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    Example and Directions
    Words (or words that have the same definition) The definition is case sensitive (Optional) Image to display with the definition [Not displayed in Glossary, only in pop-up on pages] (Optional) Caption for Image (Optional) External or Internal Link (Optional) Source for Definition
    (Eg. "Genetic, Hereditary, DNA ...") (Eg. "Relating to genes or heredity") The infamous double helix CC-BY-SA; Delmar Larsen
    Glossary Entries



    Image Caption Link Source
    absorption spectrum a series or pattern of dark lines superimposed on a continuous spectrum        
    accelerate to change velocity; to speed up, slow down, or change direction.        
    accretion the gradual accumulation of mass, as by a planet forming from colliding particles in the solar nebula        
    accretion disk the disk of gas and dust found orbiting newborn stars, as well as compact stellar remnants such as white dwarfs, neutron stars, and black holes when they are in binary systems and are sufficiently close to their binary companions to draw off material        
    active galactic nuclei, AGN galaxies that are almost as luminous as quasars and share many of their properties, although to a less spectacular degree; abnormal amounts of energy are produced in their centers        
    active galaxies galaxies that house active galactic nuclei        
    active region an area on the Sun where magnetic fields are concentrated; sunspots, prominences, flares, and CMEs all tend to occur in active regions        
    adaptive optics systems used with telescopes that can compensate for distortions in an image introduced by the atmosphere, thus resulting in sharper images        
    amino acids organic compounds that are the molecular building blocks of proteins        
    angular momentum the measure of the motion of a rotating object in terms of its speed and how widely the object’s mass is distributed around its axis        
    aperture diameter of the primary lens or mirror of a telescope        
    aphelion the point in its orbit where a planet (or other orbiting object) is farthest from the Sun        
    apogee the point in its orbit where an Earth satellite is farthest from Earth        
    apparent brightness a measure of the amount of light received by Earth from a star or other object—that is, how bright an object appears in the sky, as contrasted with its luminosity        
    apparent magnitude a measure of how bright a star looks in the sky; the larger the number, the dimmer the star appears to us        
    apparent solar time time as measured by the position of the Sun in the sky (the time that would be indicated by a sundial)        
    association a loose group of young stars whose spectral types, motions, and positions in the sky indicate a common origin        
    asteroid a stony or metallic object orbiting the Sun that is smaller than a major planet but that shows no evidence of an atmosphere or of other types of activity associated with comets        
    asteroid belt the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun        
    astrobiology the multidisciplinary study of life in the universe: its origin, evolution, distribution, and fate; similar terms are exobiology and bioastronomy        
    astrology the pseudoscience that deals with the supposed influences on human destiny of the configurations and locations in the sky of the Sun, Moon, and planets        
    astronomical unit, AU the unit of length defined as the average distance between Earth and the Sun; this distance is about 1.5 × 108 kilometers        
    aurora light radiated by atoms and ions in the ionosphere excited by charged particles from the Sun, mostly seen in the magnetic polar regions        
    bar a force of 100,000 Newtons acting on a surface area of 1 square meter; the average pressure of Earth’s atmosphere at sea level is 1.013 bars        
    baryon cycle the cycling of mass in and out of the interstellar medium, including accretion of gas from intergalactic space, loss of gas back into intergalactic space, and conversion of interstellar gas into stars        
    basalt igneous rock produced by the cooling of lava; makes up most of Earth’s oceanic crust and is found on other planets that have experienced extensive volcanic activity        
    Big Bang the theory of cosmology in which the expansion of the universe began with a primeval explosion (of space, time, matter, and energy)        
    binary stars two stars that revolve about each other        
    biomarker evidence of the presence of life, especially a global indication of life on a planet that could be detected remotely (such as an unusual atmospheric composition)        
    black hole a region in spacetime where gravity is so strong that nothing—not even light—can escape        
    blackbody an idealized object that absorbs all electromagnetic energy that falls onto it        
    brown dwarf an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 0.075 the mass of the Sun; brown dwarfs are capable of deuterium fusion, but not hydrogen fusion        
    celestial equator a great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator        
    celestial poles points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth’s polar axis        
    celestial sphere the apparent sphere of the sky; a sphere of large radius centered on the observer; directions of objects in the sky can be denoted by their position on the celestial sphere        
    central bulge (or nuclear bulge) the central (round) part of the Milky Way or a similar galaxy        
    cepheid a star that belongs to a class of yellow supergiant pulsating stars; these stars vary periodically in brightness, and the relationship between their periods and luminosities is useful in deriving distances to them        
    Chandrasekhar limit the upper limit to the mass of a white dwarf (equals 1.4 times the mass of the Sun)        
    charge-coupled device, CCD array of high-sensitivity electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens) to record an image or spectrum        
    chromatic aberration distortion that causes an image to appear fuzzy when each wavelength coming into a transparent material focuses at a different spot        
    chromosphere the part of the solar atmosphere that lies immediately above the photospheric layers        
    circumpolar zone those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon        
    closed universe a model in which the universe expands from a Big Bang, stops, and then contracts to a big crunch        
    cold dark matter slow-moving massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation, and that make up most of the mass of galaxies and galaxy clusters        
    color index difference between the magnitudes of a star or other object measured in light of two different spectral regions—for example, blue minus visual (B–V) magnitudes        
    comet a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail        
    conduction process by which heat is directly transmitted through a substance when there is a difference of temperature between adjoining regions caused by atomic or molecular collisions        
    continuous spectrum a spectrum of light composed of radiation of a continuous range of wavelengths or colors, rather than only certain discrete wavelengths        
    convection movement caused within a gas or liquid by the tendency of hotter, and therefore less dense material, to rise and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat        
    core the central part of the planet; consists of higher density material        
    corona (of the Sun) the outer (hot) atmosphere of the Sun        
    coronal hole a region in the Sun’s outer atmosphere that appears darker because there is less hot gas there        
    coronal mass ejection, CME a solar flare in which immense quantities of coronal material—mainly protons and electrons—is ejected at high speeds (500–1000 kilometers per second) into interplanetary space        
    cosmic microwave background, CMB microwave radiation coming from all directions that is the redshifted afterglow of the Big Bang        
    cosmic rays atomic nuclei (mostly protons) and electrons that are observed to strike Earth’s atmosphere with exceedingly high energies.        
    cosmological constant the term in the equations of general relativity that represents a repulsive force in the universe        
    cosmological principle the assumption that, on the large scale, the universe at any given time is the same everywhere—isotropic and homogeneous        
    cosmology the study of the organization and evolution of the universe        
    critical density in cosmology, the density that is just sufficient to bring the expansion of the universe to a stop after infinite time        
    crust the outer layer of a terrestrial planet        
    dark energy the energy that is causing the expansion of the universe to accelerate; its existence is inferred from observations of distant supernovae        
    dark matter nonluminous material, whose nature we don’t yet understand, but whose presence can be inferred because of its gravitational influence on luminous matter        
    dark matter halo the mass in the Milky Way that extends well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the center of the Galaxy; although we deduce its existence from its gravity, the composition of this matter remains a mystery        
    declination the angular distance north or south of the celestial equator        
    degenerate gas a gas that resists further compression because no two electrons can be in the same place at the same time doing the same thing (Pauli exclusion principle)        
    density the ratio of the mass of an object to its volume        
    detector device sensitive to electromagnetic radiation that makes a record of astronomical observations        
    deuterium a form of hydrogen in which the nucleus of each atom consists of one proton and one neutron        
    differential galactic rotation the idea that different parts of the Galaxy turn at different rates, since the parts of the Galaxy follow Kepler’s third law: more distant objects take longer to complete one full orbit around the center of the Galaxy        
    differential rotation the phenomenon that occurs when different parts of a rotating object rotate at different rates at different latitudes        
    differentiation gravitational separation of materials of different density into layers in the interior of a planet or moon        
    dispersion separation of different wavelengths of white light through refraction of different amounts
    DNA, deoxyribonucleic acid a molecule that stores information about how to replicate a cell and its chemical and structural components        
    Doppler effect the apparent change in wavelength or frequency of the radiation from a source due to its relative motion away from or toward the observer        
    Drake equation a formula for estimating the number of intelligent, technological civilizations in our Galaxy, first suggested by Frank Drake        
    eccentricity in an ellipse, the ratio of the distance between the foci to the major axis        
    eclipsing binary a binary star in which the plane of revolution of the two stars is nearly edge-on to our line of sight, so that the light of one star is periodically diminished by the other passing in front of it        
    ecliptic the apparent annual path of the Sun on the celestial sphere        
    electromagnetic radiation radiation consisting of waves propagated through regularly varying electric and magnetic fields and traveling at the speed of light        
    electromagnetic spectrum the whole array or family of electromagnetic waves, from radio to gamma rays        
    ellipse a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same        
    elliptical galaxy a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material        
    emission spectrum a series or pattern of bright lines superimposed on a continuous spectrum        
    energy flux the amount of energy passing through a unit area (for example, 1 square meter) per second; the units of flux are watts per square meter        
    energy level a particular level, or amount, of energy possessed by an atom or ion above the energy it possesses in its least energetic state; also used to refer to the states of energy an electron can have in an atom        
    epicycle the circular orbit of a body in the Ptolemaic system, the center of which revolves about another circle (the deferent)        
    equivalence principle concept that a gravitational force and a suitable acceleration are indistinguishable within a sufficiently local environment        
    escape speed the speed a body must achieve to break away from the gravity of another body        
    event horizon a boundary in spacetime such that events inside the boundary can have no effect on the world outside it—that is, the boundary of the region around a black hole where the curvature of spacetime no longer provides any way out        
    evolution of galaxies changes in individual galaxies over cosmic time, inferred by observing snapshots of many different galaxies at different times in their lives        
    excitation the process of giving an atom or an ion an amount of energy greater than it has in its lowest energy (ground) state        
    exoplanet a planet orbiting a star other than our Sun        
    extremophile an organism (usually a microbe) that tolerates or even thrives under conditions that most of the life around us would consider hostile, such as very high or low temperature or acidity        
    eyepiece magnifying lens used to view the image produced by the objective lens or primary mirror of a telescope        
    fault in geology, a crack or break in the crust of a planet along which slippage or movement can take place, accompanied by seismic activity        
    fission breaking up of heavier atomic nuclei into lighter ones        
    flat universe a model of the universe that has a critical density and in which the geometry of the universe is flat, like a sheet of paper        
    focus, foci (plural: foci) one of two fixed points inside an ellipse from which the sum of the distances to any point on the ellipse is constant        
    focus (of telescope) point where the rays of light converged by a mirror or lens meet        
    frequency the number of waves that cross a given point per unit time (in radiation)        
    fusion the building of heavier atomic nuclei from lighter ones        
    galactic cannibalism a process by which a larger galaxy strips material from or completely swallows a smaller one        
    gamma rays photons (of electromagnetic radiation) of energy with wavelengths no longer than 0.01 nanometer; the most energetic form of electromagnetic radiation        
    gene the basic functional unit that carries the genetic (hereditary) material contained in a cell        
    general theory of relativity Einstein’s theory relating gravity and the structure (geometry) of space and time        
    geocentric centered on Earth        
    giant a star of exaggerated size with a large, extended photosphere        
    giant molecular clouds large, cold interstellar clouds with diameters of dozens of light-years and typical masses of 105 solar masses; found in the spiral arms of galaxies, these clouds are where stars form        
    giant planet any of the planets Jupiter, Saturn, Uranus, and Neptune in our solar system, or planets of roughly that mass and composition in other planetary systems        
    globular cluster one of about 150 large, spherical star clusters (each with hundreds of thousands of stars) that form a spherical halo around the center of our Galaxy        
    grand unified theories, GUTs (GUTs) physical theories that attempt to describe the four forces of nature as different manifestations of a single force        
    granite a type of igneous silicate rock that makes up most of Earth’s continental crust        
    granulation the rice-grain-like structure of the solar photosphere; granulation is produced by upwelling currents of gas that are slightly hotter, and therefore brighter, than the surrounding regions, which are flowing downward into the Sun        
    gravitational redshift an increase in wavelength of an electromagnetic wave (light) when propagating from or near a massive object        
    gravitational wave a disturbance in the curvature of spacetime caused by changes in how matter is distributed; gravitational waves propagate at (or near) the speed of light        
    gravity the mutual attraction of material bodies or particles        
    great circle a circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center        
    greenhouse effect the blanketing (absorption) of infrared radiation near the surface of a planet—for example, by \(\ce{CO2}\) in its atmosphere        
    ground state the lowest energy state of an atom        
    H II region the region of ionized hydrogen in interstellar space        
    habitable environment an environment capable of hosting life        
    habitable zone the region around a star in which liquid water could exist on the surface of terrestrial-sized planets, hence the most probable place to look for life in a star’s planetary system        
    half-life time required for half of the radioactive atoms in a sample to disintegrate        
    halo the outermost extent of our Galaxy (or another galaxy), containing a sparse distribution of stars and globular clusters in a more or less spherical distribution        
    heliocentric centered on the Sun        
    helioseismology study of pulsations or oscillations of the Sun in order to determine the characteristics of the solar interior        
    helium flash a nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star        
    Herbig-Haro object, HH object luminous knots of gas in an area of star formation that are set to glow by jets of material from a protostar        
    Hertzsprung–Russell diagram, H–R diagram (Hertzsprung–Russell diagram) a plot of luminosity against surface temperature (or spectral type) for a group of stars        
    highlands the lighter, heavily cratered regions of the Moon, which are generally several kilometers higher than the maria        
    homogeneous having a consistent and even distribution of matter that is the same everywhere        
    horizon a great circle on the celestial sphere 90° from the zenith; more popularly, the circle around us where the dome of the sky meets Earth        
    horoscope a chart used by astrologers that shows the positions along the zodiac and in the sky of the Sun, Moon, and planets at some given instant and as seen from a particular place on Earth—usually corresponding to the time and place of a person’s birth        
    hot dark matter massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation, and that make up most of the mass of galaxies and galaxy clusters; hot dark matter is faster-moving material than cold dark matter        
    Hubble constant a constant of proportionality in the law relating the velocities of remote galaxies to their distances        
    Hubble’s law a rule that the radial velocities of remote galaxies are proportional to their distances from us        
    hydrostatic equilibrium balance between the weights of various layers, as in a star or Earth’s atmosphere, and the pressures that support them        
    igneous rock rock produced by cooling from a molten state        
    inflationary universe a theory of cosmology in which the universe is assumed to have undergone a phase of very rapid expansion when the universe was about 10–35 second old; after this period of rapid expansion, the standard Big Bang and inflationary models are identical        
    infrared electromagnetic radiation of wavelength 103–106 nanometers; longer than the longest (red) wavelengths that can be perceived by the eye, but shorter than radio wavelengths        
    interference process in which waves mix together such that their crests and troughs can alternately reinforce and cancel one another        
    interferometer instrument that combines electromagnetic radiation from one or more telescopes to obtain a resolution equivalent to what would be obtained with a single telescope with a diameter equal to the baseline separating the individual separate telescopes        
    interferometer array combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometers        
    International Date Line an arbitrary line on the surface of Earth near longitude 180° across which the date changes by one day        
    interstellar dust tiny solid grains in interstellar space thought to consist of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices        
    interstellar extinction the attenuation or absorption of light by dust in the interstellar medium        
    interstellar medium (ISM) (or interstellar matter) the gas and dust between the stars in a galaxy        
    inverse square law
    (for light) the amount of energy (light) flowing through a given area in a given time decreases in proportion to the square of the distance from the source of energy or light        
    ion an atom that has become electrically charged by the addition or loss of one or more electrons        
    ionization the process by which an atom gains or loses electrons        
    iron meteorite a meteorite composed primarily of iron and nickel        
    irregular galaxy a galaxy without any clear symmetry or pattern; neither a spiral nor an elliptical galaxy        
    isotope any of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons        
    isotropic the same in all directions        
    Kepler’s first law each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse        
    Kepler’s second law the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time        
    Kepler’s third law the square of a planet’s orbital period is directly proportional to the cube of the semimajor axis of its orbit        
    Kuiper belt a region of space beyond Neptune that is dynamically stable (like the asteroid belt); the source region for most short-period comets        
    light curve a graph that displays the time variation of the light from a variable or eclipsing binary star or, more generally, from any other object whose radiation output changes with time        
    lithium the third element in the periodic table; lithium nuclei with three protons and four neutrons were manufactured during the first few minutes of the expansion of the universe        
    Local Bubble, Local Hot Bubble a region of low-density, million degree gas in which the Sun and solar system are currently located        
    Local Fluff a slightly denser cloud inside the Local Bubble, inside which the Sun also lies        
    Local Group a small cluster of galaxies to which our Galaxy belongs        
    luminosity the rate at which a star or other object emits electromagnetic energy into space; the total power output of an object        
    luminosity class a classification of a star according to its luminosity within a given spectral class; our Sun, a G2V star, has luminosity class V, for example        
    lunar eclipse an eclipse of the Moon, in which the Moon moves into the shadow of Earth; lunar eclipses can occur only at the time of full moon        
    magnetosphere the region around a planet in which its intrinsic magnetic field dominates the interplanetary field carried by the solar wind; hence, the region within which charged particles can be trapped by the planetary magnetic field        
    magnitude an older system of measuring the amount of light we receive from a star or other luminous object; the larger the magnitude, the less radiation we receive from the object        
    main sequence a sequence of stars on the Hertzsprung–Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right        
    main-sequence turnoff location in the H–R diagram where stars begin to leave the main sequence        
    major axis the maximum diameter of an ellipse        
    mantle the largest part of Earth’s interior; lies between the crust and the core        
    mare, maria (plural: maria) Latin for “sea;” the name applied to the dark, relatively smooth features that cover 17% of the Moon’s surface        
    mass extinction the sudden disappearance in the fossil record of a large number of species of life, to be replaced by fossils of new species in subsequent layers; mass extinctions are indicators of catastrophic changes in the environment, such as might be produced by a large impact on Earth        
    mass-luminosity relation the observed relation between the masses and luminosities of many (90% of all) stars        
    mass-to-light ratio the ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity; the mass-to-light ratio gives a rough indication of the types of stars contained within a galaxy and whether or not substantial quantities of dark matter are present        
    Maunder Minimum a period during the eighteenth century when the number of sunspots seen throughout the solar cycle was unusually low        
    mean solar time time based on the rotation of Earth; mean solar time passes at a constant rate, unlike apparent solar time        
    merger a collision between galaxies (of roughly comparable size) that combine to form a single new structure        
    meridian a great circle on the terrestrial or celestial sphere that passes through the poles        
    metamorphic rock rock produced by physical and chemical alteration (without melting) under high temperature and pressure        
    meteor a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light        
    meteor a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light        
    meteor shower many meteors appearing to radiate from one point in the sky; produced when Earth passes through a cometary dust stream        
    meteorite a portion of a meteor that survives passage through an atmosphere and strikes the ground        
    microwave electromagnetic radiation of wavelengths from 1 millimeter to 1 meter; longer than infrared but shorter than radio waves        
    Milky Way Galaxy the band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Milky Way Galaxy        
    millisecond pulsar a pulsar that rotates so quickly that it can give off hundreds of pulses per second (and its period is therefore measured in milliseconds)        
    mini-Neptune a planet that is intermediate between the largest terrestrial planet in our solar system (Earth) and the smallest jovian planet (Neptune); generally, mini-Neptunes have sizes between 2.8 and 4 times Earth’s size        
    molecular cloud a large, dense, cold interstellar cloud; because of its size and density, this type of cloud can keep ultraviolet radiation from reaching its interior, where molecules are able to form        
    momentum the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force, momentum is conserved        
    near-Earth asteroid, NEA an Earth-approaching asteroid, one whose orbit could bring it on a collision course with our planet        
    near-Earth object, NEO a comet or asteroid whose path intersects the orbit of Earth        
    nebula a cloud of interstellar gas or dust; the term is most often used for clouds that are seen to glow with visible light or infrared        
    neutrino fundamental particle that has no charge and a mass that is tiny relative to an electron; it rarely interacts with ordinary matter and comes in three different types        
    neutron star a compact object of extremely high density composed almost entirely of neutrons        
    Newton’s first law every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force        
    Newton’s second law the change of motion of a body is proportional to and in the direction of the force acting on it        
    Newton’s third law Newton’s third law
    for every action there is an equal and opposite reaction (or: the mutual actions of two bodies upon each other are always equal and act in opposite directions)
    nova the cataclysmic explosion produced in a binary system, temporarily increasing its luminosity by hundreds to thousands of times        
    nucleosynthesis the building up of heavy elements from lighter ones by nuclear fusion        
    nucleus (of a comet) the solid chunk of ice and dust in the head of a comet        
    nucleus (of an atom) the massive part of an atom, composed mostly of protons and neutrons, and about which the electrons revolve        
    Oort cloud the large spherical region around the Sun from which most “new” comets come; a reservoir of objects with aphelia at about 50,000 AU        
    open cluster a comparatively loose cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of our Galaxy; sometimes referred to as a galactic cluster        
    open universe a model in which the density of the universe is not high enough to bring the expansion of the universe to a halt        
    orbit the path of an object that is in revolution about another object or point        
    orbital period the time it takes an object to travel once around the Sun        
    orbital speed the speed at which an object (usually a planet) orbits around the mass of another object; in the case of a planet, the speed at which each planet moves along its ellipse        
    organic compound a compound containing carbon, especially a complex carbon compound; not necessarily produced by life        
    organic molecule a combination of carbon and other atoms—primarily hydrogen, oxygen, nitrogen, phosphorus, and sulfur—some of which serve as the basis for our biochemistry        
    ozone (\(\ce{O3}\)) a heavy molecule of oxygen that contains three atoms rather than the more normal two        
    parallax an apparent displacement of a nearby star that results from the motion of Earth around the Sun        
    parsec a unit of distance in astronomy, equal to 3.26 light-years; at a distance of 1 parsec, a star has a parallax of 1 arcsecond        
    perigee the point in its orbit where an Earth satellite is closest to Earth        
    perihelion the point in its orbit where a planet (or other orbiting object) is nearest to the Sun        
    period-luminosity relation an empirical relation between the periods and luminosities of certain variable stars        
    perturbation a small disturbing effect on the motion or orbit of a body produced by a third body        
    phases of the Moon the different appearance of light and dark on the Moon as seen from Earth during its monthly cycle, from new moon to full moon and back to new moon        
    photochemistry chemical changes caused by electromagnetic radiation        
    photon a discrete unit (or “packet”) of electromagnetic energy        
    photon decoupling time when radiation began to stream freely through the universe without interacting with matter        
    photosphere the region of the solar (or stellar) atmosphere from which continuous radiation escapes into space        
    photosynthesis a complex sequence of chemical reactions through which some living things can use sunlight to manufacture products that store energy (such as carbohydrates), releasing oxygen as one by-product        
    photosynthesis a complex sequence of chemical reactions through which some living things can use sunlight to manufacture products that store energy (such as carbohydrates), releasing oxygen as one by-product        
    plage a bright region of the solar surface observed in the light of some spectral line        
    planet today, any of the larger objects revolving about the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly among the fixed stars        
    planetary nebula a shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life (the nebulae glow because of the ultra-violet energy of the central star)        
    planetesimals objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary objects we see today; the comets and some asteroids may be leftover planetesimals        
    plasma a hot ionized gas        
    plate tectonics the motion of segments or plates of the outer layer of a planet over the underlying mantle        
    population I star a star containing heavy elements; typically young and found in the disk        
    population II star a star with very low abundance of heavy elements; found throughout the Galaxy        
    positron particle with the same mass as an electron, but positively charged        
    precession of Earth the slow, conical motion of Earth’s axis of rotation caused principally by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge        
    prime focus point in a telescope where the objective lens or primary mirror focuses the light        
    primitive rock rock that has not experienced great heat or pressure and therefore remains representative of the original condensed materials from the solar nebula        
    prominence a large, bright, gaseous feature that appears above the surface of the Sun and extends into the corona        
    proper motion the angular change per year in the direction of a star as seen from the Sun        
    protein a key biological molecule that provides the structure and function of the body’s tissues and organs, and essentially carries out the chemical work of the cell        
    proton-proton chain series of thermonuclear reactions by which nuclei of hydrogen are built up into nuclei of helium        
    protostar a very young star still in the process of formation, before nuclear fusion begins        
    pulsar a variable radio source of small physical size that emits very rapid radio pulses in very regular periods that range from fractions of a second to several seconds; now understood to be a rotating, magnetic neutron star that is energetic enough to produce a detectable beam of radiation and particles        
    pulsating variable star a variable star that pulsates in size and luminosity        
    quasar, quasi-stellar object, QSO an object of very high redshift that looks like a star but is extragalactic and highly luminous; also called a quasi-stellar object, or QSO        
    radar technique of transmitting radio waves to an object and then detecting the radiation that the object reflects back to the transmitter; used to measure the distance to, and motion of, a target object or to form images of it        
    radial velocity motion toward or away from the observer; the component of relative velocity that lies in the line of sight        
    radiation emission of energy as electromagnetic waves or photons also the transmitted energy itself        
    radio waves all electromagnetic waves longer than microwaves, including radar waves and AM radio waves        
    radioactivity process by which certain kinds of atomic nuclei decay naturally, with the spontaneous emission of subatomic particles and gamma rays        
    reddening (interstellar) the reddening of starlight passing through interstellar dust because dust scatters blue light more effectively than red        
    redshift when lines in the spectra are displaced toward longer wavelengths (toward the red end of the visible spectrum)        
    reflecting telescope telescope in which the principal light collector is a concave mirror        
    refracting telescope telescope in which the principal light collector is a lens or system of lenses        
    resolution detail in an image; specifically, the smallest angular (or linear) features that can be distinguished        
    resonance an orbital condition in which one object is subject to periodic gravitational perturbations by another, most commonly arising when two objects orbiting a third have periods of revolution that are simple multiples or fractions of each other        
    retrograde motion the apparent westward motion of a planet on the celestial sphere or with respect to the stars        
    rift zone in geology, a place where the crust is being torn apart by internal forces generally associated with the injection of new material from the mantle and with the slow separation of tectonic plates        
    right ascension the coordinate for measuring the east-west positions of celestial bodies; the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body        
    RNA, ribonucleic acid a molecule that aids in the flow of genetic information from DNA to proteins        
    RR Lyrae one of a class of giant pulsating stars with periods shorter than 1 day, useful for finding distances        
    runaway greenhouse effect the process by which the greenhouse effect, rather than remaining stable or being lessened through intervention, continues to grow at an increasing rate        
    satellite an object that revolves around a planet        
    sedimentary rock rock formed by the deposition and cementing of fine grains of material, such as pieces of igneous rock or the shells of living things        
    seeing unsteadiness of Earth’s atmosphere, which blurs telescopic images; good seeing means the atmosphere is steady        
    seismic wave a vibration that travels through the interior of Earth or any other object; on Earth, these are generally caused by earthquakes        
    selection effect the selection of sample data in a nonrandom way, causing the sample data to be unrepresentative of the entire data set        
    semimajor axis half of the major axis of a conic section, such as an ellipse        
    SETI the search for extraterrestrial intelligence; usually applied to searches for radio signals from other civilizations        
    sidereal day Earth’s rotation period as defined by the positions of the stars in the sky; the time between successive passages of the same star through the meridian        
    sidereal month the period of the Moon’s revolution about Earth measured with respect to the stars        
    singularity the point of zero volume and infinite density to which any object that becomes a black hole must collapse, according to the theory of general relativity        
    solar day Earth’s rotation period as defined by the position of the Sun in the sky; the time between successive passages of the Sun through the meridian        
    solar flare a sudden and temporary outburst of electromagnetic radiation from an extended region of the Sun’s surface        
    solar month the time interval in which the phases repeat—say, from full to full phase        
    solar nebula the cloud of gas and dust from which the solar system formed        
    solar wind a flow of hot, charged particles leaving the Sun        
    space velocity the total (three-dimensional) speed and direction with which an object is moving through space relative to the Sun        
    spacetime system of one time and three space coordinates, with respect to which the time and place of an event can be specified        
    spectral class (or spectral type) the classification of stars according to their temperatures using the characteristics of their spectra; the types are O, B, A, F, G, K, and M with L, T, and Y added recently for cooler star-like objects that recent survey have revealed        
    spectrometer an instrument for obtaining a spectrum; in astronomy, usually attached to a telescope to record the spectrum of a star, galaxy, or other astronomical object        
    spectroscopic binary a binary star in which the components are not resolved but whose binary nature is indicated by periodic variations in radial velocity, indicating orbital motion        
    spiral arm a spiral-shaped region, characterized by relatively dense interstellar material and young stars, that is observed in the disks of spiral galaxies        
    spiral galaxy a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars, winding out from its central bulge        
    starburst a galaxy or merger of multiple galaxies that turns gas into stars much faster than usual        
    Stefan-Boltzmann law a formula from which the rate at which a blackbody radiates energy can be computed; the total rate of energy emission from a unit area of a blackbody is proportional to the fourth power of its absolute temperature: \(F = \sigma T^4\)        
    stellar wind the outflow of gas, sometimes at speeds as high as hundreds of kilometers per second, from a star        
    stony meteorite a meteorite composed mostly of stony material, either primitive or differentiated        
    stony-iron meteorite a type of differentiated meteorite that is a blend of nickel-iron and silicate materials        
    stratosphere the layer of Earth’s atmosphere above the troposphere and below the ionosphere        
    stromatolites s olid, layered rock formations that are thought to be the fossils of oxygen-producing photosynthetic bacteria in rocks that are 3.5 billion years old        
    subduction the sideways and downward movement of the edge of a plate of Earth’s crust into the mantle beneath another plate        
    sunspot large, dark features seen on the surface of the Sun caused by increased magnetic activity        
    sunspot cycle the semiregular 11-year period with which the frequency of sunspots fluctuates        
    super-Earth a planet larger than Earth, generally between 1.4 and 2.8 times the size of our planet        
    supercluster a large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies        
    supermassive black hole the object in the center of most large galaxies that is so massive and compact that light cannot escape from it; the Milky Way’s supermassive black hole contains 4.6 millions of Suns’ worth of mass        
    synchronous rotation when a body (for example, the Moon) rotates at the same rate that it revolves around another body        
    synchrotron radiation the radiation emitted by charged particles being accelerated in magnetic fields and moving at speeds near that of light        
    tail (of a comet) a tail consisting of two parts: the dust tail is made of dust loosened by the sublimation of ice in a comet that is then pushed by photons from the Sun into a curved stream; the ion tail is a stream of ionized particles evaporated from a comet and then swept away from the Sun by the solar wind        
    tectonic geological features that result from stresses and pressures in the crust of a planet; tectonic forces can lead to earthquakes and motion of the crust        
    telescope instrument for collecting visible-light or other electromagnetic radiation        
    terrestrial planet any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list        
    thermophile an organism that can tolerate high temperatures        
    tidal heating the heating of a planet or moon’s interior by variable tidal forces caused by changing gravitational pull from a nearby planet or moon        
    tide, tides alternate rising and falling of sea level caused by the difference in the strength of the Moon’s gravitational pull on different parts of Earth        
    transit when one astronomical object moves in front of another        
    transition region the region in the Sun’s atmosphere where the temperature rises very rapidly from the relatively low temperatures that characterize the chromosphere to the high temperatures of the corona        
    triple-alpha process a nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus        
    troposphere the lowest level of Earth’s atmosphere, where most weather takes place        
    type Ia supernova a supernova formed by the explosion of a white dwarf in a binary system and reach a luminosity of about \(4.5 \times 10^9\) \(L_{\text{Sun}}\); can be used to determine distances to galaxies on a large scale        
    type II supernova a stellar explosion produced at the endpoint of the evolution of stars whose mass exceeds roughly 10 times the mass of the Sun        
    ultraviolet electromagnetic radiation of wavelengths 10 to 400 nanometers; shorter than the shortest visible wavelengths        
    velocity the speed and direction a body is moving—for example, 44 kilometers per second toward the north galactic pole        
    visible light electromagnetic radiation with wavelengths of roughly 400–700 nanometers; visible to the human eye        
    visual binary a binary star in which the two components are telescopically resolved        
    void a region between clusters and superclusters of galaxies that appears relatively empty of galaxies        
    volcano a place where material from a planet’s mantle erupts on its surface        
    wavelength the distance from crest to crest or trough to trough in a wave        
    weakly interacting massive particles, WIMPS weakly interacting massive particles are one of the candidates for the composition of dark matter        
    white dwarf a low-mass star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final state of life        
    Wien’s law formula that relates the temperature of a blackbody to the wavelength at which it emits the greatest intensity of radiation        
    X-rays electromagnetic radiation with wavelengths between 0.01 nanometer and 20 nanometers; intermediate between those of ultraviolet radiation and gamma rays        
    zero-age main sequence a line denoting the main sequence on the H–R diagram for a system of stars that have completed their contraction from interstellar matter and are now deriving all their energy from nuclear reactions, but whose chemical composition has not yet been altered substantially by nuclear reactions        
    zodiac a belt around the sky about 18° wide centered on the ecliptic zenith the point on the celestial sphere opposite the direction of gravity; point directly above the observer year the period of revolution of Earth around the Sun        
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