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

    absolute magnitude - (n.)

    The magnitude that a star would appear to have if it were at a distance of ten parsecs from us.

    accretion disk - (n.)

    A rotating disk of gas surrounding a compact object (such as a neutron star or black hole), formed by material falling inward.

    albedo - (n.)

    Fraction of incident electromagnetic radiation reflected by a body such as a planet, star, or cloud.

    alpha particle - (n.)

    A cluster of two protons and two neutrons; a helium nucleus.

    Andromeda galaxy - (n.)

    The large spiral galaxy located some 700,000 parsecs from the sun; the most distant object visible to the unaided eye.

    Angstrom (A) - (n.)

    Unit of length convenient for measuring wavelengths of electromagnetic radiation: 1 A = 10-10m.

    angular momentum - (n.)

    A measure of the mass, radius, and rotational velocity of a rotating or orbiting body. In the simple case of an object in circular orbit, the angular momentum is equal to the mass of the object times its distance from the center of the orbit times its orbital speed.

    antimatter - (n.)

    A type of matter in which each particle (antiproton, antineutron, etc.) is opposite in charge and certain other properties to a corresponding particle (proton, neutron, etc.) of the same mass of the ordinary type of matter from which the solar system is made. Particles of antimatter are known to exist, but it is not known why matter is dominant in this region of the universe or whether regions exist in which antimatter is common.

    apastron - (n.)

    For an orbit around a star, the farthest point from that star.

    aperture - (n.)

    In the case of a telescope, an area open to the Universe; signifies either the maximum physical or the effective capture cross section of a telescope or radio antenna; often stated in terms of the diameter or an equivalent diameter if the aperture is noncircular.

    aphelion - (n.)

    The point in the orbit of a solar-system object where it is farthest from the sun.

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

    Apollo asteroid - (n.)

    An asteroid whose orbit brings it closer than 1 astronomical unit to the sun.

    arcsecond (arcsec) - (n.)

    Measurement of angular separation: a 1-inch stick would subtend an angle of 1 arcsec at a viewer's eye at a distance of about 6.5 miles.

    Arecibo - (n.)

    Short name for the National Astronomy and Ionospheric Center (NAIC) at Arecibo, Puerto Rico; often refers only to the NAIC 1000-ft (305-m) zenith (䔸°) antenna, the world's largest radio astronomy collector.

    asteroid - (n.)

    One of thousands of small planetlike bodies orbiting between Mars and Jupiter having diameters from a fraction of a kilometer to about 1,000 km.

    astrobiology - (n.)

    The study of life throughout the Universe, its origin, evolution, ecology and destiny.

    astrology - (n.)

    A non-scientific system based on superstition, that purports to explain or predict human actions by study of celestial positions.

    astrometry - (n.)

    Branch of astronomy that focuses on measurements, especially those relating to positions and movements.

    astronomical unit (AU) - (n.)

    Mean Earth-Sun distance: 1 AU = 1.496x10" m = 8.31 light minutes; a convenient unit for measuring distances between planets and their stars. atmosphere : Gaseous mass enveloping a planet or star.

    atmosphere - (n.)

    Gaseous mass enveloping a planet or star.

    atom - (n.)

    The smallest possible unit of a chemical element. When an atom is subdivided the parts no longer have properties of any chemical element. An atom consists of a nucleus with orbiting electrons.

    atom epoch - (n.)

    Fourth epoch in the history of the Universe, lasting from about 100 sec to 106 yr, in which matter came to dominate radiation as the principal constituent of the Universe.

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

    atomic mass unit - (n.)

    Convenient unit for measuring the mass of an atom or molecule: 1 atomic mass unit is defined as I /12 the atomic mass of the most abundant carbon isotope, 12C. A=12 for 12C.

    atomic nucleus - (n.)

    Concentrated, positively charged matter at the center of an atom; composed of protons and neutrons.

    atomic number - (n.)

    The number of protons in the nucleus of an element. It is the atomic number that defines the identity of an element. Z=6 for 12C.

    atomic theory - (n.)

    A model that offers a logical explanation for the law of multiple proportions and the law of constant composition by stating that all elements are composed of atoms, all atoms of a given element are identical, but the atoms of one element differ from the atoms of any other element; that atoms of different elements can combine to form compounds and a chemical reaction involves a change not in the atoms themselves, but in the way atoms are combined to form compounds.

    atomic weight - (n.)

    The number of protons and neutrons in an atom, averaged over the abundances of the different isotopes.

    AU - (n.)

    See astronomical unit.

    aurora - (n.)

    Glowing lights visible in the sky, resulting from processes in the earth's upper atmosphere.

    azimuth - (n.)

    The angular distance, around the horizon from the northern direction, usually expressed in angular measure from 0° for an object in the northern direction, to 180° for an object in the southern direction, around to 360°. Together, the altitude and the azimuth define the direction to an object.

    Big Bang - (n.)

    The initial singularity that started the space and time of our Universe, now thought to have occurred 13 to 15 billion years ago.

    big bang theory - (n.)

    A cosmological model, in which the universe was once compressed to infinite density and has been expanding ever since. Originally a term used by unbelieving scoffers, now as widely accepted as the model itself.

    billion - (n.)

    One-thousand million, 109 , in the USA only; elsewhere a million million, or 1012.

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

    binary system - (n.)

    Two neighboring stars that revolve around their common center of gravity; the fainter of the two stars is called the companion.

    binding energy - (n.)

    Energy derived from the conversion of mass to energy when neutrons and protons are combined to form nuclei.

    blackbody - (n.)

    Body capable of absorbing energy of all wavelengths falling on it; it is also capable of radiating all frequencies in a particular ratio to its absorbing properties. The value of the ratio depends only on the temperature of the body.

    black dwarf - (n.)

    A non-radiating ball of gas that results either when a white dwarf radiates all its energy or when gas contracts gravitationally but contains too little mass to begin nuclear fusion.

    black hole - (n.)

    An object that has collapsed under its own gravitation to such a small radius that its gravitational force traps photons of light.

    Bode's law - (n.)

    The physical laws describing the properties of a numerical scheme that roughly gives the radii of the orbits of the seven innermost planets and the radius of the asteroid belt. More numerology than science.

    Bohr atom - (n.)

    Nils Bohr's model of the hydrogen atom, in which the energy levels are depicted as concentric circles of radii that increase as (level number)2.

    bolide - (n.)

    An asteroid or comet that crashes onto the Earth or another body in the Solar System, generating a huge fire-ball.

    bolometric magnitude - (n.)

    The magnitude of a celestial object corrected to take account of the radiation in parts of the spectrum other than the visible.

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

    brown dwarf - (n.)

    An object substantially (~13 x) larger than Jupiter but with a mass no more than 40 percent that of the Sun. These objects are not big enough for gravitational collapse to heat them to the point that nuclear reactions can be triggered. Brown dwarfs may be very common in the universe and could even have planets in a habitable zone.

    burster - (n.)

    A sporadic source of intense X rays, probably consisting of a neutron star onto which new matter falls at irregular intervals.

    carbon-14 (14C) - (n.)

    A radioactive isotope of carbon produced in the upper atmosphere and present in living plants and animals that can be used in carbon-14 dating because it decays to nitrogen (14N) and a beta ray with a half-life of about 5,730 years.

    carbonaceous chondrite - (n.)

    A meteorite containing controls, with a high abundance of carbon and other volatile elements.

    carbon cycle - (n.)

    A chain of nuclear reactions, involving carbon at its intermediate stages, that transforms four hydrogen atoms into one helium atom with a resulting release in energy. The carbon cycle is only important in stars hotter than the sun.

    cassegrain (telescope or focus) - (n.)

    Some telescopes, particularly at microwave and shorter wavelengths, have a second reflector near the focus of the larger, primary mirror. This translates the focal point to a position near the apex of the primary where it is more accessible, and where practical antenna feeds are less responsive to radiations arriving from very wide angles relative to the nominal pointing direction.

    celestial sphere - (n.)

    The visible, seemingly spherical surface that appears to surround Earth and to be centered at the observer.

    Celsius scale - (n.)

    A temperature scale on which water freezes at 0 ° and boils at 100°.

    Cepheid variable - (n.)

    A type of supergiant star that oscillates in brightness in a manner similar to the star 8 Cephei. The periods of Cepheid variables, which are between 1 and 100 days, are linked to the absolute magnitude of the stars by known relationships; this allows the distances to Cepheids to be found.

    CETI - (n.)

    Acronym for communication with extraterrestrial intelligence. Sometimes pronounced with a long e and short i, to distinguish it from SETI which is favored with two short vowels.

    chain reaction - (n.)

    A self-sustaining change in which one or more products of one event cause one or more new events.

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

    chaos - (n.)

    Hypothetical first epoch in the history of the Universe, lasting 10" sec: a period about which we cannot yet even speculate.

    Charles' law - (n.)

    For a given mass of gas at constant pressure, the volume varies directly with the temperature (on the absolute scale).

    charm - (n.)

    An arbitrary name that corresponds to a property that distinguishes certain elementary particles, including types of quarks, from each other.

    chemically peculiar stars - (n.)

    Stars manifesting anomalies in the relative abundances of elements, which may arise from mechanical rather than nuclear effects; so-called manganese stars, for example, show a great overabundance of manganese and gallium, usually accompanied by excess mercury.

    chondrite - (n.)

    A type of stony meteorite that contains numerous small spherules of silicate (silica, silicon dioxide) minerals. A subset of this type of meteorites, the carbonaceous chondrites, contains several per cent organic carbon.

    closed universe - (n.)

    A possible state of the universe. In this state, the expansion of the universe will eventually be reversed; it is characterized by positive curvature, being finite in extent but having no boundaries. Recent observations indicate that this is unlikely to be the true state of our universe.

    cluster (astronomical) - (n.)

    Group of stars numbering from a few to hundreds of thousands of stars. Galactic clusters, sometimes called open clusters, contain up to a few hundred members and occur rather close to the plane of the Galaxy. Globular clusters contain tens of thousands of stars distributed about their center in a spherical manner and are found far from the plane of the Galaxy as well as in it toward the center of the Galaxy.

    CNO cycle - (n.)

    A nuclear-fusion-reaction sequence in which hydrogen nuclei are combined to form helium nuclei, and in which other nuclei, such as isotopes of carbon, oxygen, and nitrogen, appear as catalysts or by-products. The CNO cycle is dominant in the cores of stars on the upper main sequence. Same as carbon cycle.

    coherent radiation - (n.)

    Radiation in which the phases of waves at different locations in a cross-section of radiation have a definite relation to each other; in non-coherent radiation, the phases are random. Only coherent radiation shows interference.

    color - (n.)

    (a.) Of an object, a visual property that depends on wavelength; (b) an arbitrary name assigned to a property that distinguishes three kinds of quarks.

    color index - (n.)

    The difference B-V between the blue (B) and visual (V) magnitudes of a star. If B is less than V (that is, the star is brighter in blue than in visual light), then the star has a negative color index, and is a relatively hot star. If B is greater than V, the color index is positive, and the star is relatively cool.

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

    color-magnitude diagram - (n.)

    A Hertzsprung-Russell diagram in which the temperature on the horizontal axis is expressed in terms of color index.

    coma - (n.)

    The extended, glowing region that surrounds the nucleus of a comet.

    comet - (n.)

    An interplanetary body, composed of loosely bound rocky and icy material, that forms a glowing head (coma) and extended tail when it enters the inner solar system.

    comparison spectrum - (n.)

    A spectrum of known elements on earth usually photographed on the same photographic plate as a stellar spectrum in order to provide a known set of wavelengths or zero Doppler shift.

    conservation law - (n.)

    A statement that the total amount of some property (angular momentum, energy, etc.) of a body or set of bodies does not change.

    constellation - (n.)

    A prominent pattern of bright stars, historically associated with mythological figures. In modern usage, each constellation incorporates a precisely defined region of the sky.

    continuously habitable zone (CHZ) - (n.)

    Region around a star in which a planet can maintain appropriate conditions for the existence of life (including the retention of a significant amount of liquid water) for a period sufficient to allow the emergence of life.

    continuous spectrum - (n.)

    A spectrum with radiation at all wavelengths but with neither absorption nor emission lines.

    continuum - (n.)

    The continuous spectrum that we would measure from a body if no spectral lines were present.

    corona - (n.)

    The very hot, extended outer atmosphere of the sun and other cool main-sequence stars. The high temperature in the corona (l-2x 106 K) is probably caused by the dissipation of mechanical energy from the convective zone just below the photosphere.

    coronagraph - (n.)

    A type of telescope with which the corona can be seen at times other than that of a total solar eclipse.

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

    cosmic background radiation - (n.)

    The primordial radiation field that fills the universe. It was created in the form of gamma rays at the time of the big bang, but has since cooled so that today its temperature is 3 K and its peak wavelength is near 1.1 millimeters (in the microwave portion of the spectrum). Also known as the 3-degree background radiation. Also called cosmic microwave background radiation, CMBR.

    cosmic ray - (n.)

    A rapidly moving atomic nucleus from space. Some cosmic rays are produced in the sun, whereas others come from interstellar space and probably originate in supernova explosions.

    cosmogony - (n.)

    The study of the origin of the universe, usually applied in particular to the origin of the solar system.

    cosmological constant - (n.)

    A term added to the field equations by Einstein in order to allow solutions in which the universe was static; that is, neither expanding nor contracting. Although the need for the term disappeared when it was discovered that the universe is expanding, the cosmological constant is retained in the field equations by modern cosmologists, but is usually assigned the value zero.

    cosmological principle - (n.)

    The postulate, put forth by most cosmologists, that the universe is both homogeneous and isotropic; it is sometimes stated that the universe looks the same to all observers everywhere.

    cosmological redshift - (n.)

    A Doppler shift toward longer wavelengths that is caused by a galaxy's motion of recession, which in turn is caused by the expansion of the universe.

    cosmology - (n.)

    The study of the universe as a whole.

    critical mass - (n.)

    The mass of an isotope above which a self-sustaining chain reaction can occur.

    daughter isotopes - (n.)

    Isotopes formed by the radioactive decay of another isotope.

    dB - (n.)

    decibel; A unit of power ratio; the gain or loss in power in dB is equal to 10 times the logarithm of the power ratio. 1dB is approximately the smallest change in volume of sound which a normal ear can detect.

    decay constant - (n.)

    For an atom that undergoes radioactive decay, the decay constant is the proportionality factor between the time rate of decay and the total number of atoms present; it is the inverse of the mean lifetime of an atom.

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

    deceleration parameter (q0) - (n.)

    A particular measure of the rate at which the expansion of the universe is slowing down.

    declination - (n.)

    Celestial latitude, measured in degrees north or south of the celestial equator.

    degenerate gas - (n.)

    A gas in which either free electrons or free neutrons are as densely spaced as allowed by laws of quantum mechanics. Such a gas has extraordinarily high density, and its pressure is not dependent on temperature, as it is in an ordinary gas. Degenerate electron gas provides the pressure that supports white dwarfs against collapse, and degenerate neutron gas similarly supports neutron stars.

    density-wave theory - (n.)

    The explanation of spiral structure of galaxies as the effect of a wave of compression that rotates around the center of the galaxy and causes the formation of stars in the compressed region.

    detection - (n.)

    In electromagnetics, an operation converting the vector electromagnetic wave to a scalar time series proportional to either the amplitude or the power of the wave, with or without an accompanying angular time series. A crucial aspect of detection is: the signal-to-noise ratio after detection is the square of the signal-to-noise ratio before detection.

    determinism - (n.)

    The doctrine according to which like causes always produce like effects and, conversely, events are entirely explainable by their antecedent causes.

    deuterium - (n.)

    An isotope of hydrogen with a proton and a neutron in the nucleus (mass of 2 amu).

    differential gravitational force - (n.)

    A gravitational force acting on an extended object, such that the portions of the object closer to the source of gravitation feel a stronger force than the portions farther away. Such a force, also known as a tidal force, acts to deform or disrupt the object, and is responsible for many phenomena, ranging from synchronous rotation of moons or double stars to planetary ring systems to the disruption of galaxies in clusters.

    differentiation - (n.)

    The sinking of relatively heavy elements into the core of a planet or other body. Differentiation can occur only in fluid bodies, so any planet that has undergone this process must once have been at least partially molten. Also, a process whereby a stem cell acquires the characteristic features of a given cell type.

    diffraction grating - (n.)

    A very closely ruled series of lines that, through their diffraction of light, provide a spectrum of radiation that falls on it.

    disk - (n.)

    (a.) Of a galaxy, the disk-like flat portion, as opposed to the nucleus or the halo; (b) Of a star or planet, the two-dimensional projection of its surface.

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

    distance modulus - (n.)

    The difference m-M between the apparent and absolute magnitudes for a given star. This difference, which must be corrected for the effects of interstellar extinction, is a direct measure of the distance to the star.

    D lines - (n.)

    A pair of lines from sodium that appear in the yellow part of the spectrum.

    Doppler effect - (n.)

    The shift in wavelength of light that is caused by relative motion between the source of light and the observer. The Doppler shift, &#Delta&#gamma, is defined as the difference between the observed and rest (laboratory) wavelengths for a given spectral line.

    Doppler shift - (n.)

    A change in frequency resulting from relative motion along the line between the transmitter and the receiver. If the source and the receiver are approaching each other, the frequency received is higher than the frequency transmitted by a factor, depending on the actual relative velocity. Knowledge of this shift is used to determine the relative velocity.

    double bond - (n.)

    Two shared pairs of electrons.

    Drake equation - (n.)

    An approach to estimating some of the factors in guessing at the number of communicating civilizations in the galaxy.

    drifting (signal) - (n.)

    Refers to a signal with an apparent time rate of change in its typical frequency. All signals drift to some extent. In a SETI system, the dominant drift should be largely the result of only the time rate of change in the Doppler shift (q.v.).

    dwarf elliptical galaxy - (n.)

    A member of a class of small spheroidal galaxies, similar to standard elliptical galaxies except for their small size and low luminosity. Dwarf galaxies are probably the most common in the universe, but cannot be detected at distances beyond the Local Group of galaxies.

    dwarf nova - (n.)

    A close binary-star system containing a white dwarf; material from the companion star falls onto the other at sporadic intervals, creating brief nuclear outbursts.

    dynamo - (n.)

    A device that generates electricity through the effect of motion in the presence of a magnetic field. The solar dynamo explains sunspots and the solar activity cycle.

    earthshine - (n.)

    Sunlight illuminating the moon after having been reflected by the earth.

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

    eccentricity - (n.)

    A measure of the flatness of an ellipse, defined as half the distance between the foci divided by the semi-major axis.

    eclipse - (n.)

    The hiding of one celestial body by another. The hiding of a star by a body of larger angular size is usually called an occultation.

    ecliptic - (n.)

    The plane of the earth's orbit about the sun, which is approximately the plane of the solar system as a whole. The apparent path of the sun across the sky is the projection of the ecliptic onto the celestial sphere.

    electric field - (n.)

    A force field set up by an electric charge.

    electromagnetic force - (n.)

    The force created by the interaction of electric and magnetic fields. The electromagnetic force can be either attractive or repulsive, and is important in countless situations in astrophysics.

    electromagnetic radiation (wave, spectrum) - (n.)

    Energy involving electric fields and magnetic fields oscillating in phase at right angles to each other, propagated in a direction at right angles to both fields with a velocity in free space equal to c (approximately 300,000 km/sec, or 186,000 miles/sec), a universal constant.

    electron - (n.)

    A tiny (1/1830 the mass of a proton), negatively charged particle that orbits the nucleus of an atom. The charge is equal and opposite to that of a proton in the nucleus, and in a normal atom the number of electrons and protons is equal, so that the overall electrical charge is zero. It is the electrons that emit and absorb electromagnetic radiation, by making transitions between fixed energy levels.

    electron carrier - (n.)

    In a chain of chemical reactions, molecules that accept electrons from an electron donor and pass these to an electron acceptor.

    electron configuration - (n.)

    The arrangement of an atom's electrons in space.

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

    element - (n.)

    A fundamental substance in which all atoms have the same number of protons.

    elementary particle - (n.)

    Any of a number of sub-atomic particles.

    ellipse - (n.)

    A geometrical shape such that the sum of the distances from any point on it to two fixed points called foci is constant. In any bound system where two objects orbit a common center of mass, their orbits are ellipses, with the center of mass at one focus.

    elliptical galaxy - (n.)

    One of a class of galaxies characterized by smooth spheroidal forms, few young stars, and little interstellar matter.

    emission line - (n.)

    A wavelength at which radiation is emitted, creating a bright line in the spectrum.

    emission nebula - (n.)

    A cloud of interstellar gas that glows by the light of emission lines. The source of excitation that causes the gas to emit may be radiation from a nearby star, or heating by any of a variety of mechanisms.

    energy - (n.)

    The ability to do work. Energy can be in either kinetic form, when it is a measure of the motion of an object, or potential form, when it is stored but capable of being released into kinetic form.

    energy levels - (n.)

    The specific, quantized energy levels that an electron may have in an atom.

    enrichment (isotope) - (n.)

    The process by which the proportion of one isotope of an element is increased relative to the others.

    entropy - (n.)

    Tendency of systems to become more disordered (and thus more uniform) over time; also a measure of disorder; in thermodynamics, a measure of the amount of heat energy in a closed system that is not available to do work.

    equator - (n.)

    (a) Of the earth, a great circle on the earth, midway between the poles; (b) Celestial, the projection of the earth's equator onto the celestial sphere.

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

    equatorial coordinates - (n.)

    The astronomical coordinate system in which positions are measured with respect to the celestial equator (in the north-south direction) and with respect to a fixed direction (in the east-west direction). The coordinates used are declination (north- south, in units of angle) and right ascension (east-west, in units of time).

    equinox - (n.)

    An intersection of the ecliptic and the celestial equator. The center of the sun is geometrically above and below the horizon for equal lengths of time on the two days of the year when the sun passes the equinoxes; if the sun were a point and atmospheric refraction were absent, then day and night would be of equal length on those days.

    eras (geologic) - (n.)

    All of Earth's history since the appearance of the first life forms is divided roughly into four eras: Precambrian, from 3.5 billion to 570 million years ago; Paleozoic, from 570 to 225 million; Mesozoic, from 225 to 65 million; and Cenozoic, from 65 million to the present. The last two eras are broken down into the following periods: the Mesozoic into Triassic) Jurassic, and Cretaceous; the Cenozoic into Tertiary and Quaternary.

    erg - (n.)

    A unit of energy in the metric system, corresponding to the work done by a force of one dyne (the force that is required to accelerate one gram by one cm/sec2) producing a displacement of one centimeter.

    ergosphere - (n.)

    A region surrounding a rotating black hole (or other system satisfying Kerr's solution) from which work can be extracted.

    escape velocity - (n.)

    The velocity required for an object to escape the gravitational field of a body such as a planet .In a more technical sense, the escape velocity is the velocity at which the kinetic energy of the object equals its gravitational potential energy; if the object moves any faster, its kinetic energy exceeds its potential energy, and the object can escape the gravitational field.

    ETI - (n.)

    Extraterrestrial intelligence; also used to signify extraterrestrial intelligent species.

    Euclidean space - (n.)

    A space with zero curvature; a space where the sum of the angles of a triangle is 180°.

    event horizon - (n.)

    The "surface" of a black hole; the boundary of the region from within which no light can escape.

    exobiology - (n.)

    The study of life as it might occur elsewhere than on earth. Also, the study of the origin of life, at any location.

    feed (antenna feed, line feed) - (n.)

    in a reflecting antenna system, the device that converts a guided (by wire, cable, or other wave guide) electromagnetic wave intc? an electromagnetic radiation field, and vice versa, when reciprocity theorem holds as it so often does. Commonly, feeds are some form of horn antenna, but they may be dipole arrays or their.

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

    filament - (n.)

    (a) A feature of the solar surface seen in Ha as a thin, dark wavy line. A filament is a prominence projected on the solar disk. (b) In microbiology, a collective term for the cylindrical external sheath and cellular internal trichome of a filamentous prokaryote.

    filtergram - (n.)

    A photograph taken through a filter that passes only a very narrow band of wavelengths; usually applied to solar photographs.

    fireball - (n.)

    An exceptionally bright meteor. ~ See Also: bolide.

    first law of thermodynamics - (n.)

    Energy is neither created nor destroyed.

    fission, nuclear - (n.)

    The splitting of an atomic nucleus.

    flare - (n.)

    A rapid eruption of material from the surface of the sun or other star.

    flux - (n.)

    The amount of something (such as energy) passing through a surface per unit time.

    focal length - (n.)

    The distance from a lens or mirror to the point at which rays from an object at infinity are focused.

    force - (n.)

    in physics, something that can or does cause a change of momentum, measured by the rate of change of momentum with time.

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

    force field - (n.)

    A way of describing phenomena that result from action at a distance, that is, even though objects are not touching.

    Fraunhofer lines - (n.)

    The absorption lines of a solar or other stellar spectrum.

    frequency - (n.)

    The rate (in units of Hertz, or cycles per second) at which electromagnetic waves pass a fixed point. The frequency, usually designated ƒ, is related to the wavelength λ and the speed of light c by ƒ = c/λ.

    fusion - (n.)

    The amalgamation of nuclei into heavier nuclei.

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


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