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Advanced Dictionary of Astronomical Terms
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 21-cm RadiationRadio Emission Produced by cold, low-density hydrogen in
interstellar space. AAbsolute Bolometeric MagnitudeThe Absolute Magnitude we would observe if we could detect
all wavelengths Absolute Visual MagnitudeIntrinsic brightness of a star; the apparent visual
magnitude the star would have if it were 10 Parsecs away from earth. Absolute ZeroThe lowest possible temperature. The temperature at which
particles in a material, atoms or molecules, contain no energy of motion that can be
extracted from a body. Absorption LineA dark line in a spectrum. Produced by the absence of
photons absorbed by atoms or molecules. Absorption SpectrumA Spectrum that contains absorption lines. AccelerationA change in velocity Acceleration of GravityFalling objects fall with an increasing acceleration of 9.8
meters per second per second. AccretionThe sticking together of solid particles to form a larger
particle. Accretion DiskThe whirling disk of gas that forms around a compact object
such as a white dwarf, neutron star or black hole as matter is drawn in Achromatic LensA telescope lens composed of two lenses ground from
different types of glass and designed to bring two selected colors to the same focus and
correct for chromatic aberration Active GalaxyA galaxy that is a source of excess radiation, usually
radio waves, X rays, gamma rays or some combination Active Galaxy Nucleus (AGN)The central energy source of an active galaxy Active OpticsOptical elements whose position or shape is continuously
controlled by computers Adaptive OpticsComputer controlled telescope mirrors that can adjust for
some changes in seeing conditions Alt-Azimuth MountingA telescope mounting capable of motion parallel to and
perpendicular to the horizon Angstrom1 x 10 -10 meters (used to measure wavelengths) Angular MomentumMeasure of the rotation of the body around some point. Annual EclipseA type of total eclipse in which the moon is too far from
the earth to totally cover the suns surface. A ring of the photosphere surrounds the moon
during mideclipse. The corona is not visible and neither are the prominence because of the
glare from the photosphere. AnorthositeRock of aluminum and calcium silicates found in the lunar
highlands. AphelionPoint in the earth's orbit where the earth is farthest away
from the Sun (summer for us) ApogeeOrbital point of greatest distance from the earth Apparent Relative OrbitThe Orbit of one star in a visual binary with respect to
the other star as seen from earth Apparent Visual Magnitude (mv)Is the magnitude of the stars without compensating for
their distance from Earth. ArachnoidsOn Venus, one of a number of round networks of fractures in
the crust, resembles spider webs. ArchaeoastronomyThe study of astronomy by ancient peoples AssociationGroups of widely scattered stars (10 - 1000) moving
together through space; not gravitationally bound to clusters. AsterismNamed group for stars not identified as constellations AsteroidsSmall Rocky worlds, most of which lie between Mars and
Jupiter in the asteroid belt. Astrometric BinaryA binary star identified by its irregular proper motion Astronomical Unit (AU)Is the average distance from the Earth to the Sun. 1 AU =
93 million miles or 1.5 X 1011 meters or 1.5 X 109 km. AstronomyIs the study of the universe Atmospheric WindowWavelength regions in which our atmosphere is transparent -
at visual wavelengths, infrared and radio. AuroraThe glowing light display that results when a planet's
magnetic field guides charged particles toward the north and south magnetic poles, where
they strike the upper atmosphere and excite atoms to emit photons. Autumnal EquinoxThe place where the Sun crosses the celestial equator
heading south. BB Canis Majoris VariableShort Period variables stars that do not lie in the
instability strip Barred Spiral GalaxyA spiral galaxy with an elongated nuclei resembling a bar
from which the arm originate. BasaltDark, igneous rock characteristic of solidified lava. BeltsDark bands of clouds that circle Jupiter parallel to its
equator; generally red, brown, or blue-green; believed to be regions of descending gas. Big Bang TheoryThe theory that the universe began with a violent explosion
from which the expanding universe of galaxies formed. Binary StarsPairs of stars that orbit around their common center of
mass Binding EnergyThe energy needed to pull an electron away from its atom. Bipolar FlowOpposite directed jets of gas ejected by some protostellar
objects. BL Lac ObjectObjects resemble quasars; thought to highly luminous cores
of distant galaxies. Black Body RadiationRadiation emitted by a hypothetical perfect radiator. The
spectrum is continuous, and the wavelength of maximum emission depends only on the body's
temperature. Black DwarfThe end state of a white dwarf that has cooled to a low
temperature Black HoleA mass that has collapsed to such a small volume that its
gravity prevents the escape of all radiation; also, the volume of space from which the
radiation may not escape. Blue and Red ShiftsBlue shifts: Shorting of wavelengths of observed light when
an object in moving toward the observer. Red Shifts: Lengthening of wavelengths of light
as the object moves away from the observer. Bok GlobulesSmall, dark clouds only about 1 light year in diameter that
contain 10-1000 solar masses of gas and dust. Believed to be related to star formation. Bow ShockThe boundary between the undisturbed solar wind and the
region being deflected around the planet or comet BrecciaA rock composed of fragments of earlier rocks bonded
together. Brown DwarfA very cool, low luminosity star whose mass is not
sufficient to ignite nuclear fusion. BursterA source of bursts of X-rays or in some cases, gamma rays;
believed to be associated with neutron stars. CCapture HypothesisThe theory that the moon formed elsewhere in the solar
system and was later captured by the earth. Carbon DetonationThe explosive ignition of carbon burning in some giant
stars. A possible cause of some supernova explosions Cassegrain TelescopeA reflecting telescope in which the secondary mirror
reflects light back down the tube through a hole in the center of the primary mirror Celestial EquatorAn imaginary line around the sky which is directly above
the earth's equator. Celestial SphereImaginary sphere surrounding the Earth to which the stars,
planets, Sun and moon seem to be attached. Center of MassTwo bodies revolve around a common center, called the
balance point of the system. Cepheid Variable StarVariable stars with a period of 1-60 days. Their period is
related to luminosity. Chandrasekhar LimitThe maximum mass of a white dwarf, about 1.4 solar masses.
A white dwarf of greater mass can not support itself and will collapse. Charge-Coupled DeviceA CCD is an electronic device consisting of an array of
photosensitive elements, used to record images. ChromosphereA line layer of gas just above the photosphere of the sun,
it is often marked by solar eruptions called solar flares and prominence. Chromatic AberrationA distortion found in refracting telescopes because lenses
focus different colors at slightly different distances. Images are consequently surrounded
by color fringes Circular VelocityThe lateral velocity an object must have to remain in orbit Circumpolar ConstellationConstellations which appear around the celestial poles,
which never seem to rise or set. Closed OrbitA circular orbit, one which returns back on itself. Closed UniverseA model universe in which the average density is great
enough to stop the expansion and make the universe contract. Cluster MethodThe method of determining the masses of galaxies in a
cluster. CNO CycleA series of nuclear reactions that use carbon as a catalyst
to combine four hydrogen atoms to make one helium atom plus energy; effective in stars
more massive than the sun. Co-Accretion HypothesisThe theory that the moon and the earth formed together. CocoonThe cloud of gas and dust around a contracting protostar
that conceals it at visible wavelengths. Collisional BroadeningThe smearing out of a spectrum line because of collisions
among the atoms of the gas. Color IndexA numerical measure of the color of a star. CometOne of the small, icy bodies that orbit the sun and produce
tails of gas and dust when they near the sun. Compact ObjectA star that has collapsed to form a white dwarf, neutron
star or black hole. Comparative PlanetologyThe study of planets by comparing the characteristics of
different examples. Condensation SequenceThe sequence in which different materials condense from the
solar nebula as we move outward from the sun. ConstellationsApparent arrangement of stars, usually named after ancient
gods, heroes, animals or mythological beings. Continuity of Energy LawOne of the basic laws of stellar structure, The amount of
energy flowing out of the top of a shell must equal the amount coming in at the bottom
plus whatever energy is generated within the shell Continuity of Mass LawOne of the basic laws of stellar structure. The total mass
of the star must equal the sum of the masses of the shells, and the mass must be
distributed smoothly throughout the star. Continuous SpectrumA spectrum in which there are no absorption or emission
lines. CoronaThe faint outer atmosphere of the Sun that is exposed
during a total solar eclipse. CoronaeOn Venus, circular features, not caused by impacts, they
are domed plains caused by the rising plumes of molten rock from below. CoronagraphA telescope designed to photograph the inner corona of the
sun. Coronal HoleAn Area of the solar surface that is dark at X-ray
wavelengths; thought to be associated with divergent magnetic fields and the source of the
solar wind. Cosmic RayAtomic nuclei that enter earth's atmosphere at nearly the
speed of light. Some originate in solar flares, and some may come from supernova
explosions, but their true nature is not well understood. Cosmological PrincipleThe assumption that any observer in any galaxy sees the
same general features of the universe. CosmologyThe study of the nature, origin and evolution of the
universe. Coude FocusThe focal arrangement of a reflecting telescope in which
mirrors direct the light o a fixed focus beyond the bounds of the telescope's movement.
typically in a separate room, used primarily for spectroscopy. Critical DensityThe average density of the universe needed to make its
curvature flat. Critical PointThe temperature and pressure at which vapor and liquid
phases of a material have the same density. DDark NebulaA nebula consisting of dust and gas blocking our view of
more distant stars. Decameter RadiationRadio signals from Jupiter with wavelengths about 10m Decimeter RadiationRadio signals from Jupiter
with wavelengths about 0.1m DeferentThe larger circle of which an epicycle
revolves (the earth in this case). Degenerate MatterExtremely high density matter in which pressure no longer
depends on temperature due to the quantum mechanical effects. Density Wave TheoryTheory proposed to account for spiral arms as compressions
of the interstellar medium in the disk of the galaxy. Diamond Ring EffectJust as totality begins during a solar eclipse a small
portion of the Suns photosphere can peak out from behind the moon through a valley at the
edge of the lunar disk. It is not visible during every solar eclipse. Differential RotationThe rotation of a body in which different parts of the body
have different periods of rotation. This is true of the sun, Jovian planets, and the disk
of the galaxy. DifferentiationThe separation of planetary material according to density. Diffraction FringeBlurred fringe surrounding and image caused by wave
properties of light. because of this no image detail smaller than the fringe can be seen Dirty Snowball TheoryThe hypothesis that
comets are kilometer-size balls of ices with embedded impurities. Disk ComponentAll material confined to the plane of the galaxy. Distance IndicatorObjects whose luminosities or diameters are known; used to
find the distance to a star cluster or galaxy. Distance ModulusThe difference between the apparent and absolute magnitude
of a star. A measure of how far away the star is. Diurnal MotionThe apparent daily rotation of the sky. Doppler BroadeningThe smearing of spectral lines because of the motion of
atoms in the gas. Doppler EffectA change in the wavelength of radiation due to relative
radial motion of the source and the observer. Double Galaxy MethodA method of finding the masses of galaxies from orbiting
pairs of galaxies. Double StarsA pair of stars close together in the sky. Not all double
stars are necessarily in orbit around each other. Double-Exhaust ModelThe theory that double radio lobes are produced by pairs of
jets emitted in opposite directions from the centers of active galaxies. Double-Line Spectroscopic BinaryA spectroscopic binary star in which spectral lines from
both stars are visible in the spectrum Dwarf NovaA star that undergoes novalike explosions every few days or
weeks; believed to be associated with mass transfer onto a white dwarf in a binary system. Dynamo EffectThe theory that the earth's magnetic field is generated in
the conducting material of its molten core. EEccentricsAn offcenter circular path Eclipse SeasonIs the season when the Sun is close enough to a node for an
eclipse to occur, An eclipse season is 32 days. Any new moon during this period will cause
a solar eclipse. For Lunar eclipses the period is shorter only about 22 days. A full moon
occurring during this time will cause a lunar eclipse. Eclipse YearThe 346.62 days it takes the sun to return to a node Eclipsing BinaryA binary star system in which the stars eclipse each other. EclipticThe apparent path of the Sun around the Sky. EjectaPulverized rock scattered by meteorite impacts on a
planetary surface. ElectroglowThe UV radiation produced in the upper atmosphere of
Jupiter, Saturn, Uranus by high-energy particles in the planet's magnetosphere. Electromagnetic RadiationChanging electric and magnetic fields that travel through
space and transfer energy from one place to another - light, radio waves and magnetism ElectronA low mass atomic particles carrying a negative charge. EllipseA closed curve enclosing two points (foci) such that the
total distance from one focus to any other point on the curve back to the other focus
equals a constant. Elliptical GalaxyA galaxy that is round or elliptical in outline. It
contains little gas and dust, no disk or spiral arms, and a few hot, bright stars. Emission LineA bright line in a spectrum caused by the emission of
photons from atoms. Emission NebulaA cloud of gas excited by UV wavelengths of hot stars, Emission SpectrumA spectrum containing emission lines Energy LevelOne of a number of states an electron may occupy in an
atom, depending on its binding energy. Energy MachineAn object that releases energy. Commonly used to refer to
the source of energy in active galactic nuclei EpicycleAn attempt to explain the retrograde loop in the earth
centered universe, by attaching the planets to epicycles and having them revolve around
that, and in turn around the earth. EquantPtolemy placed the earth offcenter and the opposite point
from the deferent on this off center circle is called the equant. Equatorial MountingA telescope mounting that allows motion parallel to and
perpendicular to the celestial equator. ErgosphereThe region surrounding a rotating black hole within one
could not resist being dragged around the black hole. It is possible for a particle to
escape from the erosphere and extract energy from the black hole. Escape VelocityThe velocity needed to escape from the surface of a body Event HorizonThe boundary of the region of a black hole from which no
radiation may escape. No event that occurs with the event horizon is visible to a distant
observer. Excited AtomAn atom in which an electron has moved from a lower to
higher orbit. ExtinctionThe dimming of light by intervening material; commonly,
dimming by the interstellar medium. EyepieceA short focal length lens used to enlarge the image in a
telescope; the lens nearest the eye FFalse ColorA graphical representation of data in which an images is
colored to reveal additional detail FieldA way of explaining action at a distance. Filar MicrometerAn instrument that permits precise measurements at the
telescope of the position of visual binary stars and similar objects. FiltergramsA photograph (usually of the sun) taken in the light of a
specific region of the spectrum - e.g., an H-alpha filtergram. Fission HypothesisThe theory that the moon formed by breaking away from the
earth. FlareA violent eruption on the sun's surface. Flash SpectrumThe emission spectrum of the chromosphere that is visible
for the few seconds during a total solar eclipse when the moon has covered the photosphere
but has not covered the chromosphere. Flat UniverseA model of the universe in which space-time is not curved. Flatness ProblemIn cosmology the circumstance that the early universe must
have contained almost exactly the right amount of matter to close the space-time (to make
space-time flat) FlocculentWoolly, fluffy; used to refer to certain galaxies that have
a woolly appearance. Focal LengthA distance from a lens to a point where it focuses parallel
rays of light. FocusThe points around which an ellipse is drawn Forward ScatteringThe optical property of finely divided particles to
preferentially direct light in the original direction of the light's travel FrequencyThe number of times a given event occurs in a given time;
for a wave the number of cycles that pass the observer in one second GGalactic CannibalismThe theory that large galaxies absorb smaller galaxies. Galactic CoronaThe low-density extensions of the halo of a galaxy; now
suspected to extend many times the visible diameter of the galaxy. Galilean SatellitesThe four largest moons of Jupiter, named after their
discoverer Galileo. GaussA unit used to measure the strength of a magnetic field. General RelativityObservers can not distinguish locally between inertial
forces due to acceleration and uniform gravitational forces due to the presence of a
massive body. Mass tells space-time how to curve, and the curvature of space-time
(gravity) tells mass how to accelerate Geocentric UniverseAristotle believed the universe was divided into two parts,
the earth corrupt and the heavens perfect and immutable. The geocentric universe described
the universe with the earth at the center. Giant Molecular CloudVery large, cool clouds of dense gas in which stars form. Giant StarsLarge, cool, highly luminous stars in the upper right of
the H-R diagram. Typically 10-100 times the diameter of the sun. Glacial PeriodTime when glaciers advance and engulf huge sheets of land GlitchA sudden change in the period of a pulsar. Globular Star ClusterA star cluster containing 50,000 to 1 million stars in a
sphere about 75 light years in diameter; generally old metal poor and found in the
spherical component of the galaxy. Graben RilleA linear feature on a planetary surface caused by the
faulting and sinking of portions of the crust. GranulationThe fine structure visible on the solar surface caused by
rising currents of hot gas and sinking currents of cool gas below the surface. GratingMaterial onto which microscopic parallel lines are
inscribed. used to create a spectrum of colors from light Gravitational Lens EffectThe focusing of light from a distant galaxy or quasar by an
intervening galaxy to produce multiple images of the distant body. Gravitational Red ShiftA lengthening of the wavelength of a photon due to its
escape from a gravitational field. Gravitational WaveA transport of energy by the motion of waves in a
gravitational field; predicted by general relativity Greenhouse EffectThe process by which a carbon dioxide atmosphere traps heat
and raises the temperature of a planetary surface. Grooved TerrainRegions of the surface of Ganymede consisting of parallel
grooves; believed to have formed by repeated fracture and refreezing of the icy crust. Ground StateThe lowest permitted orbit of an electron in an atom. HH II RegionA region of ionized hydrogen around a hot star. H-R DiagramHalf-LifeThe time required for half of the atoms in a radioactive
sample to decay. HaloThe spherical region of a spiral galaxy containing a thin
scattering of stars, star clusters, and small amounts of gas. Head-Tail Radio GalaxyA radio galaxy with a contour consisting of a head and a
tail; believed caused by the motion of an active galaxy through the intergalactic medium. HeatThermal energy present in a body as agitation (motion)
among its particles (atoms or molecules). Heat of FormationIn planetology, the heat released by the infall of matter
during the formation of a planetary body. Heliocentric UniverseThe Sun is at the center of the universe and everything
revolves around that. Was first proposed by Copernicus. HelioseismologyThe study of the interior of the sun by the analysis of its
modes of vibration. Helium FlashThe explosive ignition of helium burning that takes place
in some giant stars. Herbig-Haro ObjectSmall nebula that vary irregularly in brightness; believed
associated with star formation. Heterogeneous AccretionThe formation of a planet by the accumulation of
planetisimals of different composition - e.g. first iron particles, then silicates High-Velocity StarA star with a large space velocity. Such stars are halo
stars passing through the disk of the galaxy at steep angles. HomogeneityThe assumption that, on the large-scale, matter is
uniformly spread throughout the universe. Homogeneous AccretionThe formation of a planet by the accumulation of
planetisimals of the same composition. Horizontal BranchIn the H-R diagram of a globular cluster, the sequence of
stars extending from the red giants toward the blue side of the diagram; includes RR Lyrae
stars. HoroscopeA chart showing the location of heavenly bodies among the
zodiacal signs and with respect to the horizon at the persons birth. Hot SpotIn radio astronomy, a bright spot in a radio lobe. Hubble's LawThe linear relation between the distance to a galaxy and
its radial velocity. Hubble Constant (H)A measure of the rate of expansion of the universe; the
average value of velocity of recession divided by distance. Hydrostatic EquilibriumThe balance between weight of the material pressing
downward on a layer in a star and the pressure in that layer. IInfrared CirrusA fine network of filaments covering the sky detected in
the far infrared by the IRAS satellite; believed associated with dust in the interstellar
medium. Infrared OutburstA sudden brightening of an object at infrared wavelengths Infrared RadiationElectromagnetic Radiation with wavelengths intermediate
between visible light and radio waves Instability StripThe region of the H-R diagram in which stars are unstable
to pulsation. A star passing through this strip becomes a variable star. Intercrater PlainThe relatively smooth terrain on Mercury. Interglacial PeriodTime when glaciers melt back (cycles are roughly 40,000
years) Interstellar Absorption LinesDark lines in some stellar spectra that are formed by the
interstellar gas. Interstellar MediumThe gas and dust distributed between the stars. Interstellar ReddeningInverse Square LawForce of gravity decreases as the square of the distance
increases Io Flux TubeA tube of magnetic lines and electric currents connecting
Io and Jupiter. IonAn atom that has lost or gained one or more electrons. IonizationThe process in which atoms lose or gain electrons. Irregular GalaxyA galaxy with a chaotic appearance, large clouds of gas and
dust, and both population I and population II stars, but without spiral arms. IsotopeAtoms that have the same number of protons but a different
number of neutrons. IsotropyThe assumption that in its general properties the universe
looks the same in every direction JJoule (J)A unit of energy roughly equivalent to the energy given up
when an apple falls on the floor. Equivalent to the force of 1 Newton acting over a
distance of 1 meter; 1 joule per second = 1 watt of power. Jovian PlanetsJupiterlike planets with large diameters and low densities. Julian DayThe Julian Day is the number of days since the year -4712.
The Julian Day begins at 12:00 Noon Greenwich mean time. Jumbled TerrainStrangely disturbed regions of the moon opposite the
locations of the Imbrium basin and Mare Oriental KKelvin Temperature ScaleThe temperature, in Celsius (Centigrade) degrees, measured
above absolute zero. Keplerian MotionOrbital motion in accord with Kepler's laws of planetary
motion. Kerr Black HoleA solution to the equations of general relativity that
describes the properties of a rotating black hole. Kiloparsec (kpc)A unit of distance equal to 1000 pc or 3260 ly. Kirchoff's LawsA set of laws that describes the Absorption and emission of
light by matter. LLagrangian PointPoints of stability in the orbital plane of a binary
system, planet, or moon. One is located 60 degrees ahead and one 60 degrees behind the
orbiting bodies. Another is located between the orbiting bodies. Large-Impact HypothesisThe theory that the moon formed from debris ejected during
a collision between the earth and a large planetisimal. Light CurveA graph of brightness versus time commonly used in
analyzing variable stars and eclipsing binaries. Light Gathering PowerThe ability of a telescope to collect light. Proportional
to the are of the telescope objective lens or mirror Light-YearIs the distance that light travels in one year.
Abbreviation is ly Lighthouse TheoryA theory that a neutron star produces pulses of radiation
by sweeping radio beams around the sky as it rotates. LimbThe edge of the apparent disk of a body, as in the
"limb of the moon". Limb DarkeningThe decrease in the brightness of the sun or other body
from its center to its limb. Line of NodesThe nodes of the moons orbit are the points where it passes
through the plane of the earth's orbit. An eclipse season occurs whenever the line
connecting these nodes (line of nodes) points toward the sun. Line ProfileA graph of light intensity verses wavelength showing the
shape of an absorption line Liquid Metal HydrogenA form of hydrogen under high pressure that is a good
electrical conductor. Lobate ScarpA curved cliff such as those found on Mercury. Local HypothesisThe theory that quasars are not at great distances but
relatively nearby. Long Period VariableA variable star with a period ranging from 100 days to over
400 days. Look Back TimeThe amount by which we look into the past when we look at a
distant galaxy; a time equal to the distance to the galaxy in light-years. LuminosityThe total amount of energy a star radiates in one second. Luminosity ClassA category of stars of similar luminosity; determined by
the widths of lines in their spectra. Lyman, Balmer and Paschen SeriesSpectral lines in the UV spectrum of hydrogen produced by
transitions whose lowest orbit of electrons is at ground state (Lyman). Spectral lines in
the visible and near UV of hydrogen produced by transitions whose lowest orbit is second
(Balmer). Spectral lines in the infrared spectrum of hydrogen produced by transitions
whose lowest orbit is the third. (Paschen) MMagellanic CloudSmall irregular galaxies that are companions to the Milky
Way; visible in the southern sky. MagnetosphereThe volume of space around a planet within which the motion
of charged particles is dominated by the planetary magnetic field rather than the solar
wind. Magnifying PowerThe ability of a telescope to make an image larger Magnitude ScaleMethod developed by Hipparchus who divided the stars into 6
classes. The brightest stars are first class and those slightly fainter are second class.
The sixth class is the faintest stars that he cold see with the unaided eye. The magnitude
scale is logarithmic like the eye. Main SequenceThe region of the H-R diagram running from upper left to
lower right, which includes roughly 90 percent of all stars. MantleThe layer of dense rock and metal oxides that lies between
the molten core and the surface of the earth; also, similar layers in other planets. MareOne of the lunar lowlands filled by successive flows of
dark lava. MassA measure of the amount of matter in an object Mass FunctionA measure of the ratio of the masses in a single-line
spectroscopic binary. Also includes the inclination, which is unknown for some systems. Maunder Butterfly DiagramA graph showing the latitude of sunspots versus time, first
plotted by W.W. Maunder in 1904. Maunder MinimumA period of less numerous sunspots and other solar activity
from 1645-1715. Megaparsec (Mpc)A unit of distance equal to 1 million pc. MetalsIN astronomical usage, all atoms heavier than helium. MeteorA small bit of matter heated by friction to incandescent
vapor as it falls into the atmosphere MeteoriteA meteor that has survived its passage through the
atmosphere and strikes the ground. MeteoroidA meteor in space before it enters the earth's atmosphere Mid-ocean RiftChasms that split the midocean rises where the crustal
plates move apart. Midocean RiseOne of the undersea mountain ranges that push up from the
seafloor in the center of the oceans. Minute of ArcA measurement of the sky which includes degrees, minutes
and seconds.. There are 60 minutes of arc in one degree. Missing MassUnobserved mass in clusters of galaxies believed to provide
sufficient gravity to bind the cluster together. ModelAn intellectual concept of how nature works MoleculeTwo or more atoms bonded together. MomentumThe measurement of the amount of motion. the product of
mass and velocity Morning and Evening StarsA planet visible in the Morning shortly before sunrise is a
morning star, a planet visible just before sunset is the Evening Star. NNanometer(nm) 1 x 10-9 meters Natural MotionObjects move toward their proper place. earth and water
downward, fire and air upward NebulaA cloud of gas and dust in space. NeutrinoA neutral massless atomic particle that travels at the
speed of light. NeutronAn atomic particles with no charge and about the same mass
as a proton. Neutron StarA small highly dense star composed almost entirely of
tightly packed neutrons; radius about 10 km. Newtonian FocusA focal arrangement of a reflecting telescope in which a
diagonal mirror reflects light out the side of the telescope for easier access NodeThe point twice a month where the moon crosses the
ecliptic. Once a month the moon crosses heading north and two week later crosses again
heading south. North and South Celestial PolesThe north ands south pivots points around which the sky
appears to rotate. NovaFrom Latin meaning "new"; a sudden brightening of
a star, making it appear as a new star in the sky; believed associated with eruptions on
white dwarfs in binary systems. Nuclear BulgeThe spherical cloud of stars that lies at the center of
spiral galaxies. NucleosynthesisThe production of elements heavier than helium by the
fusion of atomic nuclei in stars and during supernova explosions. NucleusThe central core of an atom, containing protons and
neutrons; carries a net positive charge. OObjective LensIn a refracting telescope, the long focal length lens that
forms an image of the object viewed; the lens closest to the object Objective MirrorIn a reflecting telescope the principle mirror (reflecting
surface) that forms an image of the object viewed Oblate SpheroidA sphere flattened such that its polar diameter is smaller
than its equatorial diameter. OblatenessThe flattening of a spherical body; usually caused by
rotation OccultationThe passage of a larger body in front of a smaller body. Olber's ParadoxThe conflict between observation and theory as to why the
night sky should or should not be dark. OpacityThe resistance of a gas to the passage of radiation. Open (Escape) OrbitAn orbit which leads away from the central body, never to
return Open Star ClusterA cluster of 10 to 10,000 stars with an open, transparent
appearance. The stars are not tightly grouped. Usually relatively young and located in the
disk of the galaxy. Open UniverseA model universe in which the average density is less than
the critical density needed to halt the expansion of the universe. Optical BinaryA binary star in which the stars are only apparently
associated. One star is nearby and one is more distant. Oscillating Universe TheoryThe theory that the universe begins with a big bang,
expands, slows by its own gravity, collapses to create another big bang. OutgassingThe release of gas from a planets interior. Ozone LayerIn earth's atmosphere, the layer of oxygen ions (O3) lying
15 to 30 km high that protects the surface by absorbing ultraviolet rays. PParallaxThe apparent change in the position of an object due to a
change in the location of the observer. It was because ancient astronomers did not observe
parallax that they though the earth was the center of the universe and everything in the
heavens revolved around it. ParsecA hypothetical distance to a star whose parallax is one
arcsecond; 1pc = 206,265 A.U. = 3.26 ly. Partial Eclipse (lunar or Solar)Partial eclipses are caused when the moon passes through
only part of the umbra shadow of the earth. Or when the earth passes through the only part
of the umbra shadow of the moon. Path of TotalityThe path of a total eclipse that is swept out by the umbra
shadow of the moon on the earth. PenumbraA Partial blocking of the Sun by the Earth creates an
Penumbra Shadow. The Sunlight is dimmed but not extinguished. Penumbra EclipseWhen the moon passes only through the penumbra shadow of
the earth or when the earth passes through only the penumbra shadow of the moon. PerigeePoint of closet approach to the earth PerihelionPoint in earth's orbit around the sun where it is closest
to the Sun (winter for us) Period-Luminosity DiagramA graph showing the relation-between period of pulsation
and intrinsic brightness among Cepheid variable stars. PhotometerA device used to measure the intensity and color of light PhotonA quantum of electromagnetic energy Carries an amount of
energy that depends inversely on its wavelength PhotosphereThe bright disk of the sun that is covered completely by a
total Solar eclipse. Planet MotionAlways found near the ecliptic (except Pluto) Planetary NebulaAn expanding shell of gas ejected by a star in the latter
stages of its evolution. PlanetisimalOne of the small bodies that formed from the solar nebula
and eventually grew into protoplanets. PlasticA material with properties of a solid but capable of
flowing under pressure. Plate TectonicsThe constant destruction and renewal of the earth's surface
by the motions of sections of the crust. Polar AxisThe axis of a telescope around which the celestial sphere
rotates Poor Galaxy ClusterAn irregularly shaped cluster that contains fewer than 1000
galaxies, many spiral, and no giant ellipticals. Population IStars rich in atoms heavier than helium; nearly always
relatively young stars found in the disk of the galaxy. Population IIStars poor in atoms heavier than helium; nearly always
relatively old stars found in the halo, globular clusters, or the nuclear bulge. Position AngleThe angular direction of one body with respect to another;
measured from north toward the east; typically used in the study of visual binaries. PrecessionA wobbling of the Earth's Axis. It takes 26,000 years for
the earth to complete one wobble. Pressure (P) WavesIn geophysics, mechanical waves of compression and
rarefaction that travel through the earth's interior Primary MinimumIn the light curve of an eclipsing binary, the deeper
eclipse. Prime FocusThe point at which an object mirror forms an image in a
reflecting telescope Primeval AtmosphereEarth's first air, composed of gases from the solar nebula. Primordial Background RadiationRadiation from the hot clouds of the big-bang explosion.
Because of its large red shift it appears to come from a body whose temperature is only
2.7K Prolate SpheroidA sphere stretched along its polar axis so its polar
diameter is greater than its equatorial diameter. ProminenceEruptions of the solar surface. Visible during total solar
eclipses. Proper MotionThe rate at which a star moves across the sky. Measured in
arc seconds per year. ProtonA positively charged atomic particle contained in the
nucleus of an atom. The nucleus of hydrogen atom. Proton-Proton ChainA series of three nuclear reactions that build a helium
atom by adding together protons. The main energy source in our sun. ProtoplanetMassive object resulting from the coalescence of
planetisimals in the solar nebula and destined to become a planet. ProtostarA collapsing cloud of gas and dust destined to become a
star. PulsarA source of short, precisely times radio bursts believed to
be spinning neutron stars. QQuantum MechanicsThe study of behavior of atoms and atomic particles. QuasarSmall powerful source of energy believed to be the active
core of very distant galaxies. Quasi-Periodic Object (QPO)Certain X-rays sources that "flicker" rapidly for
short intervals. RRadial Velocity(Vr) That component of an object's velocity directed away
from or toward the earth. Radial Velocity CurveA graph of the velocity of recession or approach of the
stars in a spectroscopic binary. Radiation PressureThe force exerted on the surface of a body by its
absorption of light. Small particles floating in the solar system can be blown outward by
the pressure of sunlight. Radio GalaxyA galaxy that is strong source of radio signals. Radio InterferometerTwo or more radio telescopes that combine their signals to
achieve the resolving power of a larger telescope RayEjecta from meteorite impacts forming white streamers
radiating from some lunar craters. RecombinationThe stage within 1 million years of the big bang when the
gas became transparent to radiation Recurrent NovaStars that erupt as nova every few dozen years. Red DwarfCool, low mass stars on the lower main sequence Reflecting TelescopeA telescope which uses a concave mirror to focus light into
an image Reflection NebulaA nebula produced by starlight reflecting off dust
particles in the interstellar medium. Refracting TelescopeA telescope that forms images by bending light through an
objective lens RegolithA soil made up of crushed rock fragments. Relative AgeThe age of a geological feature referred to other features.
For example, relative ages tell us the lunar maria and younger than the highlands. Relativistic Jet ModelAn explanation of superluminal expansion based on a high
velocity jet from a quasar directed approximately toward the earth. Relativistic Red ShiftThe red shift due to Doppler effect for objects traveling
near the speed of light. Resolving PowerThe ability of a telescope to reveal fine detail. depends
on the diameter of the telescope objective ResonanceThe coincidental agreement between two periodic phenomena;
commonly applied to agreements between orbital periods, which can make orbits more or less
stable. Retrograde loopThe planets sometimes speed up in their movements, slow
down, stop and even reverse direction. The motion traces out a retrograde loop. Rich Galaxy ClusterA cluster containing over 1000 galaxies, mostly elliptical,
scattered over a volume about 3 Mpc in diameter. Rift ValleyA long, straight, deep valley produced By the separation of
crustal plates. Ring GalaxyA galaxy that resembles a ring around a bright nucleus;
believed to be the result of a head-on collision of two galaxies. Roche LimitThe minimum distance between a planet and a satellite that
holds itself together by its own gravity. If a satellites orbit brings it inside the Roche
limit, tidal forces will break the satellite up. Rolling PlainsThe most common type of terrain on Venus. Rotation CurveA graph of orbital velocity versus radius in the disk of a
galaxy. RR Lyrae VariableVariable stars with periods of 12-24 hours, common in some
globular clusters. SSagittarius AThe powerful radio source located at the core of the Milky
Way. Saros CycleAfter 18 years and 11 1/3 days the eclipse pattern repeats
The saros cycle does not mean the eclipse will occur in the same place. Sine the cycle
take one third of a day. The earth will have rotated 8 hours westward from the original
location. It takes 3 saros cycles for the eclipse to repeat in the exact same place or 54
years and 1 month. Schmidt CameraA photographic telescope that take wide angle photographs Schmidt Cassegrain TelescopeA cassegrain telescope that uses a thin correcting lens as
in a schmidt camera Schwartzchild RadiusThe radius of the event horizon around a black hole. Scientific NotationA method of writing large numbers in a simple way. For
example 380,000 is written 3.8 x106. In scientific notation a number is always written as
a x 10 h Where a >= 1 and a <10. SeasonsCaused by the tilt of the earth's Axis 23.5 degrees Second of ArcSee Minute of Arc. 60 seconds of arc make up one minute. Secondary AtmosphereThe gases outgassed from a planet's interior; rich in
carbon dioxide. Secondary MinimumIn the light curve of an eclipsing binary, the shallower
eclipse Secondary MirrorIn a reflecting telescope, the mirror that reflects the
image to a point for easy observation SeeingAtmospheric conditions on a given night When the atmosphere
is unsteady, producing blurred images the seeing is considered poor Seismic WavesA mechanical vibration that travels through the earth.
Usually caused by an earthquake. SeismographAn instrument that records seismic waves. Self-Sustaining Star FormationThe process by which the birth of stars compress the
surrounding gas clouds and triggers the formation of more stars, proposed to explain
spiral arms Semimajor AxisHalf the long diameter of an ellipse Seyfert GalaxyAn otherwise normal spiral galaxy with an unusually bright,
small core that fluctuates in brightness; believed to indicate the core is erupting. Shear (S) WavesMechanical waves that travel through earth's interior by
the vibration of particles perpendicular to the direction of the wave travel. Shepherd SatelliteA satellite that, by its gravitational field, confines
particles to a planetary ring. Shield VolcanoesWide, low profile volcanic cones produced by highly liquid
lava. Shock WaveA sudden change in pressure that travels as an intense
sound wave. Sidereal DriveThe motion and gears on a telescope that turn westward to
keep it pointing at a star Sidereal Period(Sidereal Month) 27.5 Days - Actual Lunar orbit around the
earth. 1 revolution with respect to the stars. Moon drifts eastward by 13 degrees per day. Single-Line Spectroscopic BinaryA spectroscopic binary in which lines of one star are
visible in the spectrum. SingularityThe object of zero radius into which the matter of a black
hole is believed to fall. Sinuous RilleA narrow, winding valley on the moon caused by ancient lava
flows along narrow channels. Smooth PlainApparently young plains on Mercury formed by lava flows at
or soon after the formation of the Caloris Basin Solar ConstantA measure of the energy output of the sun. The total solar
energy striking 1 sq. meter just above the earth's atmosphere in 1 second. Solar Nebula TheoryThe theory that the planets formed from the same cloud of
gas and dust that formed the sun. Solar WindRapidly moving atoms and ions that escape from the solar
corona and blow outward through the solar system. Special RelativityObservers can never detect their uniform motion except
relative to other objects. The velocity of light is constant for all observers Spectral ClassA star's position in the temperature classification system
O,B,A,F,G,K, and M. Based on the appearance of the star's spectrum. Spectral SequenceThe arrangement of spectral classes (O,B,A,F,G,K, and M)
ranging from hot to cool stars. SpectrographDevice that separates light by wavelengths to produce a
spectrum Supernova Type IThe explosion of a star believed to be caused by mass
transfer to a white dwarf. Supernova Type IIThe explosion of a star believed to be caused by the
collapse of a massive star. Synchrotron RadiationRadiation emitted when high speed electrons move through a
magnetic field. Synodic Period(Synodic Month) - 29.5 days - One revolution with respect
to the Sun. This is the time frame that determines lunar phases. Used as the basis for the
first Roman Calendar. TT Tauri StarsYoung stars surrounding by gas and dust. Believed to be
contracting toward the main sequence. TemperatureA measure of the velocity of random motions among atoms or
molecules in a material. TerminatorThe dividing line between daylight and darkness on a planet
or moon. Terrestrial PlanetsEarthlike planets - small, dense, rocky. Tidal CouplingThe locking of the rotation of a body to its revolution
around another body. Tidal HeatingThe heating of a planet or satellite because of friction
caused by tides. Time DilationThe slowing of time in curved space time, believed to occur
as one approaches the speed of light or crosses the even horizon of a black hole. Titius-Bode RuleA simple series of steps that produces numbers
approximately matching the sizes of the planetary orbits. Total Eclipse (lunar or Solar)When the umbra part of the Earth's Shadow (lunar eclipse)
totally blocks the light being received by the moon, these can only occur during a Full
Moon. A Total Solar Eclipse is the earth passing through the umbra shadow of the moon and
can only occur during a New moon. TransitionThe movement of an electron from one atomic orbit to
another. Transverse VelocityThe velocity of a star perpendicular to the line of sight. Triaxial EllipsoidA geometrical solid whose three axes are equal. Triple Alpha ProcessThe nuclear fusion process that combines three helium
nuclei (alpha particles) to make one carbon nucleus. True Relative OrbitThe orbit of one star in a visual binary with respect to
the other star after correction for orbital inclination. Tuning Fork DiagramA system of classification for elliptical, spiral and
irregular galaxies. Turnoff PointThe point on the H-R diagram where a cluster's stars turn
off the main sequence and move toward the red giant region revealing the approximate age
of the cluster. UUltraviolet RadiationElectromagnetic Radiation with wavelengths shorter than
visible light but longer than X-rays UmbraIs the Earth shadow that is total. No part of the Sun can
be seen when in the umbra shadow Uncompressed DensityThe density a planet would have if its gravity did not
compress it. Uniform circular motionPlato argued that the most perfect form should be a circle
and therefore motions of the heavens should be made of up combinations of circular motion.
The most perfect motion would be uniform circular motion, so astronomers of ancient times
tried to describe the motions of the heavens in these terms UniversalityThe assumption that the physical laws observed on earth
apply throughout the universe. VVan Allen BeltsRadiation belts of high-energy particles trapped in the
earth's magnetosphere. Variable StarA star whose brightness changes periodically. VelocityA directed rate of motion Velocity Dispersion MethodA method of finding a galaxy's mass by observing the range
of velocities within the galaxy. Vernal EquinoxThe place where the Sun crosses the celestial equator
heading North. Very Long BaselineThe use of radio telescopes located thousands of miles
apart to resolve detail in radio sources. Vesicular BasaltA porous rock formed by solidified lava with trapped
bubbles. Violent MotionMotion other than natural motion Visual BinaryA binary star system in which two stars are separately
visible in a telescope. WWavelengthThe distance between successive peaks or troughs of a wave Wavelength of MaximumThe wavelength at which a perfect radiator emits the
maximum amount of energy; depends only on the object's temperature. White Dwarf StarsDying stars that have collapsed to the size of the earth
and are slowly cooling off; at the lower left of the H-R diagram. Winter SolsticeThe point on the ecliptic where the Sun reaches it most
southern point. ZZeeman EffectThe splitting of spectral lines into multiple components
when the atoms are in a magnetic field. ZenithPoint in the Sky that is directly overhead. Zero-Age Main Sequence (ZAMS)The locus in the H-R- diagram where stars first reach
stability as hydrogen burning stars. ZodiacThe 12 constellations near the ecliptic through which the
Sun passes. Zone of AvoidanceA region around the Milky Way where almost no galaxies are
visible because our view is blocked by dust in our galaxy. ZonesYellow-white regions that circle Jupiter parallel to its
equator; believed to be areas of rising gas. Compiled by Don Ware 05/25/2015
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