbiologically produced organic matter as their carbon source.

    heterotrophy - (n.)

    Literally, other-feeding; the condition of an organism that is not able to obtain nutrients by synthesizing non organic materials from the environment, and that therefore must consume other life forms to obtain the organic products necessary for life e.g., animals, fungi, most bacteria.

    high-velocity star - (n.)

    A star whose velocity relative to the solar system is large. As a rule, high-velocity stars are Population II objects following orbital paths that are highly inclined to the plane of the galactic disk.

    homeobox - (n.)

    A highly conserved sequence of 180 nucleotides common to many regulatory genes and coding for the DNA-binding part of the corresponding regulatory proteins. ~ See Also: Homeotic gene, Transcription factor.

    homeostasis - (n.)

    The ability of living organisms to keep constant certain of their physical or chemical properties by self-regulation.

    homeotic gene - (n.)

    A regulatory gene containing a homeobox sequence. ~ See Also: Homeobox.

    homogeneous - (n.)

    Having the quality of being uniform in properties throughout. In astronomy, this term is often applied to the universe as a whole, which is postulated to be homogeneous.

    homologous series - (n.)

    A series of compounds in which adjacent members of the series differ by a fixed unit of structure.

    horizontal gene transfer - (n.)

    The transfer of genes from one organism to another, as opposed to vertical gene transfer, from parent to offspring.

    horizontal branch - (n.)

    A sequence of stars in the H-R diagram of a globular cluster, extending horizontally across the diagram to the left from the red-giant region. These are probably stars undergoing helium burning in their cores, by the triple-alpha reaction.

    hormone - (n.)

    A chemical transmitter transported by the blood stream or some other humoral connection from the cells that secrete it to the cells on which it acts. ~ See Also: Chemical transmitter.

    hour angle - (n.)

    Of a celestial object as seen from a particular location, the difference between the local sidereal time and the right ascension (H.A. = L.S.T. - R.A.).

    H-R diagram - (n.)

    Hertzsprung-Russell diagram.

    Hubble constant - (n.)

    The numerical factor, usually denoted H, that describes the rate of expansion of the universe. It is the proportionality constant in the Hubble law v ~Hd, which relates the speed of recession of a galaxy (v) to its distance (d) . The present value of H has recently become fairly well known; estimates range between 47 and 63 km/sec Y Mpc, giving an age of the universe around 14 billion years.

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

    Hubble law - (n.)

    The linear relation between the velocity of recession of a distant object and its distance from us, v = H0d.

    human genome project - (n.)

    An international protect to map the entire genome of Homo sapiens.

    humectant - (n.)

    A moistening agent.

    hydrocarbon - (n.)

    Any of a diverse group of organic compounds composed of hydrogen and carbon.

    hydrogen - (n.)

    simplest atom, consisting only of one proton and one electron; the most abundant element in the Universe.

    hypotheses - (n.)

    Guesses that can be tested by experiment.

    Hz - (n.)

    see Hertz.

    IC - (n.)

    Index Catalogue, one of the supplements to Dreyer's New General Catalogue (NGC).

    ideal gas law - (n.)

    The volume of a gas is proportional to the amount of gas and its Kelvin temperature and inversely proportional to its pressure.

    IDP - (n.)

    interplanetary dust particle.

    image tube - (n.)

    An electronic device that receives incident radiation and intensifies it or converts it to a wavelength at which photographic plates are sensitive.

    impact crater - (n.)

    A crater formed on the surface of a terrestrial planet or a satellite by the impact of a meteoroid or planetesimal.

    inclination - (n.)

    Of an orbit, the angle of the plane of the orbit with respect to the ecliptic plane.

    index of refraction - (n.)

    ratio of the speed of light in a vacuum to its speed in a given medium.

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

    inertia - (n.)

    The tendency of an object to remain in its state of rest or uniform motion; this tendency is directly related to the mass of the object.

    infrared - (n.)

    Radiation beyond the red, from about 7000 angstroms up to 1 mm.

    interference - (n.)

    The property of radiation, explainable by the wave theory, in which waves that are in phase can add (constructive interference) and waves that are out of phase can subtract (destructive interference); for light, this gives alternate light and dark bands.

    interferometer - (n.)

    A device that uses the property of interference to measure such properties of objects as their positions or structure.

    interferometry - (n.)

    an astronomical technique in which the images from two or more telescopes are superimposed. Interferometry has many uses, and one of the most intriguing is its ability to cancel out the light from a point source such as a star by superimposing the images in such a way that the oscillations in light intensity from one image are the reverse of the oscillations from the other image. With this method, planets orbiting the star can be directly observed, and the composition of their atmospheres can be investigated using spectral analyses. Note that this has not yet been accomplished as of summer 2000.

    interplanetary dust particle (IDP) - (n.)

    Small, often microscopic rocky particles of interplanetary debris.

    interplanetary medium - (n.)

    Gas and dust between the planets.

    interstellar cloud - (n.)

    A region of relatively high density in the inter- stellar medium. Interstellar clouds have densities ranging between 1 and 10'' particles per cubic centimeter, and in aggregate, contain most of the mass in interstellar space.

    interstellar extinction - (n.)

    The obscuration of starlight by interstellar dust. Light is scattered off of dust grains, so that a distant star appears dimmer than it otherwise would. The scattering process is most effective at short (blue) wavelengths, so that stars seen through inter- stellar dust appear reddened and dimmed.

    inverse Compton scattering - (n.)

    A method of converting photons from lower to higher energy through interaction with electrons that are moving with velocities close to the speed of light.

    inverse-square law - (n.)

    Any law describing a force or other phenomenon that decreases in strength as the square of the distance from some central reference point. The term inverse-square law is often used by itself to mean the law staling that the intensity of light emitted by a source such as a star diminishes as the square of the distance from the source.

    ionizing radiation - (n.)

    Radiation that produces ions as it passes through matter.

    ionosphere - (n.)

    The zone of the earth's upper atmosphere, between 80- and 500-km altitude, where charged subatomic particles (chiefly protons and electrons) are trapped by the earth's magnetic field. ~ See Also: Van Allen belts.

    irregular galaxy - (n.)

    A type of galaxy that lacks defined shape or symmetry.

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

    isochron - (n.)

    A parameter used in isotopic dating of geologic materials, experimentally determined from comparison of the isotopic compositions of two or more components (usually minerals) that share a common age.

    isomerization - (n.)

    The conversion of a compound into one or more of its isomers.

    isomers - (n.)

    Compounds that have the same molecular formula but different structural formulas and properties.

    isotopes - (n.)

    different forms of an element that have the same number of protons in their nuclei, and thus the same atomic number, but that have different numbers of neutrons and thus different atomic masses. There are two kinds of isotopes, stable and unstable. Isotopes that are unstable are called radioactive and disintegrate at a constant decay rate. Examples of stable isotopes include carbon-12 and carbon-13. Carbon-14, and uranium-238 and -235, are examples of unstable isotopes.

    isotopic date - (n.)

    Age of a rock (or organic substance less than 60,000 years old) determined by measurement of the ratio of a parent isotope to one of the products of its radioactive decay.

    isotopic fractionation - (n.)

    Separation of isotopes of an element, in organisms often mediated by an enzyme.

    isotropic - (n.)

    Having the property of appearing the same in all directions. In astronomy, this term is often postulated to apply to the universe as a whole.

    Jansky (Jy) - (n.)

    convenient unit of incident spectral flux density used in radio astronomy; 1 Jy = 10-26 W/m2 Hz (named for Karl G. Jansky, initial discoverer of extraterrestrial radio radiations).

    Joule (J ) - (n.)

    unit for work in the MKS system of units: 1 J = 107 ergs = 0.239 cal.

    Jovian planet - (n.)

    Same as giant planet; in our solar system: Jupiter (Jove), Saturn, Uranus and Neptune..

    Kardashev cultures - (n.)

    N. X. Kardashev has distinguished three types of technological societies according to the amount of power they can harness: Type I can engage the power available on a planet; Type II. the power output of a star; and Type III, the power output of a galaxy.

    Kelvin - (n.)

    A unit of temperature equal to one one-hundredth of the difference between the freezing and boiling points of water, and used in a scale whose zero point is absolute zero. A Kelvin is usually denoted by K.

    kHz - (n.)

    kilohertz, 1000Hz (see Hertz).

    kilocalorie - (n.)

    A unit of energy, the amount of heat required to raise the temperature of 1 kg of water 1° C, 1000 calories.

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

    kilogram - (n.)

    The SI unit of mass, a quantity about equal to 2.2 pounds.

    kiloparsec (kpc) - (n.)

    A unit of distance equal to 1,000 parsecs.

    kinetic energy - (n.)

    The energy of motion. The kinetic energy of a moving object is equal to one-half times its mass times the square of its velocity.

    kinetic isotopic fractionation - (n.)

    Separation of isotopes of an element as a result of their speeds of movement, in organisms mediated by an enzyme that interacts more readily with one of two or more isotopes of an element.

    kinetic-molecular theory - (n.)

    A model that uses the motion of molecules to explain the behavior of the three states of matter.

    Kirkwood's gaps - (n.)

    Narrow gaps in the asteroid belt created by orbital resonance with Jupiter

    launch window - (n.)

    The time opportunity for launching a spacecraft to another body in the Solar System involving minimal energy or the shortest flight time.

    law of conservation of energy - (n.)

    The amount of energy within the universe is constant; energy cannot be created or destroyed, only transformed.

    lepton epoch - (n.)

    third epoch in the history of the Universe, lasting about 100 sec, in which the lighter elementary particles such as electrons, neutrinos, and muons were the dominant form of matter.

    life era - (n.)

    era in the history of the Universe when life emerges as the dominant element.

    light-gathering power - (n.)

    The ability of a telescope to collect light from an astronomical source. The light-gathering power is directly related to the area of the primary mirror or lens.

    light-harvesting pigment - (n.)

    Organic compounds, such as chlorophyll and bacteriochlorophyll, that absorb light energy in photosynthesis.

    light year - (n.)

    distance traveled by light in a vacuum in one year: 1 light year = 9.46>(10' 55 m.

    limb darkening - (n.)

    The dark region around the edge of the visible disk of the sun or a planet, caused by a decrease in temperature with height in the atmosphere.

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

    limestone - (n.)

    A kind of sedimentary rock consisting mainly of calcium carbonate minerals.

    line spectrum - (n.)

    The pattern of colored lines emitted by an element.

    liquid - (n.)

    A state of matter in which the substance assumes the shape of its container, flows readily, and maintains a fairly constant volume.

    liquid metallic hydrogen - (n.)

    Hydrogen in a state of semi rigidity that can exist only under conditions of extremely high pressure, as in the interiors of Jupiter and Saturn.

    liter - (n.)

    Unit of volume equal to a cube with sides 10 cm.

    lithosphere - (n.)

    The layer in the earth, moon, and terrestrial planets that includes the crust and the outer part of the mantle.

    local group - (n.)

    The cluster of about thirty galaxies to which the Milky Way belongs.

    luminosity - (n.)

    the total energy emitted by an object per second; that is, the power of the object. for stars the luminosity is usually measured in units of ergs per second.

    luminosity class - (n.)

    One of several classes to which a star can be assigned on the basis of certain luminosity indicators in its spectrum. The classes range from I for supergiants to V for main-sequence stars (also known as dwarfs).

    lunar month - (n.)

    The synodic period of the moon, equal to 27 days 7 hours 43 minutes 11.5 seconds.

    L wave - (n.)

    A type of seismic wave that travels only over the surface of the earth.

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


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