ABSOLUTE MAGNITUDE: The apparent brightness of a celestial object when observed at some standard distance. The absolute magnitude of a star is its apparent brightness at a distance of ten parsecs. The absolute magnitude of a Solar System object is its apparent brightness at a distance on one astronomical unit.

ABSORPTION LINES: Dark bands or gaps at specific wavelengths within the electromagnetic spectrum. Absorption lines are produced by gases which are opaque to light. They can be used to identify the chemical composition of an object.

ACCRETION DISK: A disk of gas orbiting a star into which matter flows from a companion star or other celestial object.

ACHONDRITE: A type of meteorite. Achondrites make up 9% of the known meteorite population. They contain no chondrules and show evidence of differentiation. Many are composed of basalt and bear a striking similarity to Earth's mantle.

ACTIVE/ADAPTIVE OPTICS: Technology allowing astronomers to compensate for instability in Earth's atmosphere by reshaping the figure, or curvature, of a telescope's primary or secondary mirror. In this way, the telescope is able to produce sharper and more detailed images than conditions would otherwise allow.

ALBEDO: The reflectivity of a celestial object. For example, the Moon's albedo is 7%. In other words, only 7% of the sunlight hitting the Moon is reflected. By contrast, Earth's albedo is 40%.

ALTITUDE: The angular distance of an object from your local horizon. For example, an object at the zenith would have an altitude of +90 degrees. An object exactly halfway from the horizon to the zenith would have an altitude of +45 degrees.

ANNULAR: Having the form of a ring. For example, an annular eclipse of the Sun occurs when the Moon's umbra is smaller than the angular size of the Sun. Thus, it's possible for the Moon to block just the inner portion of the solar disk, leaving an outer ring--an annulus--visible.

APERTURE: The diameter of a telescope's primary lens or mirror.

APHELION: The point in an orbit farthest from the Sun.

APOGEE: The point in an orbit farthest from Earth.

APPARENT MAGNITUDE: The brightness of a celestial object as measured on the magnitude scale. The apparent magnitude of an object can vary depending on the method used in the observation. Astronomers use CCD cameras with filters designed to pass blue (B), visual (V), red (R) and infrared (I) radiation. Thus, when talking about the apparent magnitude of an object, it is important to know the wavelength range in which the object was observed. For example, many galaxies have been measured for B magnitude. However since galaxies include stars across the full range of the color spectrum, their V magnitudes are often about one magnitude brighter than their B magnitudes.

ASTERISM: A stellar grouping which forms a familiar or recognizable pattern. For example, the Big Dipper is an asterism in the constellation, Ursa Major.

ASTEROID: A minor planet composed primarily of rock. The largest asteroid is Ceres with a diameter of about 570 miles. Most asteroids orbit the Sun between Mars and Jupiter in the main asteroid belt.

ASTROLOGY: The ancient art of interpreting the influence of stars and planets on people's lives.

ASTROMETRY: The study of the positions of celestial bodies.

ASTRONOMICAL UNIT: The length of the semimajor axis of Earth's orbit about the Sun. This distance is roughly 150 million kilometers or 93 million miles.

ATMOSPHERE: The gases surrounding a celestial body. The composition of Earth's atmosphere is 78% nitrogen, 21% oxygen and 1% other gases.

AURORA: A colorful light display in the upper atmosphere. Aurorae are produced by the interaction of solar particles with Earth's magnetic field.

AVERTED VISION: The observing technique in which you look at an object using the light-sensitive area of the retina offset about 18-degrees from the fovea centralis.

AZIMUTH: The angular distance of an object around your local horizon. Azimuth increases from north to east, south and west around the horizon. For example, an object positioned due east along the horizon would have an azimuth of 90 degrees which an object stationed due west would have an azimuth of 270 degrees.

BELTS: Strips of darker, lower altitude, higher temperature clouds in Jupiter's atmosphere.

BIG BANG: A theory of the creation of the universe. This theory assumes that all matter and space were initially compressed into a very small volume, then began expanding in an event called the Big Bang. The big bang theory has wide support in the scientific community because it has successfully predicted and explained many observed phenomena in the universe.

BL LACERTAE OBJECT: A radio galaxy oriented such that the jet of material emitting from the core is directed at Earth. BL Lacertae, a stellar appearing object which varies between 14th and 17th magnitude over the course of several minutes, was initially believed to be a star. However, it has been determined to be the active core of a distant elliptical galaxy. Centaurus A, the closest of the big radio galaxies, might appear as a BL Lacertae object to an observer in another galaxy.

BLACK HOLE: An object of finite mass and infinite density, from which light cannot escape.

BODE'S LAW: A numerical relationship which predicts the general positions of the seven planets closest to the Sun and the main asteroid belt. Begin with the number, 0, add three, then double for each of the succeeding numbers: 0, 3, 6, 12, 24, etc. Then, add four to each number and divide by ten. This produces a sequence of numbers in rough agreement with the semimajor axes as measured in astronomical units for Mercury, Venus, Earth, Mars, the asteroid belt, Jupiter, Saturn and Uranus.

BOLIDE: A fireball (-3 magnitude or brighter meteor) which ends in a visible explosion or burst.

BRECCIA: A rock consisting of fused pieces of smaller rocks.

BRECCIATION: Having been broken apart and reassembled.

BROWN DWARF: A failed star. Brown dwarfs are thought to form in the same manner as stars, however their mass is less than 0.08 times that of the Sun and, therefore, brown dwarfs are not able to sustain a core hydrogen burning fusion reaction.

BULGE: The thick core of a spiral galaxy where the disk and halo regions merge.

CARBONACEOUS CHONDRITE: A type of meteorite. Approximately 5% of all known meteorites fall into this category. Carbonaceous chondrites are dark stony meteorites showing no evidence of differentiation or brecciation. They are also rich in carbon and water. These appear to be remnants of the early Solar System.

CARBON STAR: A star with at least as much carbon as oxygen. Carbon stars have an intense red coloration.

CASSINI DIVISION: An apparent dark gap in Saturn's rings. The Cassini division falls between Saturn's A-ring and B-ring. Data from the Voyager I and Voyager II missions revealed that the Cassini division is not a void but a zone where the population of dust particles is much lower than either the A-ring or B-ring. The Cassini division was discovered by Giovanni Cassini in 1675.

CATADIOPTRIC TELESCOPE: A telescope that uses mirrors and lenses to collect and focus light.

CELESTIAL COORDINATES: The coordinate system used to describe the positions of celestial objects. Based on the Earth system of latitude and longitude, there are two components to an object's celestial coordinates, right ascension and declination.

CELESTIAL EQUATOR: The great circle marking the halfway point between the celestial poles. The celestial equator is Earth's equator extended outward against the celestial sphere. Any object along the celestial equator has a declination of 0 degrees. The belt stars in Orion reside along the celestial equator and can be seen throughout the civilized world.

CELESTIAL POLES: Points where Earth's poles would intersect with the celestial sphere if extended toward infinity. The north celestial pole resides at a declination of +90 degrees. Polaris resides less than one degree from the celestial north pole, hence its nickname the North Star. The south celestial pole is at a declination of -90 degrees. There are no bright stars near this position.

CELESTIAL SPHERE: The imaginary sphere of stars surrounding the Earth.

CEPHEID VARIABLE: A variable star whose period of variability is directly proportional to its mass. Cepheid variables are a valuable tool to astronomers. Measuring the period of a Cepheid variable allows one to calculate the star's mass and, therefore, intrinsic brightness. A comparison of the intrinsic and apparent brightnesses leads to an estimate of the star's distance. Cepheid variables provided the first concrete evidence that spiral nebulae are distant galaxies beyond the Milky Way.

CHANDRASEKHAR LIMIT: The theoretical upper limit on the mass of a white dwarf, 1.4 solar masses. A white dwarf exceeding the Chandrasekhar limit will go supernova.

CHARGE-COUPLED DEVICE (CCD): A computer chip with light-sensitive rows and columns of pixels. Modern astronomical cameras use CCDs to collect and store light from celestial objects over the length of an exposure. This data is then translated into images.

CHONDRITE: The most common type of meteorite. Chondrites are composed of a fine-grained silicate rock matrix. Most also show small spherical inclusions called chondrules. Chondrites show evidence of differentiation and brecciation. 86% of all known meteorites are classified as chondrites.

CHONDRULE: A spherical inclusion found in chondritic meteorites. Chondrules are composed of olivine or pyroxene.

CHROMATIC ABERRATION: A defect of optical lenses. Different wavelengths of light have different refractive properties. Therefore, an optical lens will different wavelengths of light to focus at different points. This produces colorful halos around bright stars and planets and a reduction in contrast, the visible effects of chromatic aberration.

CHROMOSPHERE OF THE SUN: The layer between the photosphere and the corona. The chromosphere, the "sphere of color," is only a few thousand kilometers thick and can only be seen during an eclipse or with special equipment. It appears as a thin reddish glow around the eclipsed Sun.

CIRCUMPOLAR: A celestial object positioned near a celestial pole such that, from your latitude, it never goes below the horizon.

COLLIMATION: The proper alignment of mirrors or lenses in an optical system.

COLOR INDEX: The quantification of a star's color. Color index is determined by subtracting the V (visual) magnitude from the B (blue) magnitude. A white star with a temperature of 9200 K will have a color index (B-V) of 0.0. Blue stars have slightly negative color indexes. Yellow, orange and red stars have increasingly positive color indexes. The full range runs from about -0.4 (blue) to +2.0 (red carbon stars).

COMA (as related to comets): A cloud of gas and dust particles forming an atmosphere around a comet. The coma appears as a diffuse sometimes yellowish glow.

COMA (as related to mirrors): A defect of optical mirrors. The imperfect figure of a mirror causes light reflected from the center to come to focus at a different point than light reflected outside the center of the mirror. Coma produces fuzzy and elongated star images outside the center of the field of view and is inversely proportional to the focal ratio of an optical system. Therefore, the faster the mirror the greater the coma.

COMET: A minor planet with exposed ices or volatiles. A comet's volatiles produce a coma and tail through solar heating and interaction with the solar wind.

CONJUNCTION: Any two or more celestial objects aligned with each other are said to be in conjunction.

CONSTELLATION: One of 88 sections into which the sky is officially divided. Common examples include the historical and mythological figures that populated the heavens of ancient societies. Ursa Major (the Great Bear) is a constellation.

CORONA OF THE SUN: The Sun's outer halo. Temperatures in the solar corona average 2 million Kelvins. However, the low density of the corona makes it optically dim, about as bright as the Moon. The only way to observe the corona visually is when the Sun is occulted, such as during an eclipse or through the use of a coronagraph.

COSMIC MICROWAVE BACKGROUND: The afterglow of the big bang. The cosmic microwave background (CMB) permeates the universe, glowing faintly at 3 Kelvins. The CMB, predicted to exist as an aftereffect of an expanding universe which was once confined to a small volume, was discovered in the early 1960's by Arno Penzias and Robert Wilson. This phenomenon provides evidence in support of the big bang theory.

COSMIC RAYS: High energy particles, mostly protons and helium nuclei, which permeate the universe. Cosmic rays are believed to be the product of supernovae, cataclysmic events which destroy stars and produce exotic particles.

COSMOLOGICAL CONSTANT: Postulated by Albert Einstein, the cosmological constant is a repulsive force which grows stronger as distance increases. Einstein introduced the cosmological constant to allow for a static universe, one that neither expands nor contracts. Years later after conclusive evidence had been gathered that the universe is expanding, Einstein referred to the cosmological constant as the greatest mistake of his career. However since the late 1990's, astronomers have gathered evidence indicating the universal expansion is accelerating. As a result, belief in a repulsive force such as that postulated Einstein has gained renewed support.

COSMOLOGY: The study of the origin, history and evolution of the universe.

CREPE RING: Saturn's innermost ring. Also known as the C-ring, the crepe ring is not nearly as opaque as the A- and B-rings. In fact when at a favorable angle, Saturn's disk can be viewed through the crepe ring. While Saturn's rings were first observed by Galileo in 1610, the charcoal grey crepe ring was not discovered until 1850.

DARK MATTER: Unilluminated mass detectable only through its gravitational interaction with normal matter. Normal matter can account for only about 10% of all matter in the universe. The remaining 90%--unobserved thus far--is considered dark matter.

DARKNESS: One of three measurements of observing conditions. Darkness simply refers to the brightness of the sky. The night sky can only get so dark as seen from Earth. Even from the most remote locations, the sky brightness will measure no lower than 22.0 magnitude per square arcsecond. During a full Moon, the sky above a remote site will appear nearly 50-times brighter than it does on a moonless night.

DECLINATION: One of two coordinates describing a celestial object's position on the celestial sphere. Declination is the celestial equivalent of latitude. An object's declination is its position north or south of the celestial equator as measured in degrees, minutes and seconds. The north celestial pole, an extension of the Earth's north pole, has a declination of +90 degrees. The south celestial pole is at -90 degrees declination. Any object located along the celestial equator has a declination of 0 degrees.

DEFERENT: A construct of the Ptolemaic system, the deferent is the circular path around the Earth or, more accurately, the equant.

DIFFERENTIATION: The separation of a planet's interior into layers of different composition and density.

DIFFRACTION: An effect of the wave nature of light, diffraction occurs when light passes through an aperture or opening. Light from a point source such as a star will be focused, not to a point, but to a diffraction disk. The diffraction disk appears as a central point surrounded by concentric diffraction rings of light. The size of the diffraction disk is dependent upon telescope aperture and seeing conditions.

DOPPLER EFFECT: The observed shift in wavelength or frequency as a result of relative radial motion.

DRAKE EQUATION: Developed by astronomer Frank Drake, the Drake equation attempts to quantify the probability that intelligent life exists elsewhere in the universe: P = f_p x n_h x f_l x f_i x f_t.

In the equation, the probability of intelligent life (P) depends on several variables. The first variable (f_p) is the fraction of stars with planets. The second (n_h) is the fraction of stars with planets orbiting within the "life zone" where liquid water can exist. The third (f_l) is the fraction of planets in the life zone harboring life. The fourth variable (f_i) represents the fraction of life harboring planets where intelligent life has evolved. The final variable (f_t) represents the expected lifespan of intelligent civilizations as a fraction of the age of the universe.

DUST TAIL: One of two types of tails associated with comets. As a comet approaches the Sun, solar energy releases ice and dust particles. These particles form a diffuse shell around the comet. The dust particles lag behind the comet tracing its orbital path, scattering sunlight and forming a dust tail.

DWARF: A stellar classification representing the main band on the Hertzsprung-Russell diagram. The Sun is a yellow dwarf star.

ECCENTRICITY: The degree of flattening of an ellipse, measured from 0 to 1. An ellipse with an eccentricity of 0 is a circle. Celestial objects in orbits with large eccentricities (close to 1.0) have highly flattened or elongated orbits.

ECLIPTIC: The apparent path of the Sun against the background stars. This path takes the Sun through the constellations of the zodiac.

ECLIPSE: An event where one celestial body passes through the shadow of another celestial body. An eclipse of the Sun occurs when the Earth passes through the Moon's shadow. An eclipse of the Moon occurs when the Moon passes through Earth's shadow.

ELLIPSE: A closed curve defined by two foci such that the sum of the distance from any point on the curve to each foci is constant. All orbits are assumed to be elliptical.

ELLIPTICAL GALAXY: A population II galaxy without spiral arms. M84 and M86 in Virgo are examples of elliptical galaxies.

ELONGATION: The angular distance separating any two or more celestial objects. Since Mercury and Venus are closer to the Sun than Earth, they never get very far from the Sun. Mercury's greatest elongation from the Sun is just 22 degrees. Venus' is somewhat greater at 46 degrees. By contrast, planets outside Earth's orbit reach solar elongations of 180 degrees when at opposition.

EMISSION NEBULA: A bright cloud of self-illuminating interstellar gas. Hot young stars within the cloud pump out intense ultraviolet radiation which ionizes the gas, causing it to fluoresce. M17, the Swan nebula, is a classic example.

EMISSION SPECTRUM: Bands of light emitted by rarefied gas at specific wavelengths. A gas's emission spectrum is unique and, therefore, can be used to identify that gas.

EQUINOX: A point where the Sun and ecliptic cross the celestial equator. The vernal equinox, when the Sun is moving northward along the ecliptic, is often referred to as the first day of spring in the Northern Hemisphere. The autumnal equinox, when the Sun is moving southward, is considered the first day autumn in the Northern Hemisphere.

EYEPIECE: A lens designed to make light rays parallel so they can be viewed by the eye.

FIREBALL: A very bright meteor. A fireball is generally considered to be -3 magnitude or brighter. A fireball which appears to burst or explode is called a bolide.

FOCAL LENGTH: The distance from the primary objective to the focal point in an optical system.

FOCAL RATIO: The relationship of a telescope's focal length to its aperture. An 80-mm refractor with a 900-mm focal length has a focal ratio of 11. This is written as f/11. Fast telescopes typically have focal ratios in the f/4 to f/6 range. Sometimes referred to as a richest field telescope (RFT), a fast telescope provides the widest possible field of view for a given aperture. Slow focal ratio telescopes usually fall in the f/8 to f/15 range and are sometimes called normal field telescopes (NFT).

GEGENSCHEIN: Brightening of the zodiacal light in the direction opposite the Sun.

GIANT: A large star which is brighter than the main sequence for a given spectral class. Giants are plotted just above the main sequence on the H-R diagram.

GLOBULAR STAR CLUSTER: A rich cluster containing tens of thousands to over a million stars. Globular clusters populating the outer galactic halo contain low metalicity population II stars, some among the oldest known stars in the universe. M13 in Hercules is a spectacular sight in a 10-inch telescope.

GREAT RED SPOT: A large oval on the southern edge of Jupiter's South Equatorial Belt. The Great Red Spot is a high pressure anticyclone which varies in color from pale yellow to crimson.

HERTZSPRUNG-RUSSELL DIAGRAM: A plot of stellar magnitudes against spectral types.

HUBBLE CLASSIFICATION SCHEME: A system for classifying galaxies based on physical appearance and structure. Classifications include elliptical, irregular, peculiar and spiral.

HUBBLE CONSTANT (H₀): The expansion rate of the universe as measured in kilometers per second per megaparsec. Observations with the Hubble Space Telescope indicate an expansion rate of approximately 65 kilometers per second per megaparsec. However, recent observations suggest the expansion rate may be accelerating.

HUBBLE, EDWIN (1889-1953): An American astronomer who revolutionized our understanding of the nature and scale of the universe. Hubble used the 100-inch Hooker telescope on Mount Wilson to prove that the "spiral nebulae" are distant galaxies far outside the Milky Way. Hubble also discovered the radial velocity-distance relation, which led to the theory of the expanding universe.

INCLINATION: The angle an orbital plane makes with respect to the ecliptic plane.

INFERIOR CONJUNCTION: An alignment of the Sun and any two planets such that an observer on the outer planet sees the inner planet positioned between himself and the Sun. For example, Venus is at inferior conjunction when it is positioned between Earth and the Sun.

INFLATION: The theory which states that the universe underwent a brief period of super-rapid expansion immediately after the big bang. One of the outcomes of an inflationary universe is that the observable portion is just a very small part of the whole. As a result, the observable universe should appear flat.

IRREGULAR GALAXY: A galaxy lacking visible organized structure. Irregular galaxies often show evidence of rapid star formation. M82 in Ursa Major is a classic example of an irregular galaxy.

KIRKWOOD GAPS: Gaps in the distribution of asteroids within the main belt, which were first discovered by Daniel Kirkwood in 1866. Kirkwood gaps occur where resonances with Jupiter produce unstable orbits.

KUIPER BELT: The disk-shaped region of the outer Solar System where periodic comets (orbital period < 200 years) originate. Gerard Kuiper postulated the existence of this region to explain the observed distribution of short-period comets.

LIGHT-GATHERING POWER: The amount of light collected by a mirror or lens. Light-gathering power is determined by the primary objective's surface area. A doubling of aperture produces a four-fold increase in light-gathering power.

LIGHT-YEAR: The distance light travels in one year. Light travels at approximately 186,000 miles (300,000 kilometers) per second, which makes a light-year roughly 6 trillion miles (9.5 trillion kilometers) in distance.

LIMITING MAGNITUDE: The faintest stellar object visible. A telescope's limiting magnitude is directly proportional to its light-gathering power. The larger a telescope's aperture, the fainter its limiting magnitude.

LOCAL GROUP: A group of approximately 33 galaxies including the Milky Way, M31 and M33, which are gravitationally bound to each other. The local group is approximately 5 million light-years across. Most local group galaxies are dwarf ellipticals and irregulars associated with M31 or the Milky Way.

MAGNIFICATION: The focal length of the telescope divided by the focal length of the eyepiece. A 10-mm eyepiece in a telescope with a 900-mm focal length yields 90X magnification. The longer a telescope's focal length, the more magnification a given eyepiece will produce.

MAGNITUDE: The logarithmic scale used to quantify the brightness of celestial objects. Each change of five magnitudes is equivalent to a 100-fold change in brightness. Each change of one magnitude equals roughly a 2.52 change in brightness.

MARE: A dark area on the Moon. Most maria are impact basins filled with solidified molten rock.

MESSIER, CHARLES (1730-1817): A French astronomer and comet hunter who prepared one of the earliest catalogs of nebulous objects and star clusters. The entire Messier catalog of 110 star clusters, nebulae and galaxies can be seen with a 3-inch telescope under dark skies.

METEOR: The light display resulting when a meteoroid passes through Earth's atmosphere. The intense heat produced during this event causes the surrounding atmospheric gases to fluoresce and glow.

METEORITE: Rocky or metallic debris from space that hits the ground.

METEOROID: Debris from an asteroid or comet in space.

METEOR SHOWER: A display in which numerous meteors appear to originate from a specific point in the sky. A meteor shower occurs when Earth passes through the debris stream left in the wake of a comet or asteroid. As many as 100 meteors per hour can be observed during a meteor shower. A meteor storm is an intense meteor shower where thousands of meteors per hour are observed.

NEAR-EARTH ASTEROID: An asteroid in an orbit which brings it sufficiently close to Earth's orbit that gravitational interaction may put the asteroid on a collision course with Earth. NEAs are divided into several families, including Apollos, Atens and Amors.

NEUTRON STAR: The remnant of a supernova, a neutron star is supported by degenerate neutrons and has a mass near the Chandrasekhar limit. Neutron stars spin rapidly and, if aligned just so, are visible as pulsating radio sources or pulsars.

NOVA: A sudden stellar brightening. Novae occur in binary star systems involving a white dwarf and main sequence star. The white dwarf siphons hydrogen from the main sequence star. The hydrogen is compressed on the surface of the white dwarf until it detonates, producing a nova.

OCCULTATION: An event where one celestial body blocks or passes in front on another.

OORT CLOUD: The spherical region of the outermost Solar System which is home to long-period (orbital periods > 200 years) comets. This region was postulated by Jan Oort to explain the observed distribution of long-period comets.

OPEN STAR CLUSTER: A cluster containing several hundred to several thousand stars. Open clusters reside in the galactic disk and contain high metalicity population I stars. NGC 7789 is a beautiful open cluster.

OPPOSITION: The alignment of the Sun and any two planets such that an observer on the inner planet sees the Sun and outer planet opposite each other in the sky. For example, Mars is at opposition when it lies opposite the Sun as viewed from Earth. Outer planets are closest and, therefore, best observed when at opposition.

PARALLAX: The apparent shift in position of a celestial object when viewed from different directions.

PARSEC: The distance at which one astronomical unit (150 million kilometers) subtends one second of arc. One parsec is equal to 3.26 light-years in distance.

PECULIAR GALAXY: Any galaxy not classified as elliptical, irregular or spiral. Most peculiar galaxies appear to be interacting gravitationally with other galaxies. The Ringtail, NGC 4038 and NGC 4039, is an interacting galaxy pair.

PENUMBRA: A region of partial shadow.

PERIGEE: The point in an orbit closest to Earth.

PERIHELION: The point in an orbit closest to the Sun.

PHOTOSPHERE OF THE SUN: The bright apparent surface of the Sun.

PLANETARY NEBULA: A dying star with a shell of gas surrounding an exposed core. Ultraviolet radiation from the core causes the surrounding gas to fluoresce and glow. Eventually this outer shell will expand and dissipate, leaving the core completely exposed as a white dwarf. The Ring nebula, M57, in Lyra is the most observed planetary nebula.

PLASMA TAIL: One of two types of tails associated with comets. As a comet approaches the Sun, solar energy causes exposed ices or volatiles to sublimate, to transition from a solid to a gas. Charged particles in the solar wind ionize these gas molecules causing them to fluoresce and glow. These particles are shaped by the solar wind, forming a plasma tail.

POPULATION I STARS: Stars with high metalicity content, these are typical of the disk population in the Galaxy.

POPULATION II STARS: Stars with low metalicity content, these are typical of the halo population in the Galaxy.

PROTOSTAR: A star in the process of forming.

PULSAR: A pulsating radio source, a pulsar is a rapidly spinning neutron star oriented such that it appears to be a radio beacon.

QUASAR: A quasi-stellar radio source. Quasars are believed to be the active cores of very distant galaxies. 3C 273 in Virgo is the brightest known quasar and is estimated to be more than 1 billion light-years distant.

RADIO GALAXY: An active version of an elliptical galaxy, which is very bright in the radio spectrum at cosmological distances. M87 in Virgo is a well-known example.

REFLECTING TELESCOPE: A telescope that uses mirrors to collect and focus light.

REFLECTION NEBULA: A bright cloud of interstellar gas illuminated by scattered light from nearby stars. The Trifid nebula, M20, features reflection and emission components.

REFRACTING TELESCOPE: A telescope that uses lenses to collect and focus light.

RETROGRADE MOTION: The apparent westward motion of a celestial body with respect to the stars. Retrograde motion is a perspective effect produced when an inner body passes an outer body in their respective orbits.

RIGHT ASCENSION: One of two coordinates defining an object's location, right ascension describes an object's position along the celestial equator as measured in hours, minutes and seconds. The sky is divided into 24 hours of right ascension, each hour being 15 degrees wide. Right ascension increases as you move east along the celestial equator.

SEEING: One of three measurements of observing conditions. Seeing refers to atmospheric steadiness and its impact on a telescope's ability to resolve fine detail. Good seeing describes conditions where the air is steady and high magnification produces stable, well-defined views. Poor seeing describes conditions where the air is turbulent and telescopic images are only sporadically stable.

SEMIMAJOR AXIS: Half the major axis characterizing the size of an ellipse.

SEYFERT GALAXY: A spiral galaxy with an extremely active, bright core with strong radio emission lines. M77 in Cetus is a Seyfert galaxy.

SIDEREAL PERIOD: The orbital period relative to the stars.

SOLAR FLARE: A sudden release of magnetic energy in the solar corona which brightens the chromosphere.

SOLAR PROMINENCE: An arching loop of chromospheric gas confined by the Sun's magnetic field.

SPIRAL GALAXY: A galaxy with spiral arms and a high population I component. M51 in Canes Venatici is a stunning example of a face-on spiral galaxy. Barred spiral galaxies show a central bar running through the nuclear region. NGC 7479 in Pegasus displays classic barred spiral features in my 10-inch.

SUNSPOT: Dark cool regions in the photosphere. Sunspots are caused by magnetic fields that inhibit the flow of energy to the sunspot regions.

SUPERCLUSTER: A cluster of galaxy clusters.

SUPERGIANT: Huge, luminous stars plotted along the top of the H-R diagram.

SUPERIOR CONJUNCTION: An alignment of the Sun and any two planets such that an observer on one planet sees the Sun between himself and the other planet. For example, Venus is at superior conjunction when it lies on the other side of the Sun from the Earth.

SUPERNOVA: The sudden, intense brightening produced by the cataclysmic destruction of a star. A core-collapse supernova occurs when fusion stops in the iron core of a high-mass star. Electron degeneration is not able to support the core, which crushes inward upon itself releasing an incredible amount of energy. In binary systems, a white dwarf which accumulates mass beyond the Chandrasekhar limit (1.4 solar masses) from a gravitationally bound companion will undergo a cataclysmic collapse. This produces what is known as a type IA supernova.

SUPERNOVA REMNANT: An expanding cloud of debris produced by a supernova. If this cloud is irradiated by the remnant neutron star, molecular gases in the debris cloud are ionized and glow. The Crab nebula, M1 in Taurus, is perhaps the best known supernova remnant.

SYNODIC PERIOD: The orbital period relative to the Sun.

T TAURI STARS: Active, young, variable, pre-main sequence stars grouped into T associations.

TELESCOPE: A device used to collect and focus electromagnetic radiation, commonly visible light.

TERMINATOR: The boundary on a celestial object separating day from night.

TRANSPARENCY: One of three measurements of observing conditions. Transparency refers to the clarity of the sky. A transparent sky shows minimal haze around bright objects or near the horizon. When the transparency is good and the night sky is dark, thousands of faint stars are visible and a telescope is able to discern objects to the theoretical limit of its aperture.

UMBRA: A region of full shadow. The darkest part of the Moon's shadow is the umbra.

VARIABLE STAR: A star whose magnitude changes over time. Cepheid variable stars are considered standard candles, objects which can be used to estimate distances in the universe.

WHITE DWARF: The remnant core of a red giant star. White dwarfs are about the size of Earth but are much more dense than Earth. They are plotted along the bottom of the H-R diagram.

WOLF-RAYET STAR: A high mass star (10 to 50 solar masses) showing emission lines of helium, and hydrogen or carbon.

ZENITH: The point directly overhead on the celestial sphere.

ZODIAC: The band of constellations through which the ecliptic runs.

ZODIACAL LIGHT: A band of light visible along the ecliptic, the zodiacal light is produced by asteroid and comet debris scattering sunlight.

ZONES: Strips of lighter higher altitude, lower temperature clouds in Jupiter's atmosphere.

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Revised: June 11, 2003 [WDF]