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G1GLOSSARY

GLOSSARY

aberration of light: A small shift in the apparent direc-tion to an object caused by the motion of Earth andthe finite speed of light. This effect was used by Bradleyin 1727 to make the first historical proof that Earthmoved through space.absorption-line spectrum: The spectrum formed bynarrow regions in a spectrum when a continuous spec-trum passes through a cooler gas.acceleration: The rate of change of velocity with time(the change in velocity divided by the elapsed time).accretion: An aggregation of material that is increasingits mass by gathering and incorporating smaller pieces ofmaterial—either by gravity or by collisions.accretion disk: Material accreted by a larger mass andcaused to spiral into a flattened “disk” shape by theconservation of angular momentum.Active Galactic Nuclei (AGN): One of a class of galaxieswith very energetic behavior observed in the nuclearregion, including high levels of nonthermal continuousemission and unusual radio jets.active optics: Telescopic systems that can adjust theshape and focus of their optical components rapidly,to compensate for distortion in the incoming signal dueto turbulence and other motions in the Earth’s atmo-sphere. Also called adaptive optics.Algol: An Arabic name meaning “Demon Star.” Algol isan eclipsing binary system whose variations in brightnesscan be seen by the naked eye.Allende meteorite: A meteorite with unusual isotopicratios suggesting that a supernova may have played a partin forming the early solar system.Almagest: The Arabic title of Ptolemy’s magnum opus.almanac: A table giving the position at various times ofa celestial body. Also called an ephemeris.alpha-capture reactions: Nuclear reactions in whichalpha particles are added to a heavier nucleus.Alpha Centauri: A bright triple-star system close tothe Sun.alpha particle: The nucleus of a helium atom, composedof two protons and two neutrons.altitude: The angular distance of an object above thehorizon.amino acids: Complex molecules that form the basicbuilding blocks of protein molecules. Conditions inside theinterstellar clouds are favorable for the formation of thesemolecules.

Anaximander: A member of the Pythagorean school,credited with early theories on the composition andmotion of the universe.Andromeda Galaxy: (also Andromeda Nebula and M31).This diffuse, naked-eye object is revealed by telescopicobservations to be a giant, nearby spiral galaxy nearly 3million light-years away.angle of incidence: The angle between an incident rayof light and a perpendicular to the surface at the point ofincidence.angle of reflection: The angle between a reflected rayof light and the perpendicular to the surface at the point ofreflection.angstrom: (abbreviated Å) A unit for measuring thewavelengths of light. It is 10–8 cm. On this scale blue lightis around 4500 Å and red light at 6500 Å.angular momentum: A measure of the amount of spinor orbital motion possessed by a body due to its inertia.Physicists describe it as a “conserved” property. Seeconservation.angular size: The angle at the eye subtended by raysdrawn from the outside of an object. Same as apparent size.annihilation: The total conversion of mass to energybrought about by the collision of matter with anti-matter.annular eclipse: An eclipse of the Sun in which theangular size of the Moon’s disk is not quite large enoughto block the solar photosphere. In an annular eclipse, theSun is not dimmed enough to allow the corona to be seen.antielectron: The antiparticle of an electron.antimatter: Material with charge and various otherproperties reversed when compared with normal matter.When matter and antimatter collide, they annihilate eachother.Ap (A-peculiar stars): A-type stars that have someanomalies in their chemical composition when comparedwith normal A-stars.aphelion: The greatest distance from the Sun reached byan object orbiting the Sun. See also perihelion.

apogee: The most distant point in the orbit of an Earth-orbiting body. See also perigee.Apollo asteroids: A group of asteroids with orbits thatcome close to the Earth.Apollonius of Perga: Early Greek credited with thefirst suggestion of epicycles and deferents as a device toexplain the motions of celestial bodies.

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G2GLOSSARY

apparent magnitude: The brightnesses of celestialbodies measured on a scale first suggested by Hipparchus.In this system (which turns out to be logarithmic), fainterstars are characterized by larger numbers. Two starsthat differ by one magnitude differ by a factor of 2.512 intheir brightness ratio. A difference of 5 magnitudescorresponds to a factor of 100 in brightness.apparent size: See angular size.apparent solar time: Also called sundial time. The hoursof the day as measured by the apparent progress of theSun across the sky.Ariel: One of the larger moons of Uranus.Aristarchus: An early Greek astronomer who carried outestimates of the relative sizes and distances of the Earth,Moon, and Sun and espoused a sun-centered solar systemmodel.Aristotle: The most influential of Greek philosophers.He produced enduring studies in biology, geometry, andlogic. His work in physical science and astronomy,however, was less successful.asteroid: A body whose diameter ranges from that ofa grain of sand up to that of Texas. Asteroids populate theregion between Mars and Jupiter. Also called minorplanets.asteroid belt: A region of asteroids located between theorbits of Mars and Jupiter.asthenosphere: A region of soft material in the Earth’supper mantle.Astronomical Unit (AU): The average distance fromEarth to the Sun—about 150 million km (or 93 million mi).asymptotic giant branch (AGB): A region in the H-Rdiagram of a globular cluster. It is a post-main-sequenceevolutionary track followed by stars having both an H- andan He-burning shell.atmosphere: The gaseous outer regions of a planet ora star.atmospheric extinction: A reduction in the apparentbrightness of the light from a celestial object due to theabsorption and scattering of light out of the incomingbeam of energy by the Earth’s atmosphere.atmospheric windows: Spectral regions of the atmo-sphere that are largely or partly transparent to radiation.atom: The smallest subdivision of matter (the smallestpiece) that still retains the chemical properties of theelement.atomic number: The number of protons in the nucleusof an element.atomic weight: A scale of the relative weights of theatoms of different chemical elements. For an individualatom, the atomic weight would be the atomic mass num-ber—the number of protons plus neutrons in the nucleus.For example, the most frequently occurring form ofcarbon has a nucleus with 6 protons and 6 neutrons andan atomic weight of 12. There is a less frequently occur-

ring form of carbon with 6 protons and 7 neutrons and anatomic weight 13. The “average” atomic weight for carbondepends upon the frequency of occurrence in nature of thedifferent types of carbon.AU: Abbreviation for Astronomical Unit.aurora: High-energy charged particles from space impactupon and excite atmospheric molecules causing them toradiate. This radiation is more commonly seen near themagnetic poles, since that is where the magnetic field linestend to direct the incoming particles. The aurora borealisis seen around the north magnetic pole, and the auroraaustralis is seen in south magnetic regions.auroral ring: Auroral radiation occurring in a ring-shapedregion around the magnetic pole.autumnal equinox: The moment when the Sun crossesthe celestial equator from north to south, marking thebeginning of fall. See also equinox.average: From a series of measurements, the value mostlikely to be closest to the true value. It helps to negate theeffects of high and low values.axis: A line about which a body rotates.azimuth: Angular distance along the horizon of a celestialobject, measured clockwise (eastward) from north.background 3 K radiation: Faint radio static with ahighly isotropic distribution, thought to be remnantradiation from the early stages after the big bang.Balmer lines: A set of spectral lines due to transitions ofthe hydrogen atom between level 2 and higher energylevels.barium stars: Red giant stars showing abnormally largeabundances of barium and other chemical elements notnormally found in most stars.Barnard’s star: A star close to the Sun with “wiggles” inits motion, suggesting the possibility of other masses (inparticular, planets) orbiting the star.barred spirals: Galaxies with a “bar” of stars runningthrough the nucleus. The spiral arms appear to start fromthe bar.Barringer Crater: A one-mile diameter crater in Arizonacreated by meteoric impact.basalt: Igneous rock (produced at high temperatures)that is common in the outer parts of the Earth and theMoon.basin: Large, shallow depression seen on terrestrialplanets, created by impact or plate tectonics.Bayeux tapestry: Woven at the time of the Battle ofHastings, it shows a representation of the 1066 C.E.appearance of Halley’s comet.B.C.E.: Before the Common Era, which coincides with thetime before Christian era. The meaning is the same as B.C.Becklin-Neugebauer source (B-N source): A brightpoint source of infrared emission seen in the OrionNebula. It is probably either a new star or a new clusterof stars in the process of formation.

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G3GLOSSARY

Beta Lyrae: A star whose rapid rotation changes theshape of its spectral line profiles.Beta Persei: See Algol.Beta Pictoris: A star circled by a disk of dusty materialthat may form planets.big bang: A model that postulates the origin of the uni-verse in a state of enormous temperature and density (seeprimeval fireball) that produced the expanding universewe observe today.big crunch: What results when an expanding universereverses direction, so that density and temperature againincrease.Bighorn medicine wheel: A construction of rockshigh in the Wyoming Big Horn Mountains that may havebeen constructed by Plains Indians for astronomical,calendrical, and ritualistic purposes.binary accretion model: Postulates that the formationof the Earth-Moon system occurred by accretion of bothbodies from the same dust and gas cloud.binary pulsar: A highly evolved binary pair of stars inwhich one of the two has become a pulsar.binary star: Two stars in mutual orbit under the attrac-tion of gravitational forces.binary X-ray source: A highly evolved binary pair ofstars in which one star has become an X-ray-emitting,collapsed object such as a white dwarf, neutron star, orblack hole. The X-rays are thought to be emitted by ahot accretion disk in the system.bipolar flow: Gas flowing outward in opposite directionsseen in objects that may be Young Stellar Objects (YSOs).blackbody: A conceptualized “perfect radiator,” which isfound useful in theoretical studies of radiation. A black-body is one that absorbs 100% of the radiation it encoun-ters. It re-emits that energy in a spectral pattern thatdepends only upon temperature.blackbody spectrum: A continuous distribution ofenergy given off by a blackbody. The amount of energyemitted at each wavelength by a blackbody is given byPlanck’s Law.black holes: Collapsed objects whose gravitational fieldis so intense that not even light can escape from them.They cannot be observed directly, but their effect upontheir environment can be studied.blink microscope (blink comparator): An instrumentthat projects two different photos of the same area of thesky in rapid succession. Stars with brightness variationswill be seen to “blink” through the viewer.BL Lac objects: Active galaxies that show rapidlyvarying, highly polarized non-thermal emission fromtheir nuclei.Bohr model of the atom: An early “solar system”model for the atom. Negatively charged electrons wereenvisioned as orbiting the massive and positively chargednucleus of the atom. Only certain orbits were allowed.

Bok globules: Small dark regions seen in interstellarclouds, many of which are observed to surround a youngstar.bolide: A meteor that explodes in the Earth’s atmosphere.bottom-up hypothesis: A model of galaxy clusterformation in which galaxies form before clustering itselfoccurs.bow shock: A shock wave of pressure and densitycreated by material moving at supersonic speed througha medium. Sonic booms are a terrestrial example.Brackett series: A set of spectral lines due to transitionsof the hydrogen atom between level 3 and higher energylevels.Brahe, Tycho: Sixteenth-century astronomer famousfor his accurate naked-eye observations. Considered to bethe father of modern observational astronomy. Collabo-rated with Johannes Kepler to complete the CopernicanRevolution.braided F-ring: One of Saturn’s rings that shows time-variable shape and behavior.breccia: A rock composed of different types of mineralfragments pressed together.bright-line spectrum: Same as emission-line spectrum.brown dwarf: A low-mass object that is larger than atypical planet but that cannot have hydrogen fusion in itscore. As a consequence, it is an “almost-star” with lowtemperature and luminosity.burster: Sporadic outbursts of high-intensity X-rayemission. The sources are still a matter for argument.butterfly diagram: The pattern created when the latitudeat which sunspots form are graphed as a function of time.Callisto: The outermost Galilean satellite.Caloris Basin: A large multi-ring basin on Mercury.canali (“canals”): Long, dark features seen on Mars bysome astronomers. Schiaparelli called them canali, whichis Italian for channels. Other people made the transitionto “canals” in their hypothesizing and went on to postulatelife and modern civilizations on Mars.capture model: Imagines the Moon to have beencaptured by the Earth’s gravity.carbonaceous chondrites: A group of meteorites with arelatively high carbon abundance and chondrules. Seechondrules.Cas A: A strong radio source that is the remnant of asupernova explosion that took place in 1667.Cassegrain: A telescope arrangement that uses a second-ary mirror and a small hole in the primary mirror to focuslight behind the primary mirror.Cassini division: A gap in Saturn’s rings named after itsdiscoverer—Giovanni Cassini.cataclysmic variables: Stars that become unstable andviolent in their final evolutionary stages.

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CCD (charge coupled device): A two-dimensionalsemiconductor chip that is used as an extremely sensitivephoton detector.C.E.: The Common Era, which coinsides with the Christianera. The meaning is the same as A.D.celestial equator: This great circle on the celestialsphere is the projection of the Earth’s equator onto theplane of the sky. It is halfway between the celestial poles.celestial pole: The projection onto the celestial sphereof the spin axis of the Earth. The North Celestial Pole(NCP) is currently located near the star Polaris, while theSCP is not near a bright star. The long, slow motion ofthe Earth called precession will cause these two celestialpoles to slowly move with respect to the stars over a26,000-year cycle.celestial sphere: The “Bowl of the Sky.” To the ancients,this is the apparent sphere of sky encircling the Earth onwhich the stars appear to be fixed.Cen A: A strong radio source resembling an ellipticalgalaxy but with large regions of dust obscuration.center of mass: The imaginary point on the sky aboutwhich two gravitationally bound bodies will orbit.centripetal force or acceleration: A force or accelera-tion directed towards the center of a circular path.Cen X-3: A powerful X-ray source in Centaurus composedof a giant star orbited by an X-ray-emitting neutron star.Cepheid instability strip: The region of the H-R diagramwhere Cepheid variable stars and Cepheid-like variablesare located.Cepheid variables: Luminous giant stars whose energyoutput varies in a regular and recognizable manner. Theyare important for determining the distances to nearbygalaxies.Ceres: The first discovered and largest asteroid.Chandrasekhar limit: The largest mass that theory willpermit a white dwarf to acquire, roughly 1.4 solar masses.Charon: Pluto’s moon.Chiron: An object discovered in 1977 in an orbit that liesmostly between Saturn and Uranus. Initially classifiedas an asteroid but later exhibited behavior more like thenucleus of a comet.chondrites: See carbonaceous chondrites.chondrules: Silicate nodules found in a particular class ofmeteorites that may be a good sample of primeval solarsystem composition.chromatic aberration: A defect of telescopes made withlenses. Different colors of light are refracted (bent) bydifferent amounts, so that they are brought to differentfocal points.chromosphere: The layer of the Sun’s atmosphere justabove the photosphere where most of the absorption linesare formed.circumpolar stars: Those stars that never go below thehorizon in their 24-hour daily motion because of theirproximity to the celestial pole.

Circus Maximus: A surface feature on Uranus’s satellite,Miranda.closed universe: A universe that possesses enough massthat the curvature of space would be positive, and theuniverse would be finite.CNO cycle: A cycle of nuclear reactions in which H fusionproduces He through reactions that use C nuclei as acatalyst.COBE: COsmic Background Explorer satellite.collisional dissociation: The breakup of a moleculethrough a collision.collisional ionization: An atom or an ion loses anelectron due to an energetic collision.coma: The bright diffuse halo of a comet. Also calledthe head.comets: Small (a few miles in size) solar system bodiescomposed of dust and frozen gases that evaporate andglow as they near and go around the Sun. Some cometshave regular and predictable reappearances, whereasothers enter the solar system without warning and fromunpredictable directions.comparative planetology: An approach to the study ofplanetary bodies, having the goal of understanding theirsimilarities and differences.conduction: The transfer of heat by particle (oftenelectron) collisions.conjunction: Two bodies getting as close to each otherin the sky as possible.conservation: A conserved quantity is one whosenumerical value stays constant in a system with noexternal forces present. Examples are linear and angularmomentum, and energy. The momentum can be passedbetween bodies but it does not disappear.constellations: Arbitrary groupings of stars that havepassed down to us from ancient times. Different cultureson Earth historically evolved different constellations andmyths related to them. For convenience, astronomers haveadopted the ancient Greek constellations and given themwell-defined boundaries.constructive interference: When two similar wavesencounter each other and interact in such a way as tocombine their strength. See also destructive interference.

continental drift: A slow movement of Earth’s greatland masses that has produced the present continentswe now see from the breakup and drifting apart of onegiant landmass.continental plates: The large-scale, surface crust layersof Earth.continuous creation of matter: A hypothesis that triesto explain the expansion of the universe as being due tothe pressure caused by new matter that is hypothesizedto be constantly and spontaneously appearing. See alsosteady state hypothesis.

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continuous spectrum: A spectrum in which energy isemitted at all wavelengths. With the naked eye, we seea “rainbow” of color. It is distinct from the absorption- andemission-line spectrum.convection: The transfer of heat energy by the move-ments of hot material.co-orbital satellites: Satellites inhabiting the same orbits.Examples occur with Saturn’s satellites Dione and Tethys.Copernican hypothesis: The hypothesis put forth byCopernicus in 1543 that the planets all orbited around theSun.Copernicus satellite: An orbiting telescope for observingstars in the ultraviolet spectral region.Cordillera Mountains: Lunar mountains forming themulti-ringed Orientale Basin.coriolis effects: Observed effects due to living on arotating frame of reference. Two examples would be (1)the deflections exhibited by artillery shells, and (2) thecirculations of winds on the Earth.corona: The Sun’s tenuous outer atmosphere, which isvisible only at the time of a total solar eclipse. It has atemperature near 2 million degrees.coronagraph: A telescopic device to block most of thelight from the bright surface of the Sun and reveal itsfainter, outer layers without an eclipse.coronal holes: Regions of the Sun’s corona where strongloops of magnetic field produce a lower than normaldensity in the hot coronal plasma.cosmic abundance: Most of the universe exhibits achemical abundance that is (by number of atoms) roughly90% hydrogen, almost 10% helium, and less than 1 or 2%of all the other elements. The planets form an exception tothis generalization.cosmic background radiation: See background 3 Kradiation.cosmic rays: Particles from space moving at close to thespeed of light, impinging on the Earth with tremendousenergies from all directions in space.cosmological constant: A term added by Einstein tohis equations of general relativity to force the mathematicsto provide a static solution rather than a collapsing one.cosmological distance: If an object’s distance is sogreat that the effects of general relativity are important,the object is said to be at a cosmological distance.cosmological redshift: A redshift caused by the expan-sion of space rather than by relative motion.cosmology: The study of the structure and evolution ofthe universe.coudé: A focal arrangement for a telescope that sendsthe light down the polar axis into an observing room. Thisis the longest possible focal length for a telescope andproduces the largest possible image.

Crab Nebula: An energetic gas cloud with a pulsar inthe center, which has been found to be the remains leftover from the spectacular 1054 C.E. supernova explosion inTaurus.crater: A depression in a planetary or satellite surface,created by the impact of a fast-moving body or byvolcanism.crater chains: A line of craters that would be unlikely tobe produced by impact. In many cases, they provideevidence for some sort of “volcanic” activity. Some arecaused by secondary impacts.critical density: The particular value for the density ofmatter in the universe that would just bring the observedexpansion of the universe to a halt.critical mass: The mass of material required to reachthe critical density.curved space: According to Einstein’s general theory ofrelativity, the distribution of mass in the universe deter-mines the curvature of space. As bodies move, they mustfollow the local curvature of space, which will not neces-sarily be the familiar three dimensions of Euclidiangeometry.Cyg A: A powerful source of synchrotron radiation inCygnus that may be two galaxies colliding with each other.Cygnus X-1, X-3: X-ray sources in Cygnus that probablyconsist of a collapsed object emitting X-rays and orbitinga giant star.dark cloud: An interstellar cloud of dust and gas that isdense enough to be opaque.dark-line spectrum: See absorption-line spectrum.dark matter: Not yet detected or explained, the existenceof this material is revealed by the rapid accelerations ofgalaxies seen in clusters of galaxies. The amount of thismaterial will be critical in determining the future fate ofthe universe.Death Star: See Nemesis.declination: The angular distance of a celestial objectnorth or south of the celestial equator.decoupling (of radiation and matter): The epoch earlyin the life of the universe when the matter becomes largelytransparent to the radiation in the universe.deductive reasoning (deduction): Inference from ageneral statement to a specific prediction.deferent: In the ancient geocentric theory of the universe,this is a large circle approximately centered upon theEarth, along which a smaller circle, the epicycle, wouldmove.degenerate electron gas: Electrons compressed into astate of density so high that the ordinary gas laws breakdown. In the degenerate state, a change of temperaturedoes not result in a change of pressure.Deimos: One of the two small moons of Mars (the otheris Phobos).

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density: The amount of mass in a unit volume; massdivided by the volume (mass per unit volume).density wave: A giant (galaxy-sized), spiral-shaped com-pression wave that may create spiral arm structure bycompressing giant gas clouds and causing the onset ofstar formation.destructive interference: If two similar interacting wavesare out of phase with each other, the result will be amutual cancellation.deuterium: An isotope of hydrogen having one neutron.It is useful in age-dating celestial objects.diffraction: The bending or spreading-out of waves whenthey pass through apertures or move close to edges ofobjects and obstacles.diffraction grating: A surface that is marked withthousands of fine lines per inch. These lines diffract lightas if they were many single slits working in unison,causing a spectrum to form.diffuse nebula: A gas cloud excited to radiate byabsorbing the ultraviolet emission from a hot star in ornear the gas cloud. The gas is ionized, and as it recom-bines, it emits a bright-line spectrum.dipole anisotropy: An apparent anisotropy in the 3 Kmicrowave background radiation caused by the motion ofEarth.dirty snowball; dirty iceberg: A model for the nucleusof a comet that envisions its structure to be a frozen ballof gas, dust, and rocks that tends to break down when thecomet nears the Sun. The evaporated material forms thetail of the comet.discrete emission: Energy given off only at certainspecific wavelengths.dispersion: The separation of light into its componentcolors by the action of a prism or a grating.dissociation: Breaking a molecule apart either by theabsorption of a photon or a collision with something.distance pyramid: A diagram showing how the methodsfor determining distances to distant objects depend on thedistances to nearby ones.Doppler effect: A shift in the observed wavelength ofradiation caused by radial motion of the source of lightwith respect to the observer.double-lined spectroscopic binary: A spectroscopicbinary in which absorption lines from both stars arevisible. Such systems provide vital information on thestellar masses.double quasar: An optical illusion produced by a stronggravitational field in space. The light from a quasar is bentby the field, producing an extra, spurious image.double shell source: When fusion occurs in two shellssurrounding a star’s nucleus.DQ Herculus: The first stellar nova to be shown to bea binary star system.

dredge-up: Products of fusion reactions deep inside astar, sometimes carried up closer to the surface of thestar by convective currents, changing the chemicalcomposition of the material and affecting the reactionsthat are possible.dwarf: A small, low-luminosity star. Sometimes used todescribe main-sequence stars of luminosity class V.dwarf elliptical: Small elliptical galaxies containing insome cases as few as a million stars.dynamo model: A model that postulates the generationof magnetic fields in planets and other objects by the circu-lation of conducting fluids inside the object.eccentricity: The deviation from circular shape (or“flattening”) of an ellipse.eclipse: When one celestial body passes in front ofanother, blocking its light as seen from the Earth.eclipsing binary: A binary system where the plane ofthe orbit is nearly in our line of sight, causing each star toperiodically pass in front of the other, blocking its light.The light curve of the system will exhibit dips at regularintervals.ecliptic: The apparent path of the Sun on the celestialsphere with respect to the “fixed stars.”Einstein observatory: An X-ray telescope that orbitedoutside the atmosphere from 1978 to 1981.ejecta blanket: Material excavated by an impact explo-sion and deposited over the surface.electromagnetic radiation: Energy in the form of a wave,propagating through space at the speed of light.electromagnetic waves: See electromagnetic radiation.

electron: A small, negatively charged particle. In theBohr model of the atom, the electrons of an element orbitthe positively charged, massive nucleus of the atom.electroweak force: At the high energies permeatingthe universe in the initial moments after the big bang, theelectromagnetic force and the weak nuclear force wereinitially combined into one force, the electroweak force.ellipse: A geometric curve in the shape of a “squashed”circle. The shape is determined by the eccentricity. Seeeccentricity.elliptical orbit: Bodies that are gravitationally bound toother bodies and moving in closed orbits will exhibit anorbital path that is an ellipse.elongation: The angular distance between a planet andthe Sun as viewed from the Earth.emission-line spectrum: Energy emitted at specific,discrete wavelengths by transparent gases. Each chemicalelement has its own pattern of emission lines.emission nebula: A rarified gas cloud excited to radiateby absorbing radiaton from a nearby hot star.Enceladus: A satellite of Saturn that may be significantlyaffected by tidal friction.

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encounter theory: Proposed explanations of the originof the solar system in which a collision or near approach ofanother star to the Sun gravitationally pulls off somematerial from the Sun. This material is hypothesized tocondense and form planets.energy level diagram: A graphical display of the energylevels of an atom.energy levels: A specific amount of energy associatedwith each possible electron orbit. Each chemical elementhas its own set of stable energy levels.epicenter: A point on the Earth’s surface from whichseismic waves appear to radiate, located directly above thetrue center of the earthquake disturbance.epicycle: In the ancient Ptolemaic (geocentric) theory ofthe universe, the epicycle is a small circle that movesaround the deferent, while the planet moves around theepicycle. This arrangement can produce the retrogrademotion shown by planets.equatorial bulge: The equatorial diameter of the Earthis slightly larger than the polar diameter.equatorial coordinate system: A coordinate grid on thesky analogous to latitude and longitude on the Earth. Thenorth-south position of an object is called its declination,while the east-west location is called its right ascension.equatorial mounting: A mount for a telescope thatenables it to follow the motion of a celestial object acrossthe sky. Most mounts will feature a polar axis that pointstoward the NCP.equinox: Two points on the celestial sphere where theecliptic plane and the celestial equator intersect. Thespring (vernal) equinox occurs near March 21 when theSun is crossing the celestial equator going from south tonorth. The autumnal equinox, at about September 21,occurs when the Sun crosses the celestial equator goingfrom north to south. At these times, day and night are ofequal length everywhere on the Earth (hence the name,meaning “equal night”).Eratosthenes: An ancient Greek astronomer whoproduced an accurate estimate of the size of the Earth bycomparing shadows at two locations on Earth.erg: A tiny unit of energy.Eros: An Apollo asteroid that passes close to the Earth.Measurements of its parallax gave us our first accuratedeterminations of the scale of the orbits in the solarsystem.escape velocity: If an orbiting body possesses a speedgreater than this velocity, then it will escape from the bodyit is orbiting. Such orbiting bodies will move in parabolicor hyperbolic paths.Europa: The smallest of the Galilean satellites of Jupiter.event horizon (Schwarzschild radius): A criticaldistance characterizing a gravitating mass collapsing intoa black hole. When the object shrinks beyond this criticaldistance, it is no longer visible to us, and light and otherinformation is no longer able to escape from within theobject itself.

evolutionary track: The path traced out with time on anH-R diagram by an evolving star, which is changing itstemperature and luminosity.excitation: The process of exciting an atom to a higherenergy level, either by absorbing a photon (photo-excitation) or by collisions (collisional excitation).excited state: An energy level (state) other than theground state.expansion of the universe: Everything in the universe isobserved to be moving away from everything else.extinction: The dimming of light observed when it passesthrough a medium. The interstellar medium causesdimming and reddening of starlight passing through it,as does the atmosphere of the Earth.extrasolar planet: a planet around a star other than theSun.faint object camera; faint object spectrograph:Two instrument systems of the Hubble Space Telescope.fireball: An unusually bright meteor.fission: The nucleus of a heavy atom splitting apart andforming other, lighter nuclei (and giving off energy in theprocess).fission model: A theory of the origin of the Moon, whichproposes that the material for the Moon was thrownoutward by a rapidly spinning Earth.five-minute oscillation: An oscillation (expansion andcontraction) of the outer layers of the Sun.flare: Unpredictable energetic outbursts seen in somestars and on the surface of the Sun.flash spectrum: The instant before a total solar eclipsewhen the dark-line spectrum is replaced by a spectrum ofcurved emission lines from the hot gases above theobservable surface.flat space: Ordinary three-dimensional Euclidean space.flatness problem: The cosmological problem of explain-ing why the universe appears to be so flat, that is, why thedensity of the universe is so close to the critical density.fluorescence: A radiation process whereby an atom ormolecule absorbs a photon and then re-emits the energy atlonger wavelengths.focal length: The distance from the primary mirror orlens at which the light from a distant source is brought toa focus.focus: The point in space where the light gathered bytelescope is formed into a clear image.forbidden lines: Certain emission lines that are only seenin very low-density gases such as astronomical nebulae.These spectral features are not normally seen on Earthbecause of the higher densities (hence the misnomer“forbidden”).frequency: The number of waves passing the observerper second. The frequency of electromagnetic waves ismeasured in Hertz, which is the number of cycles persecond.

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full Moon: When the Moon is geometrically opposite tothe Sun in the sky, Earth observers can see the disk ofthe Moon fully illuminated.FU Orionis star: A small group of stars observed toincrease brightness strongly in a short time. They areobjects associated with star formation.fusion: Light nuclei merging together to form a heaviernucleus (and give off energy).galactic cannibalism: Smaller galaxies consumed bycolliding with or being drawn into larger galaxies. Thisprocess may cause changes in the shape and form ofthe larger galaxy and change its evolutionary pattern.galactic cluster: Small (a few hundred stars at most)groups of associated stars found in the galactic plane.They have formed from gas clouds in the plane andexhibit a variety of ages, from billions of years to recentlyformed.galactic corona (galactic halo): State-of-the-artimaging of many galaxies reveals the existence of exten-sive, spherical regions of emission surrounding the bright,obvious portions of galaxies. The Milky Way appears tohave such a region. Depending upon physical conditions,these regions could contain large amounts of mass.galactic equator: The great circle on the sky that tracesout the fundamental plane of the Milky Way.galactic plane (galactic disk): The flattened, pancake-shaped distribution of gas, dust, and young stars in ourgalaxy.galactic rotation curve: A graph of the speed of the starsand other material versus their distance from the centerof the galaxy.galaxy: Giant aggregates of stars, gas, and dust in avariety of forms. They range in size from objects that con-tain perhaps a million solar masses worth of material upto perhaps a trillion stars.galaxy cluster: Clusters of galaxies, ranging from smallgroups of only a few up to thousands of galaxies.Galilean satellites: The four brightest, largest satellitesof Jupiter.gamma ray: Extremely high-energy photons with wave-lengths on the order of the size of an atom (approximately1 Å).gamma-ray burster: An object that suddenly emits ahuge amount of gamma-ray energy in a short burst.Ganymede: The largest of the Galilean satellites.gas (ion) tail: A comet tail consisting of ions. Its struc-ture and shape are determined by the material’s interac-tion with the solar wind.gegenschein: A small region of diffuse light, located180 degrees away from the Sun in the sky, caused bysunlight reflected from dust particles in the ecliptic plane.Geminga: A powerful source of high-energy gamma raysand pulsed X-rays.

general theory of relativity: Einstein’s theory ofgravity. General relativity is used to describe the motionof bodies in the presence of a strong mass, which warpsthe surrounding space and time, thus giving space acurvature.geocentric theory: Early theories that imagined theEarth to be the unmoving center of the universe, and thatthe stars and planets revolved around the Earth.geodesic: The shortest distance between two points.A generalization of the concept of a straight line.geomagnetic axis: The line connecting the two magneticpoles on Earth.giant elliptical galaxies: Huge extragalactic aggregatesof stars that present an elliptical distribution of light. Thelargest of these may contain many trillions of stars.giant impact theory: A theory for the formation of theMoon that postulates a collision between the young Earthand a Mars-sized object. Material from the Earth andthe passing object combine to form a disk, which thencondenses into the Moon.giant molecular cloud: Aggregations of molecules intoclouds of more than 100,000 solar masses of material,these objects are the largest discrete entities in the Galaxy.They are thought to be the site of star formation.giant star: After leaving the main sequence, normal starsundergo changes in their physical state that cause themto swell in size, to sizes on the order of 10 solar radii. Seealso supergiant stars.gibbous: The phase between the quarter phases and fullin which the Moon’s surface as seen from Earth is morethan half illuminated.Global Oscillation Network Group (GONG): Aworldwide network of solar telescopes for continuousmonitoring of the Sun.globular cluster: A star cluster, often containing morethan 100,000 stars. Within the Milky Way, they orbit inelliptical orbits and have ages in excess of 10 billion years.globules: See Bok globules.gnomon: A stick or rod held perpendicular to Earth’ssurface for the purpose of casting shadows and determin-ing time.Grand Unified Theory: A theory that attempts to unifythe four forces of nature as different manifestations of asingle force.granulation: The cellular appearance of the solar photo-sphere, brought about by a boiling motion in which brightblobs of hot material are rising and cooler, dark materialis sinking back into the Sun.gravitation: The mutual attraction between any twobodies possessing mass. In nonrelativistic environments,the size of the attraction and the accelerations of thebodies involved can be calculated from Newton’s Law ofGravity.gravitational instability: A physical condition wherematerial is on the verge of gravitational collapse.

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gravitational lens: An object whose mass is sufficient tobend radiation from background objects and form imagesof the background object.gravitational potential energy: The energy residing ingravitating masses that can be called upon to do work.gravitational radiation (gravitational waves): Wavespredicted by the general theory of relativity that shouldbe emitted when large masses accelerate in stronggravitational fields.gravitational redshift: A redshift caused by a photonlosing energy when escaping from a large gravitationalfield.Great circle: The largest circle one can trace on a sphere.Its center is at the center of the sphere.Great Dark Spot: An atmospheric phenomenon observedin the atmosphere of Neptune.Great Red Spot: A cyclonic storm in the upper atmo-sphere of Jupiter that has been observed for severalhundred years.Great Wall: In a figure of the distribution of galaxies withdistance, an extensive alignment of galaxies appearing asa wall separating the nearby universe from more distantregions.greatest (eastern or western) elongation: When theangular distance of Mercury or Venus from the Sun (asseen from Earth) achieves its maximum value.greenhouse effect: The trapping of radiation near thesurface of a planet due to the absorbing properties of theoverlying atmosphere.ground state: The lowest energy level of an atom.GUT: See Grand Unified Theory.

half-life: The time it takes for one-half of a quantity ofradioactive material to decay.Halley’s comet: A comet that approaches the Sun every75–76 years. It has been observed repeatedly since ancientChinese astronomers recorded it.halo: A spherical region around the galaxy containingglobular clusters and some stars. There may be otherconstituents of which we are unaware.H alpha (H�): The red line of the hydrogen Balmerseries, located at 6563Å.head-tail galaxy: A radio galaxy moving through interga-lactic space will have its outer portions (where the radioemission comes from) swept backwards into the shape ofa tail.heat: The thermal energy of an object—the sum totalenergy of the motions of all of its particles.Hebrew calendar: A lunar calendar that requires theinsertion of additional months from time to time to keeppace with the solar calendar.heliocentric theory: A model of the solar system with theSun at the center and the planets (including the Earth)going around it. In ancient Greece, this hypothesis was putforward by Aristarchus.

helioseismology: The study of the interior of the Sun asrevealed by vibrations (oscillations) seen at its surface.helium burning: The fusion of helium into carbon viathe triple-alpha process.helium flash: An explosive episode of helium burningtaking place in the degenerate core of a low-mass star.Herbig-Haro objects: Emission regions observed tomove away from young stellar objects in a bipolar flow.hertz: A unit of frequency for electromagnetic waves,equal to one cycle per second.Hertzsprung-Russell diagram: See H-R diagram.Hidalgo: An asteroid with an elongated orbit that isinclined 40° to the ecliptic plane.highlands: On the Moon, the regions of higher elevationthat were not covered by the lava flows that created themaria.high-velocity stars: Stars that show unusually largevelocities relative to the Sun. They are part of the galactichalo and not moving in the plane of the galaxy as is theSun.Hipparchus: A Greek astronomer with many achieve-ments: the first star catalogue, the magnitude scale,discovery of precession.HIPPARCOS: A satellite that measured stellar distances.Hirayama asteroid families: Groups of asteroids withhighly similar orbits.H I region: Regions and gas clouds primarily composedof neutral atomic hydrogen.H II region: Regions and gas clouds primarily composedof ionized hydrogen, symbolized by H II. The clouds ofionized hydrogen are produced by nearby, high tempera-ture O and B stars.horizon: The great circle that represents the intersectionof the Earth and the sky.horizon problem: The cosmological problem of explain-ing why the microwave background is so uniform, giventhat the size of the universe at the time the backgroundradiation was formed was larger than the light travel timeacross it. In other words, why is the background radiationso uniform, given that two separate regions emitting itare too far apart for information to be transferred betweenthem.horizontal branch: A region of the H-R diagram ofglobular clusters. It consists of evolved low-mass starsburning helium in the core and hydrogen in a shell.H-R diagram: A graph of the luminosity of stars plottedagainst their surface temperatures (or spectral types, orcolor). The many forms of the diagram are often calledcollectively by the initials of its codiscoverers,Hertzsprung and Russell.H2: Molecular hydrogen—two H atoms bound together.Hubble constant: The constant H in the mathematicalexpression of Hubble’s law, V = Hd. See also Hubble law.

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Hubble Deep Field: Two areas of the sky (one north,the other south) that the Hubble Space Telescopeobserved for about 100 hours to observe fainter thanpreviously done.Hubble law: Describes the expansion of the universe inwhich we see that the speed of recession of a galaxy isproportional to its distance from us.Hubble Space Telescope: The first complete observa-tory in space, equipped with all the instrumentationneeded to observe celestial objects in many different ways.Hyades: A nearby, open cluster of stars in the constella-tion Taurus.hydrogen burning: Nuclear fusion in which 4 protons(hydrogen nuclei) are converted to helium with theemission of energy.hydrostatic equilibrium: A stable condition where theinward force of gravity at every point in a star is balancedby the outward forces of gas and radiation pressure.hyperbolic orbit: The mathematical curve that will befollowed by an object that is orbiting at speeds greaterthan the velocity of escape.Hyperion: A satellite of Saturn.Iapetus: A satellite of Saturn, having its forward-facinghemisphere dark-colored and the other one light-colored.ICE: International Comet Explorer satellite.ideal gas: Under normal conditions, the pressure of a gasis directly dependent upon its temperature and density.Also known as a perfect gas.igneous rock: Rock formed by the cooling of moltenmaterial.inductive reasoning (induction): Reasoning fromparticular observations to general conclusions orprinciples.inertia: The resistance to any change in an object’smotion. Inertia is caused by the property of mass.inferior conjunction: When Mercury and Venus comeinto conjunction with the Sun and are at their closest toEarth.inferior planets: Planets closer to the Sun than Earthis—i.e., Mercury and Venus.inflationary universe: The hypothesis that a period ofextremely rapid expansion occurred early in the historyof the universe (followed by the slower expansion we seetoday).instability strip: The region of the H-R diagram whereCepheid variables and Cepheid-like variables are found.interference: The interaction of two similar waves. Seeconstructive and destructive interference.interferometer: An instrument that uses the propertiesof interference to increase resolution. Radio interferom-eters consist of two or more telescopes examining thesame source at the same time. The interference betweenthe different dishes enables astronomers to examine thetarget with very high resolution.

intergalactic matter: Material in space between clustersof galaxies.interstellar absorption lines: Sharp absorption linesseen in the spectra of some stars because of absorption ofenergy by interstellar gas.interstellar extinction: Radiation removed from a beamof light due to its passage through the dust component ofthe interstellar material.interstellar medium: The atoms, ions, and molecules inspace between the stars.inverse square law of light: The brightness of aradiating body varies inversely with the square of itsdistance from us.Io: The innermost Galilean satellite of Jupiter, heated bygravitational and magnetic forces into an active volcanicstate.ion: An atom with a net charge, due to the loss or gainof one or more electrons.ionization: The process in which an electron in an atomabsorbs sufficient energy for the electron to escape.ionization energy: The energy necessary to remove anelectron from an atom or ion.ionosphere: The layer of the Earth’s atmosphere (at andbeyond 100 km up) consisting of charged particles.IRAS: The InfraRed Astronomical Satellite, which mappedmost of the sky at long wavelengths. The data now serveas an archive for astronomical research.iridium: A chemical element found in greater abundancein meteorites than on the Earth. It may allow us to locateancient impacts more successfully and draw more conclu-sions about their consequences for life on Earth.irregular galaxies: Galaxies with odd shapes that donot fit into the conventional schemes of classification byappearance.Ishtar Terra: A large plateau in the northern hemisphereof Venus.isotope: Two atoms having the same number of protonsbut different numbers of neutrons are said to be isotopes.isotropy: Having equal value in all directions. Isotropicradiation would be radiation impinging on the Earthequally from all directions.IUE: The International Ultraviolet Explorer satellite.Jean’s length: Condensations and perturbations in a gasthat are larger than the Jean’s length will be inclinedtoward gravitational collapse.Jean’s mass: The amount of material in a length equal tothe Jean’s length.jet: A narrow, high-energy beam of particles and radiation.jet stream: High-speed flows of material in the upperatmosphere.Jovian planets: Solar system planets that are larger,rotate more rapidly, and are less dense than the terrestrialplanets such as Earth. They have dense atmospheres andchemical compositions similar to that of the Sun.

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Juno: The third asteroid discovered.Kapteyn Universe: A model of the galaxy derived fromstar counts. The model, which suffered from systematicerrors, showed the Sun to be at the center. Named afterthe 20th-century Dutch astronomer J. C. Kapteyn.Keck telescope: As of 1994, the world’s largest telescopewith a 10-m diameter mirror.kelvin: A temperature scale with no negative numbers.X°C is equal to X � 273 kelvins.Kelvin-Helmholtz contraction time: The timerequired for a body to collapse to half its size under itsown self-gravity.Kepler: Johannes Kepler was the 17th-century astrono-mer who used Tycho Brahe’s observations to discoverthe laws governing the motions of the planets in the solarsystem.Keplerian (orbital) motion: Motion that follows Kepler’slaws.Kepler’s laws: Three empirically determined laws, basedon observations of Tycho Brahe, that describe the motionsof planets around the Sun.kinetic energy: Energy possessed by a body by virtue ofits motion.Kirchhoff ’s laws: Extensive laboratory work by Kirch-hoff discovered several modes of emitting radiation(continuous, dark-line, and bright-line) and the conditionsunder which each would be produced.Kirkwood gaps: Regions in the asteroid belt that containno asteroids.KL (Kleinman-Low) Nebula: A source of strong infraredradiation near the Orion Nebula.KREEP: Lunar material with a composition high inpotassium (K), rare earth elements (REE), and phospho-rus (P).Kuiper Airborne Observatory: A flying infraredtelescope system.Kuiper belt: A hypothesized disk-shaped region sur-rounding the solar system thought to contain short-periodcomets.Lagrangian points: Five points in space where thecombined gravitational attraction of two bodies will canceleach other out.Large Magellanic Cloud (LMC): See Magellanic Clouds.latitude: Angular distance north or south of the equator.law of parsimony: Of two equally successful hypotheses,the simplest one is preferred.law of superposition: When interpreting the surface ofthe Moon or other firm surfaces, the assumption that thetopmost features are the most recently formed.Leonids: An August meteor shower famous for itsoccasional, spectacular displays.light-gathering power: A measure of the ability of a lensor telescope to gather light. It depends upon its area,which for a circular mirror is �R2, where R is the radius ofthe lens or telescope.

light-year: The distance light travels in one year (9.5 �1012 km).limb: The edge of an object.limb darkening: The edges of the Sun are less brightbecause the radiation we receive from there comes fromouter, cooler layers. The center of the disk of the Sun isbrighter because the radiation comes from deeper, hotterlayers.line profile: A detailed display with wavelength of theamount of energy absorbed at each and every point withinan absorption line. Such details can reveal atmosphericdensity, rotation, magnetic fields, and other effects.lithosphere: The solid, rocky layers of Earth just abovethe aesthenosphere.Local Group: Our galaxy is a member of a small groupof about two dozen galaxies. This group also contains theMagellanic Clouds and the Andromeda Galaxy.Local Supercluster: Our Local Group is also a memberof a cluster of galaxy clusters dominated by the extensiveVirgo Cluster of galaxies.longitude: East-west location on Earth along the equator,measuring from the circle that runs through Greenwich,England.long-period variable star: A cool, giant star that maytake more than a year to go through its cycle of brightnessvariation.luminosity: The total energy emitted per second by acelestial object.luminosity class: Stars of the same surface temperatureshow considerable variation in total energy output. Theluminosity classification was invented to be able to labelthose differences. Class I contains superluminous super-giants while class V designates feeble stars from thebottom of the main sequence.lunar eclipse: The Moon orbits through Earth’s shadow,and we see a circular line of shadow cross the Moon.lunar occultation: The passing of the Moon in front ofa star.Lunar Orbiter: A series of satellites that provided anextensive survey of most parts of the lunar surface in the1970s.Lunar Ranger: The earliest explorations of the Moonfeatured rockets with TV cameras in the nose that wouldcrash into the lunar surface, transmitting pictures as longas they were able.Lyman series: Transitions of the H atom involvingthe ground state. The resulting spectral lines are in theultraviolet spectral region.Maat Mons: A volcanic mountain on Venus.Magellanic Clouds (Large and Small): Two smallirregular galaxies that are probably bound gravitationallyto the Milky Way. The SMC actually consists of twogalaxies. They are naked-eye objects in the southernhemisphere.

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Magellanic Stream: A ribbon of hydrogen gas runningfrom the Milky Way to the Magellanic Clouds.magma: Molten rock.magnetar: Neutron star with a super-high magnetic field100–1000 times stronger than that of a normal neutronstar.magnetic braking: A process in which particles ejectedby a star interact with its magnetic field to slow the star’srotation.magnetic field: A force field that can affect the motionof charged particles and magnetic materials.magnetic lines of force: Lines of equal magnetic fieldintensity.magnetosphere: A zone surrounding a planetary bodyin which solar wind particles are trapped by the planet’smagnetic field.magnification or magnifying power: An increase in theapparent size of an object. In a telescope system, themagnification is the focal length of the objective lensdivided by the focal length of the eyepiece.magnitude: A scale of apparent brightness used byastronomers since the time of Hipparchus. If two objectsdiffer by one magnitude in apparent brightness, they differin brightnesses by a factor of 2.512. (Note: this is not thesame as “order of magnitude.”)main sequence: The region of the H-R diagram con-sisting of stable stars of different masses. A normal starspends most of its life on the main sequence.major axis: The longest dimension of an ellipse, mea-sured through the foci.mantle: The region of Earth between the crust and thecore.maria: Singular: mare. Latin for “seas,” these are dark-colored lowlands on the Moon’s surface that have beencovered by lava flows.Mariner: Spaceprobes to Venus and Mars in the late1960s.mascons: Concentrations of mass just under the Moon’ssurface discovered by the anomalous motions of LunarOrbiters.mass: A measure of the amount of material in a body. It isa measure of the concept of inertia.mass-luminosity relationship: The luminosity of a staris approximately proportional to the 4 power of its mass;that is, L � M4.mass-radius relation: For white dwarfs. The moremassive the white dwarf, the smaller its radius.Maunder minimum: A decline in sunspot and other solaractivity from 1645 to 1715, which had consequences forEarth weather.meridian: The great circle of the sky through the twocelestial poles and the zenith. It divides the sky intoeastern and western halves.

metal-poor stars: Stars whose chemical composition islow in metal abundance relative to the Sun.metal-rich stars: Stars with above average abundancesof metals.metamorphic rock: Rock that originally formed byigneous or sedimentary processes and changed in formdue to the presence of high pressures or temperatures.metastable states: Low-lying energy states where theatom will remain for an unusually long time beforeemitting radiation and undergoing transitions to theground state.meteor: A streak of light left by a meteoroid passingthrough Earth’s atmosphere.meteorite: A meteoroid that survives the passage throughEarth’s atmosphere and reaches the surface.meteorite fall: A meteorite that is observed to hit thesurface of Earth and then is recovered.meteorite find: A meteorite that attracts our attentionby its unusual appearance and hence is discoveredserendipitously.meteoroid: A small chunk of material (rocky or metallic)that travels through interplanetary space.meteor shower: An encounter between Earth and astream of particles that have evaporated from a comet butcontinued to orbit the Sun. This encounter produces alarge number of meteors that appear to be radiating fromone particular direction called the radiant.micrometeorites: Minute particles of meteoritic materialthat are strongly decelerated by the atmosphere. They areable to radiate away their heat before striking the surface.Milky Way: Originally, the prominent band of diffuselight that runs across the sky. More recently, the spiralsystem of perhaps 1 trillion stars in which our solarsystem is located.millisecond pulsars: Pulsars whose signals vary inperiods of milliseconds.Mimas: A moon of Saturn showing a giant impact crater.minor planet: An asteroid.Mira variables: A long-period variable star, of whichMira is the prototype.Miranda: A small Uranian moon with a highly “dis-tressed” surface.model: It is often a metaphor that provides a mentalpicture to explain phenomena. A model, as is the case of atheory or working hypothesis, makes predictions that canbe tested. Examples of models include the wave andparticle models of light.molecular bands: Groups of closely spaced absorptionlines due to molecules that are seen in some spectra.molecular clouds: Giant interstellar clouds with a largefraction of their matter in the form of molecules.molecules: Two or more atoms bound togetherchemically.

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momentum: For an object moving in a straight line,the linear momentum is mass times velocity. For an objectmoving in a circular orbit about the Sun, the angularmomentum is mass times speed times distance from theSun.multiple mirror telescope: A telescope that acquires alarge light-gathering power by collecting photons in manysmall mirrors that are oriented to bring all of the differentbeams of light to a common focal point.nadir: The opposite direction from the zenith (straightdown). The direction that is indicated by a weight hangingfreely in Earth’s gravitational field.nebula: A diffuse, nonstellar object.nebular hypothesis: The theory suggesting that the Sunand planets were formed as condensations out of a cloudof dust and gas that was flattened into a disk of material bythe rotation of the cloud.negative curvature: A geometry in which two parallelbeams of light would diverge from each other withdistance and in which the sum of the angles in a triangleare less than 180 degrees.Nemesis: A star hypothesized to orbit the Sun and toperturb the orbits of comets to produce a suggested 65-million-year period of mass extinctions on Earth.Nereid: A satellite of Neptune.neutrino: An elusive elementary particle with no electriccharge and little if any mass. Neutrinos are produced incertain nuclear reactions and carry energy away from thestar.neutrino oscillation: The ability of a neutrino to changefrom one type to another.neutron: A nuclear particle with no electric charge andapproximately the same mass as a proton.neutron star: A star that derives most of its pressuresupport from degenerate neutrons. It is a collapsed objectwhose material approaches the density of the nucleus ofan atom.new Moon: The phase where the Moon and the Sun arein conjunction but it is the back side of the Moon that isilluminated.Newtonian reflector: A telescope that employs a smalldiagonal mirror to produce a focus outside the side of thetelescope tube.Newton’s first law: A body will maintain its current stateof motion until it is disturbed by an external force. Alsocalled law of inertia.Newton’s second law: The famous F = ma describeshow forces cause accelerations. It shows that the accelera-tion a body experiences, a = F/m, is directly proportionalto the force and inversely proportional to its mass.Newton’s third law: If object 1 exerts a force on object 2,then object 2 exerts an equal and opposite force on object1—action � reaction.Nix Olympica: A giant but now inactive volcano on Mars.Also known as Olympus Mons.

nonthermal radiation: Radiation produced by interac-tions between charged particles moving near the speed oflight in a magnetic field. The intensity of the radiationdoes not follow the blackbody curves exhibited by thermalradiation and is polarized.noon: Apparent noon is when the Sun reaches its maxi-mum altitude and shows the shortest shadow for theday. Noon on the civil clock incorporates effects such aslocation in a time zone and Daylight Savings Time.north celestial pole: See celestial pole.

nova: A star that increases its brightness dramatically in ashort time, and then slowly declines in light output. Manynovae are recurrent. Current models of novae postulatethat they are binary systems where one star dumps massonto a nearby collapsed object.nuclear bulge: The mass concentration at the center ofa galaxy.nuclear reaction: An interaction between a nucleusand another particle that results in energy emission orabsorption and a change in the type of particles present.See fusion and fission.nucleosynthesis: The manufacture (synthesis) ofelements through a variety of nuclear reactions insidestars.nucleus of an atom: The central concentration of massin an atom, composed of protons and neutrons and havinga net positive charge. It is surrounded by a distributionof electrons that are bound to it by means of a negativeelectrical charge.nucleus of a comet: The small core of frozen gases anddust that evaporates when the comet is near the Sun toproduce the bright halo and tail features.nucleus of a galaxy: The central regions where one findsa concentration of mass and often violent activity.OB associations: Recently formed loose associations ofO and B stars. The associations do not have enough massto prevent them from drifting apart in a relatively shorttime.Oberon: A satellite of Uranus.objective: The large lens or mirror that gathers light ina telescope system.objective prism spectrograph: A telescope-prism com-bination in which a large prism is placed in front of thetelescope objective to produce a photograph of the spectraof numerous stars rather than one of the stars themselves.Occam’s Razor: See law of parsimony.occultation: When one celestial body passes in front ofanother. See, for example, lunar occultation.Olbers’ paradox: In a uniform distribution of starsthroughout an infinite universe, all lines of sight from theEarth would terminate on the surface of a star, thuscausing the sky to be everywhere as bright as the surfaceof the Sun. This simple conjecture leads quickly tosophisticated cosmological conjectures.

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Olympus Mons: A large extinct volcano on Mars.Oort Cloud: A hypothesized cloud of primeval materialsurrounding the solar system at a considerable distancefrom the Sun and thought to be a source of comet nuclei,which are perturbed by stars to pass near the Sun.open cluster: Clusters of stars that form in the galacticplane. Also called galactic clusters.open universe: A universe that possesses insufficientmass to halt the observed expansion. The curvatureof space would be negative, and the universe would beinfinite.opposition: When two objects are separated in the skyby 180 degrees as seen from Earth.optical doubles: Two stars close together in the sky,which give the impression of being affiliated but are reallyat very different distances from the observer.orbital angular momentum: See momentum.orbital inclination: The angle between an orbital planeand some reference plane. For example, in the solarsystem the reference plane would be the ecliptic plane.Orbiting Astronomical Observatory: One of the earliertelescope systems sent into high Earth orbit to escape thedetrimental effects of the Earth’s atmosphere.order of magnitude: A concept that allows makingapproximate comparisons of numbers.Orientale Basin (Mare Orientale): A giant, multi-ringedbasin on the Moon.Orion Nebula: A giant cloud of gas and dust illuminatedby a cluster of hot stars in the cloud. The hot stars areyoung, and there is evidence that new stars are continuingto form inside the nebula. The Orion Nebula is visible tothe naked eye as the middle star in the Sword of Orion.oscillating universe: A hypothesis of the evolution ofthe universe in which it expands and contracts endlessly,undergoing repeated fireballs.outgasing: An expulsion of material from the inner, hotterregions of a planet.ozone layer: A layer of Earth’s atmosphere in whichsunlight ionizes O2 molecules and creates O3 (ozone).pair production: The interaction of two high-energygamma rays to produce a pair of particles—an atomicparticle and its antiparticle.Pallas: The second largest asteroid, discovered in 1802.Pangaea: The name given to the hypothetical superconti-nent of crustal material on Earth that eventually separatedand moved about to form the continents we see today.parabola: Mathematically, a curve on which every pointis equidistant from some point and some line. Dynamically,it is the orbit that would be followed by a body that justexactly achieved escape velocity.

paradigm: The state of a science at any particular time,comprising its current theories and observations andthe conclusions drawn from them. A scientific revolutionoccurs when a field of science goes through a majorchange of paradigm, as for example when the Ptolemaicmodel was displaced by the Copernican model.parallax: A shift in the apparent direction of an object dueto a change in the position of the observer. Most com-monly, the movement of nearby stars on the backgroundof unmoving distant stars due to the motion of the Eartharound the Sun. The term is often loosely used synony-mously with distance.parent molecule: A molecule inferred to exist fromobservations of the “daughter” molecules that we are ableto see. The “daughter” is produced by a modification ofthe original “parent” molecules.parsec: 3.26 ly, the distance at which the parallax of astar will equal one second of arc.partial eclipse: When the eclipsed body is not totallycovered by the eclipsing body.Paschen series: The transitions of the hydrogen atominvolving the third energy level and appearing in the nearinfrared.penumbra: The region of partial shadow in an eclipse.Also, the lighter dark region surrounding a sunspot.perfect gas: See ideal gas.perigee: The nearest approach to the Earth by an Earth-orbiting body. See also apogee.perihelion: The nearest approach to the Sun by a solarsystem object. See also aphelion.period: The time interval required for a certain cyclicbehavior to repeat itself. For example, the synodic periodof Mars (the time interval between two successiveoppositions) is 780 days.period-luminosity relation: The correlation between theperiod of light variation and the luminosity of a Cepheidvariable star. This property enables astronomers to judgethe distances of nearby galaxies that have Cepheids inthem.Perseids: A meteor shower at its strongest in early tomid-August.Pfund series: The series of transitions of the hydrogenatom that involve the fourth energy level. The lines occurin the infrared.phases: The variation in the appearance of a solar systemobject seen by an observer as the object takes up differentpositions with respect to the observer and the Sun.Phobos: One of the two small moons of Mars.Phoebe: A moon of Saturn, consisting of a dark surface.photodissociation: The separation of a molecule into twoor more atoms or molecules caused by the absorption ofa photon.

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photoelectric effect: The process in which certainphotosensitive materials will eject electrons when illumi-nated by blue light but not when illuminated by red light(no matter how strong the irradiation is). This behaviorwas explained by Einstein as implying that radiationcomes in discrete particles, or chunks, known as photons.photoionization: The absorption of a photon leads tothe removal of an electron from an atom or ion.photons: Discrete particles (bundles, chunks) of electro-magnetic energy.photosphere: The visible surface of the Sun. Theregion of the Sun’s atmosphere that contributes most ofthe visible light that we see when we look at the Sun.Pioneer 10, 11: 1970s spacecraft probes to Jupiter.pixel: A picture element of a detector. It is the smallestdetecting element on a photographic film or a CCD chip.Planck’s constant: The energy of a photon is E = h f,where f is the frequency of the radiation and h is Planck’sconstant.Planck curve: A mathematical expression describingthe emission given off by radiating bodies as a function oftemperature. Describes the spectrum of a blackbody.Planck time: The first 10–43 second after the big bang,when the laws of quantum mechanics and relativity bothdetermined what occurred.planet: The name means “wanderer” in Greek, sincethe ancients observed these bodies to move around withrespect to the sphere of “fixed stars.”planetary nebula: An expanding shell of gas ejected fromthe outer atmosphere of red giant stars in a late stage ofstellar evolution.planetesimals: Aggregates of mass (about asteroid-size)that are thought to combine to form protoplanets in theearly history of the solar system.Plaskett’s star: A star of high mass (approximately 70times the mass of the Sun). It is perhaps the most massivestar known.plasma: A gas in which all or many of the componentsare charged particles (electrons and ions).plate tectonics: Many changes in the surface of theEarth can be explained in terms of the slow movementsof the Earth’s crustal plates.Pleiades: A small, open cluster of stars in Taurus that areeasily visible to the naked eye and are known as the SevenSisters. This star cluster has been singled out as worthyof observation by virtually every skywatching culture overthe ages. Its appearances just before dawn in the east andits disappearances just after dusk in the west just happento coincide with important phases of the agriculturalcycles in the northern temperate zone.polar axis: A line pointing toward the NCP about whichtelescopes can rotate to follow the daily motion of a star.polar caps: Regions of ice and snow at the poles of aplanet.

Polaris: The end star in the handle of the Little Dipper,Polaris is a second-magnitude star that coincidentallyhappens to be located within 1° of the NCP at the presenttime.polarization: The alignment of the planes of vibration ofelectromagnetic waves.polarization of starlight: Starlight that has beenpolarized by scattering from dust particles in space.pole star: See Polaris.

Population I stars: Stars associated with the galacticplane that are (on the average) younger in age and have ahigher percentage of heavy elements in their chemicalcompositions.Population II stars: Older stars with a lower percentageof heavy elements in their chemical compositions, prefer-entially located in the galactic halo.positive curvature: Spacetime that closes on itself,forming a finite configuration. If two parallel beams oflight set out in positively curved space, the two beams willslowly approach each other, like lines drawn on thesurface of the Earth.positron: An antielectron, a unit of antimatter with thesame mass as the electron but with opposite charge.potential energy: Energy that is stored in some way andavailable to do work—e.g., a ball held in the air has poten-tial energy; when it is released, that potential energy isconverted to kinetic energy of motion. See gravitationalpotential energy.precession of the equinoxes: The 26,000-year revolutionof the Earth’s rotation axis causes the equinox points tomove slowly around the celestial sphere with the sameperiod. Precession of the equinoxes produces a change inthe coordinates of stars with time.pressure: The force per unit area exerted by a gas ora liquid.pressure wave (P-wave): Waves that are compressionalin nature—in particular, a seismic wave in the Earth ora sound wave.prime focus: The first focus that is produced by a lens ora mirror, unmodified by other extra reflecting surfaces.The prime focus allows short photographic exposure timesand results in small images.primeval fireball: The state of incredibly high tempera-ture and pressure out of which our expanding universebegan.prominences: Solar material elevated above the surfaceby the magnetic forces found in active regions on the solarsurface. Most easily seen at the limb.proton: A positively charged nuclear particle of approxi-mately the same mass as a neutron.proton-proton (p-p) chain: A series of nuclear fusionreactions in low-mass stars. The chain produces energy inthe process of creating a helium nucleus from fourprotons.

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protoplanets: An early stage in the process of making aplanet where the body is still accreting mass.protostar: A cloud of gas and dust that is collapsing underits own gravity and destined to become a star when itscentral temperature becomes high enough to start nuclearreactions in its central regions.protosun: The Sun when it was a protostar.Proxima Centauri: The nearest star to the Earth.Ptolemaic system: The model of the cosmos, passed onfrom ancient Greece, in which the Earth is the unmovingcenter of the universe and the planets and stars revolvearound it.pulsars: Objects that emit energetic and rapid pulsesof radio radiation. They are thought to be rapidly rotatingneutron stars surrounded by plasma and a strong mag-netic field.Pythagoras: An early Greek philosopher-scientist whoformed a school arguing that numbers are the basis of allunderstanding.Q stars: Hypothetical objects to explain certain obser-vations in a way other than calling on the concept of theblack hole. They consist of protons, electrons, and neu-trons held together by the strong nuclear force rather thangravity.quadrature: The configuration of a superior planet inwhich the Sun and the planet are 90 degrees apart in thesky, when viewed from the Earth.quantum mechanics (quantum theory): A theoryof physics, which says that, on the subatomic level, theproperties of the universe are not continuous but areinstead found in discrete units called quanta. A photon isan example of quantized energy.quarks: A fundamental particle in subnuclear physics. Itis proposed that other particles are composed of the unionof different types of quarks.quasar: A star-like appearing object observed to have alarge redshift in a spectrum containing emission andabsorption lines. The word is a contraction for quasi-stellarradio source but is applied even to objects that do not emitradio radiation. They are now known to be the nuclei ofyoung galaxies.radar observations: By bouncing radar waves off thesurfaces of nearby celestial objects and determiningthe time it takes to return, details about the structureand elevation of the surface can be learned.radial velocity: The relative line-of-sight velocity betweena celestial object and the observer.radian: A unit of angular measurement equal to 57.3degrees.radiant: A point in the sky from which objects in a meteorshower appear to diverge. It is not a physical point but aneffect of perspective.

radiation: Historically, this term was applied to bothparticles (for example, beta radiation, which turned out tobe emitted electrons) and energy propagating throughspace in the form of electromagnetic waves.radiation belts: Equatorial regions of energetic chargedparticles trapped in the magnetic field of a planet. Thestrongest radiation belts in the solar system belong toJupiter. For the Earth’s belts, see Van Allen belts.radiation, cosmic 3 K background: See background3 K radiation.radiation, hydrogen 21-cm: Low-energy photonsemitted at a radio wavelength of 21 centimeter by hydro-gen atoms undergoing a “spin-flip” transition within thelowest energy level of the H atom.radiation pressure: The “push” exerted by the impactof photons.radiative transport: The movement of energy fromone place to another by the emission and absorption ofradiation.radio galaxies: Galaxies that give off anomalously largequantities of radio emission.radio interferometer: Two radio telescopes linkedtogether electronically. By comparing the interferencepatterns between the two signals, fine details in thepattern of the object’s radio image can be discerned.radio jets: Material that appears to be ejected at highspeeds from active regions in radio galaxies.radio telescope: A telescope constructed to detectthe emission of electromagnetic energy of wavelengthsranging from millimeters to meters.radioactive dating: A method of dating rocks by exam-ining the quantity of radioactive decay by-products inrelation to the decaying material.radioactive decay: A fission process in which a heavyelement spontaneously splits into simpler components.The speed of the decay is governed by the “half-life” of theelement. Every half-life, half of the remaining radioactiveatoms will break down.radioactive element: An element whose nucleus isunstable and spontaneously decays (changes) to a lighterelement.radioactivity: The spontaneous radiation of emission froman unstable element. Particles emitted in a decay processmay include electrons, nuclei, and gamma rays.random error: An error in a set of measurements causedby random variations in the way a set of measurements isobtained. All measurements have random errors, whichare measured by the standard deviation.ray: A thin pencil of light. Also, a plume of material ejectedradially from the point of a meteoric impact.real image: An image formed at a definite point in spaceby the convergence of light rays. Energy is concentratedat this point and can be examined by eye or captured on aphotographic plate or other detector.

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recombination: An atomic process in which an electronand an ion reunite, giving off energy.recurrent novae: Stars that undergo the nova phenom-enon repeatedly.reddening: In the scattering process that removes pho-tons from a beam of radiation passing through a medium,the short wavelength radiation undergoes more scatter-ing, leaving the color of the object to appear redder than itreally is.red giant: Low surface temperature, highly evolved starsof great size and luminosity.redshift: The Doppler shifting of spectral features tolonger wavelengths due to the relative motion of thesource of light away from the Earth.redshift, cosmological: See cosmological redshift.reflecting telescope: A telescope that uses a mirror togather radiation.reflection nebula: A dust cloud that reflects somestarlight in our direction.refracting telescope (refractor): A telescope that usesa lens to gather radiation.refraction: The bending of light that passes from onemedium into a different medium.regolith: The crumbling and shattered top layer of lunardirt created by constant bombardment of particles fromspace.relativistic jets: Jets moving outward with speeds nearthat of light from active regions in galaxies.resolving power (resolution): The ability of an opticalsystem to observe fine detail, such as two stars closetogether.resonance: A situation in which the orbital and rota-tional periods of an object are either equal or relatedby a simple ratio such as 2:1, 3:2, 3:1, etc. Examplesinclude the Moon’s rotation and revolution periods (1:1)and Mercury’s rotation and revolution periods (3:2).A resonance also occurs when the orbital period of anobject is a simple fraction of the orbital period of anotherbody.retrograde motion: The occasional westward movementin the motion of planets that generally move eastwardrelative to the background stars. The westward motionsare caused by the Earth’s movement in its orbit, when itlaps or is lapped by another planet.retroreflector: A reflector that will return a beam of lightin exactly the same direction from which it came.revolution: Orbital motion around a body or a centerof mass.Rhea: A cratered, icy moon of Saturn.right ascension: The east-west coordinate in the equato-rial coordinate system. It is the distance along the celestialequator (in hours of time running from 0 to 24) eastwardfrom the vernal equinox.

rille: See sinuous rille.ring galaxy: A galaxy containing a nucleus surrounded bya ring of material. Ring galaxies appear to form fromgalaxy collisions.Ring Nebula: A well-known planetary nebula in theconstellation Lyra.ring system: A system of icy rocks and particles orbitingeach of the giant planets.Roche limit: A critical distance from a planet, inside ofwhich tidal forces prevent material from accreting to formlarger bodies. Inside the Roche limit, tidal forces arelarger than the mutual forces between nearby gravitatingbodies; outside the limit, mutual gravity allows bodies toaccumulate more material and grow.Roche lobe: An imaginary surface formed by theoverlapping gravitational fields of the two stars in a binarysystem. The lobes look like the parts of a figure eight.Material inside each lobe is considered to belong to thenearest star; material can readily flow from one star tothe other through the point of intersection.ROSAT: An X-ray satellite currently in orbit. It is namedfor Wilhelm Röntgen, who discovered X-rays.rotation: The movement of a body about an axis.rotation curve: A graph of the orbital speeds of objectsin a spiral galaxy as a function of distance from the centerof the galaxy.r-process (rapid process): A nuclear reaction that buildsup heavy elements by the rapid accumulation of neutrons.RR Lyrae stars: Variable stars whose luminosity variesin regular cycles of less than 24 hours.Russell-Vogt theorem: The structure of a normal star inequilibrium depends only upon the mass, chemicalcomposition, and age of the star.SAGE: Soviet American Gallium Experiment, an experi-ment searching for solar neutrinos.Sagittarius A (Sgr A): A group of several extremelyenergetic radio and infrared sources found at or near thecenter of our Milky Way galaxy.scattering: The change in direction suffered by photonspassing through gas atoms or dust particles.Schmidt telescope: A telescope employing a sphericalmirror and an aspherical correcting lens. Its advantagesare short photographic exposure times and a wide field ofview.Schroeter’s Valley: A lunar valley running from a craterto a mare. It is thought to be a channel for the flow of lava.Schwarzschild radius: See event horizon.Sculptor: A small spheroidal galaxy in the Local Group.secondary craters: Craters formed by the impact ofmaterial ejected from the impact that formed the primarycrater.sedimentary rock: A rock formed when rock debris,transported to a new site by water, wind, ice, or gravity,settles and becomes cemented.

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seeing: The distortion, blurring, and artificial enlarge-ment of stellar images caused by the turbulent motions ofgases in the Earth’s atmosphere.seismic waves: Waves caused by earthquakes thatpropagate through and around the Earth. See also pressurewave and shear wave.seismometer: An instrument to detect seismic events.selection effect: A bias in data collection that prefer-entially exhibits certain evidence and withholds otherevidence from observers. A simple example: the assump-tion that most meteoroids are composed largely of iron,because iron meteorites are easier to recognize anddiscover when they are lying on the ground.self-gravity: The gravitational attraction each part of anobject has on all the other parts.semi-major axis: Half the longest axis of an ellipse,which is also equal to the average distance of the orbitingobject from the center of mass.sexagesimal system: A number system devised inancient Mesopotamia using a base of 60 rather than thebase of 10 that is used in the decimal system. It is par-ticularly applied to situations involving angles and time.Sextans: A small spheroidal galaxy in the Local Group.Seyfert galaxies: Spiral galaxies with brilliant, star-likenuclei exhibiting violent activity.shear wave (s-wave): A type of seismic wave where themotion of the material is perpendicular to the direction ofthe movement of the wave.shepherd satellites: Small moons that may be instru-mental in shaping and maintaining the rings around giantplanets.shock wave: A compressional wave traveling at greaterthan the speed of sound creates a discontinuity propagat-ing ahead of it. Sonic booms are a terrestrial example of ashock wave.short-period comets: Comets having periods less than200 years. They may have come from the hypothesizedKuiper belt.sidereal day: The time interval between two successivemeridian passages of a star (23 hours and 56 minutes).sidereal period: The period of an object’s movement withrespect to the stellar background.silicates: Rocks and compounds formed with silicon (e.g.,sand).single-line spectroscopic binary: A binary star in whichthe spectral lines of only one star are observed to shiftback and forth due to the Doppler effect.singularity: A point in space where extreme conditionsare found and normal physical laws break down.sinuous rille: A winding channel on a planetary orsatellite surface, generally thought to be caused by pastlava flows.Sirius (Sirius A): The brightest star in the sky.

Sirius B: A white dwarf star in orbit with Sirius A.Small Magellanic Cloud: See Magellanic Clouds.Snedecor rough check: An approximation to the stan-dard deviation. Given by the range in the observationsdivided by the square root of the number of observationsminus 1.S0 galaxy: A galaxy with a large nucleus and a flatteneddisk but no indication of spiral arms in the disk.solar day: The time interval between two successivemeridian passages of the Sun (= 24 hours).solar neutrino problem: Theory predicts a largernumber of neutrinos from the Sun than is measured bycurrent detectors. This has spawned a variety of possi-ble explanations, including the possibility that neutrinosactually have a small mass.solar wind: An outward flow of high-speed particlesstreaming from the Sun.solstice, summer and winter: The moment of the yearwhen the Sun is as far north of the celestial equator(summer in the northern hemisphere) as it can be and asfar south (winter) as it can be. The northern hemispheresummer solstice occurs about June 21 (the longest day ofthe year), and the winter solstice occurs around December21.space-time: In Einstein’s general theory of relativity, timeis no longer an absolute quantity. It becomes anothercoordinate like the spatial coordinates. Instead of discuss-ing space and time, we must now speak of space-time.Spacewatch Camera: A telescope system, at the Univer-sity of Arizona, designed to search for asteroids that comenear the Earth.spectral class (spectral type): A classification schemefor spectra that uses letters and numbers to express thesurface temperatures of stars.spectral line: A feature seen at a discrete wavelength ina spectrum.spectral sequence: OBAFGKM is a sequence of lettersthat order the stars by decreasing temperature in theastronomical classification scheme of spectral types.spectrograph: An instrument that disperses the radia-tion from celestial objects into a spectrum, which is thenrecorded on film or electronically.spectroheliograph: An instrument that produces animage of the Sun in one single wavelength, using asophisticated filtering technique.spectroscopic binary: An apparently single star that isdiscovered to be a multiple system by the detection of avariable Doppler shift in the spectrum.spectroscopic parallax: A method of estimating thedistances of stars from spectroscopic information, whichprovides a star’s temperature and the luminosity class.spectroscopy: The specialized study of spectra.spectrum: Plural: spectra. The energy of differentwavelengths radiated by celestial objects. See bright linespectrum, continuous spectrum, and dark line spectrum.

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spherical aberration: Distortions in images formed byspherical mirrors and lenses.spicules: Jets of bright gas dancing in the chromosphereof the Sun, looking like a waving field of wheat.spin angular momentum: Angular momentum of a bodydue to its rotational motion about a spin axis.spiral arm: A spiral-shaped region in a spiral galaxy,where bright gas clouds and recently formed, young starsare found.spiral galaxies: Galaxies that show spiral patterns ofbrightness.spiral tracers: Young objects that trace out the locationsof the arms of a spiral galaxy.s-process: A nuclear reaction involving the slow accu-mulation of neutrons.SS 433: A complex star system exhibiting both redshiftsand blueshifts in its spectrum. It may be a normal starlosing matter to an accretion disk around a collapsedobject, with rapidly moving jets of material in the system.standard candle: An object whose luminosity is well-known, so that it can be used as a stable reference toestimate the luminosities, and thus distances, of distantobjects.standard deviation: A statistical measurement of theprecision of a set of measurements. A measure of therandom errors of a measurement.starburst galaxy: A class of galaxy characterized byperiodic outbursts of star formation over widespreadregions in the galaxy.starspots: Dark, magnetically active regions seen on starsthat may be similar to sunspots in their origin.steady state hypothesis: A hypothesis that proposes thatthe expansion of the universe is caused by the pressure ofnew matter that is continually being spontaneously pro-duced. In this model, the universe would look the same atall times past, present, and future. It would be infinitelyold, without a beginning or an end.Stefan-Boltzmann law: The total energy per unit areaemitted each second by a radiating object varies as thefourth power of its temperature (E � T4).stellar model: A theoretical model of the interior proper-ties of a star. The model is expressed as a table or graph ofimportant stellar properties (temperature, pressure,density, and so on) as a function of distance from thecenter.stellar wind: An outflow of charged particles from a star;similar to the solar wind.stochastic star formation: See supernova-induced starformation.Stonehenge: An ancient and huge stone structure inEngland. Apparent astronomical alignments have beenfound in the structure, and many hypotheses have beenput forward about the astronomical sophistication of thebuilders. Many of the hypotheses remain controversial.

stony-iron meteorite: A meteorite with some iron andnickel mixed into its rocky composition.stony meteorite: A rocky meteorite that on casualexamination looks like an Earth rock.Straight Wall: A long cliff on the Moon.stratosphere: A region of the Earth’s atmosphere,between about 20 to 50 km, in which temperatureincreases with height.strong nuclear force: The force responsible for bindingparticles together in the nucleus.subduction: The process by which cold lithosphericmaterial sinks into the asthenosphere.sunspot cycle: A regular variation in the number ofsunspots, which repeats itself every 11 years or so.If variations in magnetic orientation are included, thesunspot cycle is 22 years.sunspots: Dark regions on the photosphere of the Sunthat are cooler than their surroundings and appear darkby contrast. Sunspots also possess strong magnetic fields.supercluster: A cluster of galaxy clusters.supergiant star: Highly evolved star with low surfacetemperature, high luminosity, and very large size.superior conjunction: When a planet is in conjunctionwith the Sun, but on the other side of the Sun, as viewedfrom the Earth.superior planets: Planets with orbits larger than Earth’s.supermassive black hole: A black hole containingmillions, perhaps billions of solar masses. The HubbleSpace Telescope has provided overwhelming evidence in1994 of the existence of a supermassive black hole inthe center of M87, the giant elliptical galaxy in the Virgocluster of galaxies.supernova: A star that undergoes a spectacular increasein its energy output due to a catastrophic explosion of thecore.supernova-induced collapse: The compressional wavefrom a supernova explosion can squeeze gas clouds thatit encounters and induce them to begin a gravitationalcollapse.supernova-induced star formation: A statistical modeof star formation in which the supernova explosion of amassive star triggers the formation of other massive stars,which evolve rapidly to become supernovae in turn,causing the process to continue.supernova 1987A: A relatively nearby supernova in theLarge Magellanic Cloud.supernova remnant: The energetic and rapidly expand-ing gases produced by a supernova explosion. Theappearance and the amount of radio emission as comparedto visible wavelength emission depend upon the time sincethe explosion.synchronous rotation: A satellite whose rotation periodis exactly equal to its period of orbital revolution.

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G20GLOSSARY

synchrotron radiation: Radiation emitted by chargedparticles moving in a strong magnetic field near the speedof light.synodic period: For the Moon, the time required forthe Moon to go from one phase back to the same phase(29.5 days). For a planet, the time required for it to gofrom a particular configuration with respect to the Sunback to that same configuration—e.g., from superiorconjunction to superior conjunction.systematic error: An error that causes a measurementto be always too large or too small.T Tauri star: A young star that is still collapsing onto themain sequence.technetium: A radioactive element with a cosmicallyshort half-life that is seen in stellar atmospheres. Its verypresence indicates that nuclear reactions are taking placein a star.temperature: A measure of the speeds of the particlesin a gas.terminator: The line dividing light and dark on anilluminated body.terrestrial planets: Small, high-density planets that arerocky in composition and have relatively thin atmospheres(Mercury, Venus, Earth, Mars).Tethys: A satellite of Saturn with a crater 40% its size.thermal equilibrium: An equilibrium in which theenergy radiated by a star equals the energy produced inthe interior.thermal radiation: Energy emitted by a body as aconsequence of its temperature; the same as blackbodyradiation.thermal runaway: An unstable situation where anuncontrolled energy buildup occurs.3 K background radiation: See background 3 Kradiation.tidal force: A force resulting from unequal gravitationalforces (called differential gravitational forces) on oppositesides of a body.tidal friction: Water slowed by the tidal forces on it fromthe Moon will cause a drag force on the Earth by rubbingacross the continental shelves.Titan: Saturn’s largest moon—large enough to retain anatmosphere of its own.Titania: One of the five large moons of Uranus.top-down hypothesis: A model of galaxy cluster forma-tion in which clustering occurs prior to the formation ofthe galaxies making up the cluster.total eclipse: The complete blocking of one body byanother when spatial alignments are just right as seenfrom the Earth.transition region: The region between the chromosphereand the corona in which temperature rises dramatically.

Trapezium: A cluster of four young, luminous stars,ionizing and illuminating the central regions of the OrionNebula.trigonometric parallax: A method of determining thedistance to an object by noting the angle of displacementit shows with respect to distant background objects whenit is viewed from two different locations of the observer.triple-alpha process: A fusion reaction that convertsthree helium nuclei into a carbon nucleus.Triton: Neptune’s largest moon.Trojan group: A group of asteroids that orbit the Sun inJupiter’s orbit such that they maintain a 60-degree angleahead of and behind Jupiter. The asteroids are located attwo Lagrangian points.troposphere: The lowest level of the Earth’s atmosphere,where weather occurs.Tully-Fisher relation: The larger the luminosity of agalaxy, the larger the width of its 21-cm emission feature.Tunguska: A region in Siberia that suffered a massiveimpact of some sort in 1908.tuning fork diagram: A spatial display of the variousshapes and forms exhibited by galaxies.turbulence: A chaotic, convective motion of material.turnoff point: When the stars in a cluster are plotted inan H-R diagram, they will generally exhibit a lower portionof a main sequence, terminated at the top end in a pointwhere the evolution of more massive stars has causedthem to move away from the main sequence towards thered giant region. This turnoff point can be used todetermine the age of the cluster.two-armed spiral shock model: Applying the densitywave model to explain the simplest form of spiral structurethat we observe—a plain, two-armed spiral system.Tycho Brahe: The 16th-century astronomer who came tobe called the “father of modern observational astronomy”by diligently obtaining observations that were as good asthe naked eye can resolve. Brahe provided the observa-tions that allowed Kepler to deduce for the first time thecorrect laws of motion for the planets.Type I supernova: A supernova in which H lines are notseen in the spectrum. They show a sharp peak in bright-ness followed by a slow decline.Type II supernova: A supernova in which strong H linesare seen in the spectrum. The brightness shows a broaderpeak up until a sharp decline in emission sets in.ultraviolet radiation: Electromagnetic waves not visibleto the eye, with wavelengths that are shorter than bluevisible light.Ulysses: The first spacecraft to orbit the Sun in a polar,rather than equatorial, orbit.umbra: The region of total shadow in an eclipse. Also,the darkest part of a sunspot.universe: The totality of everything that can conceivablybe observed.

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Valhalla: A multi-ringed basin on Jupiter’s satellite,Callisto.Valles Marineris: The extensive canyon system on Mars.Van Allen belts: Regions of charged particles circlingthe Earth above the equator. The particles are capturedand held by the Earth’s magnetic field.variable star: A star that varies in brightness.velocity: Speed is distance traveled per unit time, butvelocity is a vector quantity that includes both speed anddirection of motion.velocity dispersion: The range of velocities exhibitedby some group of objects being studied—e.g., the stars ina cluster or the galaxies in a galaxy cluster.velocity-distance relation: See Hubble law.Venera: A series of Russian spacecrafts sent to studyVenus in the 1970s.vernal equinox: See equinox.Very Large Array (VLA): A giant radio interferometer,consisting of 27 antennas mounted on railroad tracks uponwhich they move.Very Long Baseline Interferometry (VLBI): Theprocess of combining the signals from radio telescopes ondifferent continents (and possibly even telescopes in Earthorbit) to achieve high resolution radio views of celestialobjects.Viking: American spacecraft that landed on the surfaceof Mars in 1976.Virgo Cluster: A very large cluster of galaxies relativelynear the Local Group.virtual image: An image with no definite location inspace. No energy passes through the apparent image.viscosity: The property of a fluid that has a highresistance to flowing.visual binaries: Double stars that can be seen as separatestars in a telescope.voids: Large empty regions of intergalactic space.volatiles: Material that condenses at relatively lowtemperatures.volcanic domes: Raised regions on the Moon and planetsthat resembles the domes left on the Earth by volcanicactivity.Voyager: A pair of sophisticated interstellar probes thatvisited the Jovian planets in the 1980s.wavelength: The distance between two crests of a wave(or any two similar points on the waveform). It is usuallyrepresented in astronomy by the Greek letter lambda (�).wave-particle duality: A conclusion of quantum mechan-ics—that all objects have an admixture of both wave andparticle properties. This duality is most easily seen inlight, where it shows a wave nature (from diffraction andinterference) and a particle nature (from the photoelectriceffect).

weak nuclear force: The force responsibile forradioactivity.weight: The force exerted on an object by gravity.weightlessness: Not the lack of gravity, but the lack ofacceleration relative to a local reference frame. Forexample, astronauts whose acceleration around the Earthis the same as that of their spacecraft will be weightless.white dwarfs: A class of hot stars that are near the endof their evolution. They have lost their sources of nuclearenergy and as a consequence have collapsed into ex-tremely small configurations—the size of the Earth orsmaller.Widmanstätten patterns: Crystalline patterns seen iniron meteorites that indicate a past history of heating andslow cooling.Wien’s law: The wavelength at which the maximumamount of radiation is emitted is inversely proportional tothe temperature of the radiating object.winter solstice: See solstice.Wolf-Rayet stars: Hot, large, and luminous stars withbroad emission lines seen in their spectra caused by high-velocity stellar winds.W Virginus stars: Cepheid variables that belong toPopulation II.X-ray burster: Brief episodes of powerful X-ray emissionfrom a celestial source.X-rays: Highly energetic photons and electromagneticwaves of short wavelength (1 to a 100 angstroms).Young Stellar Object (YSO): Point sources of infraredradiation associated with dark clouds and other phenom-ena connected with star formation.Zeeman effect: The splitting of atomic energy levelscaused by a magnetic field. The result is that numerousspectral lines are formed rather than a single line; whenseen in low resolution, a line is broadened.zenith: The point in the sky directly overhead, 90 degreesfrom the horizon.Zero Age Main Sequence (ZAMS): The location of starsof different mass in the HR diagram as they leave thecontracting phase and first settle into an equilibrium statepowered by nuclear fusion.zodiac: Twelve constellations along the ecliptic thathave been passed down from ancient cultures. Theseconstellations divide the Sun’s motion approximately into12 months.zodiacal light: A region of diffuse illumination stretchingalong the ecliptic and centered on the Sun—also knownas the “False Dawn.” It is caused by scattering of sunlightoff of dust particles in the ecliptic plane.zone of avoidance: A region near the galactic plane ofthe Milky Way where few galaxies can be observed, dueto extinction caused by the dense dust clouds in the planeof the galaxy.

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