absolute magnitude See luminosity.
accretion The gradual accumulation of mass; usually refers to the build-up of larger masses from smaller ones through mutual gravitational attraction.
active galaxy A galaxy that has a more luminous nucleus than most galaxies.
altazimuth coordinates Altitude (angular distance above the horizon) and azimuth (compass direction expressed in angular measure from due north).
altitude See altazimuth coordinates.
angular momentum The rotating version of linear momentum. Depends on the mass distribution and rotational velocity of an object.
angstrom Abbreviated A (or A). A small size, equal to one one ten billionth of a meter, or 10-10 meter.
angular separation Distance between objects on the sky expressed as an angle (such as degrees) rather than in distance units (such as feet or meters).
angular size Size expressed as an angle (such as degrees) rather than in distance units (such as feet or meters).
annihilate Used as an intransitive verb by astronomers. Particles and antiparticles annihilate when they meet, converting their mass into energetic photons. Electrons and anti-electrons (positrons) were continually created and annihilated in the early universe.
aperture The diameter of the objective lens (that is, the main lens) or primary mirror of a telescope and the main lenses of binoculars.
apollo asteroids Asteroids with sufficiently eccentric orbits to cross paths with the Earth (and other terrestrial planets).
apparent magnitude A value that depends on the distance to an object. See luminosity.
arcminute One sixtieth of an angular degree. arcsecond One sixtieth of an arcminute ('Aoo of a degree).
assumption of mediocrity A scientific assumption that we on Earth are not so special. The conditions that have allowed life to arise and evolve on the Earth likely exist in many other places in the Galaxy and universe. Related to the more lofty cosmo-logical principle.
asterism An arbitrary grouping of stars within or associated with a constellation, perceived to have a recognizable shape (such as a Teapot or Orion's Belt) and, therefore, readily serve as celestial landmarks.
asteroid One of thousands of small, rocky members of the solar system that orbit the Sun. The largest asteroids are sometimes called minor planets.
astronomical unit (A.U.) A conventional unit of measurement equivalent to the average distance from the earth to the sun (149,603,500 kilometers, or 92,754,170 miles).
autumnal equinox On or about September 21; on this date, the beginning of fall, day and night are of equal length because the Sun's apparent course against the background stars (the ecliptic) intersects the celestial equator.
azimuth See altazimuth coordinates.
barred-spiral galaxy A spiral galaxy that has a linear feature, or bar of stars, running through the galaxy's center. The bar lies in the plane of the spiral galaxy's disk, and the spiral arms typically start at the end of the bar.
Big Bang The primordial explosion of a highly compact universe; the origin of the expansion of the universe.
binaries Also called binary stars. Two-star systems in which the stars orbit a common center of mass. The way the companion stars move can tell astronomers much about the individual stars, including their masses.
black body An idealized (theoretical) object that absorbs all radiation that falls on it and perfectly re-emits all radiation it absorbs. The spectrum (or intensity of light as a function of wavelength) that such an object emits is an idealized mathematical construct called a black-body curve, which can serve as an index to measure the temperature of a real object. Some astronomical sources (like stars) can be approximated as black bodies.
black dwarf A burned-out star that has cooled from the white dwarf stage through yellow and red.
black hole A stellar-mass black hole is the end result of the core collapse of a highmass (greater than 10 solar mass) star. It is an object from which no light can escape within a certain distance. Although space behaves strangely very close to a black hole, at astronomical distances, the black hole's only effect is gravitational.
Bode's Law Also called the Titius-Bode Law, Bode's Law is a numerical trick that gives the approximate interval between some of the planetary orbits in our solar system.
brightness The measured intensity of radiation from an object. The brightness of astronomical objects falls off with the square of the distance.
brown dwarf A failed star; that is, a star in which the forces of heat and gravity reached equilibrium before the core temperature rose sufficiently to trigger nuclear fusion.
calderas Craters produced not by meteoroid impact but by volcanic activity. See also corona.
cardinal points The directions of due north, south, east, and west.
Cassini division A dark gap between rings A and B of Saturn. It is named for its discoverer, Gian Domenico Cassini (1625-1712), namesake of the mission that will arrive at Saturn in 2004.
cataclysmic variable See variable star.
celestial equator An imaginary great circle dividing the Northern and Southern Hemispheres of the celestial sphere.
celestial sphere An imaginary sphere surrounding the Earth into which the stars are imagined to be fixed. For hundreds of years, people believed such a sphere (or bowl) really existed. Today, however, astronomers use the concept as a convenient way to describe the position of stars relative to one another.
chain reaction See nuclear fission.
circumpolar stars Stars near the celestial North Pole; from many locations on Earth, these stars never set.
closed universe A universe that is finite and without boundaries. A universe with density above the critical value is necessarily closed (see critical density). An open universe, in contrast, will expand forever because its density is insufficient to halt the expansion.
comet Also thought of as a "dirty snowball." Small celestial bodies composed mainly of ice and dust, comets complete highly eccentric orbits around the sun. As a comet approaches the Sun, some of its material is vaporized and ionized to create a gaseous head (coma) and two long tails (one made of dust, and one made of ions).
conjunction The apparent coming together of two celestial objects in the sky.
constellations Arbitrary formations of stars that are perceived as figures or designs. There are 88 official constellations in the Northern and Southern Hemispheres. (See Appendix C)
convective motion A gas-flow pattern created by the rising movement of warm gases (or liquids) and the sinking movement of cooler gases (or liquids).
core The innermost region of a planet or star.
core-collapse supernova The extraordinarily energetic explosion that results when the core of a high-mass star collapses under its own gravity.
core-halo galaxy See radio galaxy.
core hydrogen burning The principal nuclear fusion reaction process of a star. The hydrogen at the star's core is fused into helium, and the small amount of mass lost is used to produce enormous amounts of energy.
corona In astronomy, a corona may be a luminous ring appearing to surround a celestial body, the luminous envelope of ionized gas outside the Sun's chromosphere, or a large upswelling in the mantle of the surface of a planet or the Moon that takes the form of concentric fissures and that is an effect of volcanic activity. See also calderas.
cosmic microwave background The highly redshifted photons left behind by the Big Bang and detectable today throughout all space as radio-wavelength radiation, indicating a blackbody temperature for the universe of 2.73 K.
cosmological principle A cornerstone assumption about the nature of the universe. It holds that the universe exhibits two key properties: homogeneity (sameness of structure on the largest scale) and isotropy (looking the same in all directions).
cosmological redshift The lengthening of the wavelengths of electromagnetic radiation caused by the expansion of the universe.
cosmology The study of the origin, structure, and evolution of the universe.
crater The Latin word for "bowl," it refers to the shape of depressions in the Moon or other celestial objects created (mostly) by meteoroid impacts.
critical density The density of matter in the universe that represents the division between a universe that expands infinitely (unbound) and one that will ultimately collapse (bound). The density of the universe determines whether it will expand forever or end with a conflagration as dramatic as the Big Bang.
crust The surface layer of a planet.
dark halo The region surrounding the Milky Way and other galaxies that contains dark matter. The shape of the dark halo can be probed by examining in detail the effects its mass has on the rotation of a galaxy.
dark matter A catch-all phrase used to describe an apparently abundant substance in the universe of unknown composition. Dark matter is 100 times more abundant than luminous matter on the largest scales.
differential rotation A property of anything that rotates and is not rigid. A spinning CD is a rigid rotator. A spinning piece of gelatin is not as rigid, and a spinning cloud of gas is even less so. For example, the atmospheres of the outer planets and of the Sun have equatorial regions that rotate at a different rate from the polar regions.
Drake Equation Proposes a number of terms that help us make a rough estimate of the number of civilizations in our Galaxy, the Milky Way.
dust lanes Dark areas sometimes visible within emission nebulae and galaxies; the term most frequently refers to interstellar absorption apparent in edge-on spiral galaxies.
eccentric An ellipse (or elliptical orbit) is called eccentric when it is noncircular. An ellipse with an eccentricity of 0 is a circle, and an ellipse with an eccentricity of close to 1 would be very oblong.
eclipse An astronomical event in which one body passes in front of another so that the light from the occulted (shadowed) body is blocked. When the Sun, Moon, and the Earth align, the Moon blocks the light of the Sun (solar eclipse).
eclipsing binaries See visual binaries.
ecliptic The ecliptic traces the apparent path of the Sun against the background stars of the celestial sphere. This great circle is inclined at 23V2 degrees relative to the celestial equator, which is the projection of the Earth's equator onto the celestial sphere.
ejecta blanket The debris displaced by a meteoroid impact on a planetary surface. Such debris are apparent on the lunar surface.
electromagnetic radiation Energy in the form of rapidly fluctuating electric and magnetic fields and including visible light in addition to radio, infrared, ultraviolet, x-ray, and gamma-ray radiation. EM radiation often arises from moving charges in atoms and molecules, though high-energy radiation can arise in other processes.
electromagnetic spectrum The complete range of electromagnetic radiation, from radio waves to gamma waves and everything in between.
ellipse A flattened circle drawn around two foci instead of a single center point.
elliptical galaxy A galaxy with no discernible disk or bulge and that looks like an oval or circle of stars on the sky. The true shapes of ellipticals vary from elongated ("footballs") to spherical ("baseballs") to flattened ("hamburger buns"). Elliptical galaxies consist of old stars and appear to have little or no gas in them.
emission lines Narrow regions of the spectrum where a particular substance is observed to emit its energy. These lines result from basic processes occurring on the smallest scales in an atom (like electrons moving between energy levels).
emission nebulae Glowing clouds of hot, ionized interstellar gas located near a young, massive star. (Singular is emission nebula.)
ephemerides Special almanacs that give the daily positions of various celestial objects for periods of several years.
escape velocity The velocity necessary for an object to escape the gravitational pull of another object.
event horizon Coincides with the Schwarzschild radius and is an imaginary boundary surrounding a collapsing star or black hole. Within the event horizon, no information of the events occurring there can be communicated to the outside, as at this distance not even the speed of light provides sufficient velocity to escape.
fireball A substantial meteoroid that can create a spectacular display when it burns up in the atmosphere.
flat universe The universe that results if its density is precisely at the critical level. It is flat in the sense that its space is defined by the rules of ordinary Euclidean geometry—parallel lines never cross.
focal length The distance from a mirror surface to the point where parallel rays of light are focused.
focus The point at which a mirror concentrates parallel rays of light that strike its surface.
frequency The number of wave crests that pass a given point per unit of time. By convention, this is measured in hertz (equivalent to one crest-to-crest cycle per second and named in honor of the nineteenth-century German physicist Heinrich Rudolf Hertz).
galactic bulge Also called nuclear bulge. A swelling at the center of spiral galaxies. Bulges consist of old stars and extend out a few thousand light-years from the galactic centers.
galactic disk The thinnest part of a spiral galaxy. The disk surrounds the nuclear bulge and contains a mixture of old and young stars, gas, and dust. In the case of the Milky Way, it extends out some 50,000 light-years from the Galactic center but is only about 1,000 light-years thick. It is the dust in the disk (a few hundred light-years thick) that creates the dark ribbon that runs the length of the Milky Way and limits the view of our own Galaxy (see dust lanes).
galactic halo A large (50,000 light-year radius) sphere of old stars surrounding the Galaxy.
galactic nucleus The core of a spiral galaxy. In the case of Seyfert (active) galaxies, the nucleus is extremely luminous in the radio wavelengths.
galaxy cluster A gravitationally bound group of galaxies.
geocentric Earth-centered; the geocentric model of the universe (or solar system) is one in which Earth is believed to be at the center of the universe (or solar system).
giant molecular clouds Huge collections of cold (10 K to 100 K) gas that contain many millions of solar masses of molecular hydrogen. These clouds also contain other molecules (like carbon dioxide) that can be imaged with radio telescopes. The cores of these clouds are often the sites of the most recent star formation.
gibbous A word from Middle English that means "bulging"—an apt description of the Moon's shape between its first and third quarter phases.
globular clusters Collections of a few hundred thousand stars, held together by their mutual gravitational attraction, that are found in highly eccentric orbits above and below the galactic disk.
gnomon Any object designed to project a shadow used as an indicator. The upright part of a sundial is a gnomon.
go-to computer controller A handheld "paddle" that stores location data on celestial objects. Select an object in the database or punch in right ascension and declination coordinates, and the controller will guide motors to point the telescope (if properly aligned) at the desired object or coordinates.
H-R diagram Short for Hertzsprung-Russell diagram. A graphical plot of luminosity versus temperature for a group of stars. Used to determine the age of clusters of stars.
heliocentric Sun-centered; describes our solar system, in which the planets and other bodies orbit the Sun.
helium flash A stellar explosion produced by the rapid increase in the temperature of a red giant's core driven by the fusion of helium.
homogeneity See cosmological principle.
Hubble's Law The linear relationship between the velocity of a galaxy's recession to the galaxy's distance from us. Simply stated, the law says that the recessional velocity is directly proportional to the distance. Used to determine the age of the universe.
inflationary epoch A time soon after the Big Bang when the universe was puffed up suddenly, increasing in size by a factor of 10 in an instant. This inflation could account for the incredible sameness, or uniformity, of the universe, even in regions that (without inflation) could never have been in contact with one another.
interferometer A combination of telescopes linked together to create the equivalent (in terms of resolution) of a giant telescope. This method (while computing-intensive) greatly increases resolving power.
interstellar matter The material found between stars. Refers to the gas and dust thinly distributed throughout space, the matter from which the stars are formed. About 5 percent of our Galaxy's mass is contained in its gas and dust. The remaining 95 percent is in stars.
interstellar medium See interstellar matter. intrinsic variable See variable star.
irregular galaxy A galaxy type lacking obvious structure but containing lots of raw materials for the creation of new stars, and often, many young, hot stars.
isotropy See cosmological principle.
jovian planets The gaseous planets in our solar system farthest from the Sun (with the exception of Pluto, which is classed as neither terrestrial nor jovian): Jupiter, Saturn, Uranus, and Neptune.
Kelvin scale The Kelvin (K) temperature scale is tied to the Celsius (C) temperature scale and is useful because there are no negative Kelvin temperatures. 0 C is the temperature at which water at atmospheric pressure freezes. 100 C is the temperature at which water boils. Absolute zero (0 K) is the coldest temperature that matter can attain. At this temperature, the atoms in matter would stop jiggling around all together. 0 K corresponds to approximately -273 C.
kiva A Native American underground ceremonial chamber, partially open at the top. Associated with various rituals, many are clearly oriented to the sun.
leading face and trailing face Moons that are tidally locked to their parent planet have a leading face and a trailing face; the leading face always looks in the direction of the orbit, the trailing face away from it.
libration The slow oscillation of the Moon (or other satellite, natural or artificial) as it orbits a larger celestial body. Lunar libration gives us glimpses of a very small portion of the far side of our Moon.
light pollution The effect of poorly planned lighting fixtures that allow light to be directed upward into the sky. Light pollution washes out the contrast between the night sky and stars.
light-year The distance light travels in one year: approximately 5.88 trillion miles (9.46 trillion km). For interstellar measurements, astronomers use the light-year as a basic unit of distance.
(The) Local Group A galaxy cluster, a gravitationally bound group of galaxies that includes the Milky Way, Andromeda, and other smaller galaxies.
luminosity The total energy radiated by a star each second. Luminosity is a quality intrinsic to the star; magnitude may or may not be. Absolute magnitude is another name for luminosity, but apparent magnitude is the amount of energy emitted by a star and striking some surface or detection device (including our eyes). Apparent magnitude varies with distance.
magnetosphere A zone of electrically charged particles trapped by a planet's magnetic field. The magnetosphere lies far above the planet's atmosphere.
magnitude A system for classifying stars according to apparent brightness (see luminosity). The human eye can detect stars with magnitudes from 1 (the brightest) to 6 (the faintest). A 1st magnitude star is 100x brighter than a 6th magnitude star.
main sequence When the temperature and luminosity of a large number of stars are plotted, the points tend to fall mostly in a diagonal region across the plot. The main sequence is this well-defined region of the Hertzsprung-Russell diagram (H-R diagram) in which stars spend most of their lifetime. The Sun will be a main sequence star for about 10 billion years.
mantle The layer of a planet beneath its crust and surrounding its core.
maria (pronounced MAH-ree-uh) The plural of "mare" (pronounced MAR-ay, with a long a at the end), this word is the Latin for "sea." Maria are dark-grayish plains on the lunar surface that resembled bodies of water to early observers.
meteor The term for a bright streak across the night sky—a "shooting star." Mete-oroid is the object itself, a rocky object that is typically a tiny fragment lost from a comet or an asteroid. A micrometeoroid is a very small meteoroid. The few mete-oroids that are not consumed in the Earth's atmosphere reach the ground as meteorites.
meteor shower When the Earth's orbit intersects the debris that litters the path of a comet, we see a meteor shower. These happen at regular times during the year.
meteorite See meteor.
meteoroid See meteor.
micrometeoroid See meteor.
millisecond pulsar A neutron star rotating at some 1,000 revolutions per second and emitting energy in extremely rapid pulses.
minor planet See asteroid.
nebula A term with several applications in astronomy but used most generally to describe any fuzzy patch seen in the sky. Nebulae are often (though not always) vast clouds of dust and gas.
neutron star The superdense, compact remnant of a massive star, one possible survivor of a supernova explosion. It is supported by degenerate neutron pressure, not fusion. It is an entire star with the density of an atomic nucleus.
nova A star that suddenly and very dramatically brightens, resulting from the triggering of nuclear fusion caused by the accretion of material from a binary companion star.
nuclear fission A nuclear reaction in which an atomic unit splits into fragments, thereby releasing energy. In a fission reactor, the split-off fragments collide with other nuclei, causing them to fragment, until a chain reaction is under way.
nuclear fusion A nuclear reaction that produces energy by joining atomic nuclei. While the mass of a nucleus produced by joining two nuclei is less than that of the sum of the original two nuclei, the mass is not lost; rather, it is converted into large amounts of energy.
objective lens See aperture.
open universe A universe whose density is below the critical value (see critical density). In contrast to a closed universe, an open universe will expand forever because its density is insufficient to halt the expansion.
optical window An atmospheric property that allows visible light to reach us from space.
orbital period The time required for an object to complete one full orbit around another object. The orbital period of the Earth around the Sun, for example, is a fraction over 365 days.
planetary nebula The ejected gaseous envelope of a red giant star. This shell of gas is lit up by the ultraviolet photons that escape from the hot, white dwarf star that remains. (This term is sometimes confusing as it has nothing to do with planets.)
planetesimals Embryonic planets in an early formative stage. Planetesimals, which are probably the size of small moons, develop into protoplanets, immature but full-scale planets. It is the protoplanets that go on to develop into mature planets as they cool.
precession The slow change in the direction of the axis of a spinning object (such as the Earth), caused by an external influence or influences (such as the gravitational fields of the Sun and the Moon).
primary mirror See aperture.
primordial synthesis The fusion reactions that occurred in the early universe at temperatures of about 10 K. These fusion reactions produced helium and a small amount of lithium.
proper motion Motion of a star determined by measuring the angular displacement of a target star relative to more distant background stars. Measurements are taken over long periods of time, and the result is an angular velocity (measured, for example, in arcseconds/year). If the distance to the star is known, this angular displacement can be converted into a transverse velocity in km/s (see transverse component).
protoplanet See planetesimals.
pulsar A rapidly rotating neutron star whose magnetic field is oriented such that it sweeps across the Earth with a regular period.
pulsating variable See variable star.
quasar Short for "quasi-stellar radio source." Quasars are bright, distant, tiny objects that produce the luminosity of 100 to 1,000 galaxies within a region the size of a solar system.
radar Short for "radio detection and ranging." In astronomy, radio signals are sometimes used to measure the distance of planets and other objects in the solar system.
radial component See transverse component.
radio galaxy A member of an active galaxy subclass of elliptical galaxies. Radio galaxies are characterized by strong radio emission and, in some cases, narrow jets and wispy lobes of emission located hundreds of thousands of light-years from the nucleus.
radio jets Narrow beams of ionized material that have been ejected at relativistic velocities from a galaxy's nucleus.
radio lobe The diffuse or wispy radio emission found at the end of a radio jet. In some radio galaxies, the lobe emission dominates; in others, the jet emission dominates.
radio telescope An instrument, usually a very large dish-type antenna connected to a receiver and recording and/or imaging equipment, used to observe radio-wavelength electromagnetic radiation emitted by stars and other celestial objects.
radio window A property of our atmosphere that allows some radio waves from space to reach Earth and that allows some radio waves broadcast from Earth to penetrate the atmosphere.
radioactive decay The natural process whereby a specific atom or isotope is converted into another specific atom or isotope at a constant and known rate. By measuring the relative abundance of parent-and-daughter nuclei in a given sample of material (such as a meteorite), it is possible to determine the age of the sample.
red giant A late stage in the career of stars about as massive as the Sun. More massive stars in their giant phase are referred to as supergiants. The relatively low surface temperature of this stage produces its red color.
redshift An increase in the detected wavelength of electromagnetic radiation emitted by a celestial object as the recessional velocity between it and the observer increases. The name derives from the fact that lengthening the wavelength of visible light tends to redden the light that is observed, but is used generally.
refracting telescope Also called a refractor. A telescope that creates its image by refracting (bending) light rays with lenses.
resolving power The ability of a telescope (optical or radio telescope) to render distinct, individual images of objects that are close together.
retrograde An orbit that is backward or contrary to the orbital direction of the planets.
Schwarzschild radius The radius of an object with a given mass at which the escape velocity equals the speed of light. As a rule of thumb, the Schwarzschild radius of a black hole (in km) is approximately three times its mass in solar masses, so a 5-solar mass black hole has a Schwarzschild radius of about 5 x 3 = 15 km.
seeing The degradation of optical telescopic images as a result of atmospheric turbulence. "Good seeing" denotes conditions relatively free from such atmospheric interference.
Seyfert galaxy A type of active galaxy, resembling a spiral galaxy, whose strong radio-wavelength emissions and emission lines come from a small region at its core.
sidereal day A day measured from star rise to star rise. The sidereal day is 3.9 minutes shorter than the solar day.
sidereal month The period of 27.3 days that it takes the moon to orbit once around the Earth. See also synodic month.
sidereal year The time it takes the Earth to complete one circuit around the Sun with respect to the stars.
singularity The infinitely dense remnant of a massive core collapse.
solar day A day measured from sunup to sunup (or noon to noon, or sunset to sunset). Slightly longer than a sidereal day. See sidereal day.
solar flares Explosive events that occur in or near an active region on the Sun's surface.
solar nebula The vast primordial cloud of gas and dust from which (it has been theorized) the Sun and solar system were formed.
solar wind A continuous stream of radiation and matter that escapes from the Sun. Its effects may be seen in how it blows the tails of a comet approaching the Sun.
sounding rockets Early suborbital rocket probes launched to study the upper atmosphere.
spectral lines See spectroscope.
spectral classification A system for classifying stars according to their surface temperature as measured by their spectra. The presence or absence of certain spectral lines is used to place stars in a spectral class.
spectrometer See spectroscope.
spectroscope An instrument that passes incoming light through a slit and prism, splitting it into its component colors. A spectrometer is an instrument capable of precisely measuring the spectrum thus produced. Substances produce characteristic spectral lines or emission lines, which act as the "fingerprint" of the substance, enabling identification of it.
spectroscopic binaries See visual binaries.
spicules Jets of matter expelled from the Sun's photosphere region into the chromosphere above it.
spiral arms Structures found in spiral galaxies apparently caused by the action of spiral density waves.
spiral density waves Waves of compression that move around the disk of a spiral galaxy. These waves are thought to trigger clouds of gas into collapse, thus forming hot, young stars. It is mostly the ionized gas around these young, massive stars that we observe as spiral arms.
spiral galaxy A galaxy characterized by a distinctive structure consisting of a thin disk surrounding a galactic bulge. The disk is dominated by bright, curved arcs of emission known as spiral arms. The rotation curves of spiral galaxies indicate the presence of large amounts of dark matter.
standard candle Any object whose luminosity is well known. Its measured brightness can then be used to determine how far away the object is. The brightest standard candles can be seen from the greatest distances.
standard solar model Our current picture of the structure of the Sun. The model seeks to explain the observable properties of the Sun and also to describe properties of its mostly unobservable interior.
stellar occultation An astronomical event that occurs when a planet passes in front of a star, dimming the star's light (as seen from the Earth). The exact way in which the light dims can reveal details, for example, in the planet's atmosphere.
summer solstice On or about June 21; the longest day in the Northern Hemisphere, marking the beginning of summer.
sunspots Irregularly shaped dark areas on the face of the Sun. They appear dark because they are cooler than the surrounding material. They are tied to the presence of magnetic fields at the Sun's surface.
Supercluster A group of galaxy clusters. The Local Supercluster contains some 1015 solar masses.
superluminal motion A term for the apparent "faster than light" motion of blobs of material in some radio jets. This effect results from radio-emitting blobs moving at high velocity toward the observer.
supernova The explosion accompanying the death of a massive star as its core collapses.
synchronous orbit A celestial object is in synchronous orbit when its period of rotation is equal to its average orbital period; the Moon, in synchronous orbit, presents only one face to the Earth.
synchrotron radiation Synchrotron radiation arises when charged particles (e.g., electrons) are accelerated by strong magnetic fields. Some of the emission from radio galaxies is synchrotron.
synodic month The period of 29.5 days that the Moon requires to cycle through its phases, from new Moon to new Moon. See also sidereal month.
telescope A word from Greek roots meaning "far-seeing." Optical telescopes are arrangements of lenses and/or mirrors designed to gather visible light efficiently enough to enhance resolution and sensitivity. See also radio telescope.
terminator The boundary separating light from dark, the daytime from nighttime hemispheres of the Moon (or another planetary or lunar body).
terrestrial planets The planets in our solar system closest to the Sun: Mercury, Venus, Earth, and Mars.
thought experiment A systematic hypothetical or imaginary simulation of reality, used as an alternative to actual experimentation when such experimentation is impractical or impossible.
tidal bulge The deformation of one celestial body caused by the gravitational force of another extended celestial body. The Moon creates an elongation of the Earth's oceans—a tidal bulge.
time dilation The apparent slowing of time (as perceived by an outside observer) as an object approaches the event horizon of a black hole, or moves at very high velocity.
trailing face See leading face.
transverse component Stellar movement across the sky, perpendicular to our line of sight. The radial component is motion toward or away from us. True space motion is calculated by combining the observed transverse and radial components.
triangulation An indirect method of measuring distance derived by geometry or trigonometry using a known baseline and two angles from the baseline to the object.
tropical year A year measured from equinox to equinox. See sidereal year.
Tully-Fisher relation A correlation of the rotational velocity of any spiral galaxy with its luminosity.
universal recession The apparent general movement of all galaxies away from us. This observation does not mean we are at the center of the expansion. Any observer located anywhere in the universe should see the same redshift.
Van Allen belts Named for their discoverer, American physicist James A. Van Allen, these are vast doughnut-shaped zones of highly energetic, charged particles that are trapped in the magnetic field of the Earth. The zones were discovered in 1958.
variable star A star that periodically changes in brightness. A cataclysmic variable is a star, such as a nova or supernova, that changes in brightness suddenly and dramatically as a result of interaction with a binary companion star, while an intrinsic variable changes brightness because of rapid changes in its diameter. Pulsating variables are intrinsic variables that vary in brightness in a fixed period or span of time and are useful distance indicators.
vernal equinox On or about March 21; on this date, the beginning of spring, day and night are of equal duration because the sun's apparent course intersects the celestial equator at these times.
Very-Long Baseline Interferometry (VBLI) A system combining signals from radio telescopes distant from one another in order to achieve very high degrees of image resolution.
visual binaries Binary stars that can be resolved from the Earth. Spectroscopic binaries are too distant to be seen as distinct points of light, but they can be observed with a spectroscope. In this case, the presence of a binary system is detected by noting Doppler-shifting spectral lines as the stars orbit one another. If the orbit of one star in a binary system periodically eclipses its partner, it's possible to monitor the variations of light emitted from the system and thereby gather information about orbital motion, mass, and radii. These binaries are called eclipsing binaries.
water hole The span of the radio spectrum from 18 cm to 21 cm, which many researchers believe is the most likely wavelength on which extraterrestrial broadcasts will be made. The name is a little astronomical joke—the hydrogen (H) and hydroxyl (OH) lines are both located in a quiet region of the radio spectrum, a region where there isn't a lot of background noise. Since H and OH add up to H2O (water), this dip in the spectrum is called the water hole.
wavelength The distance between two adjacent wave crests (high points) or troughs (low points). By convention, this distance is measured in meters or decimal fractions thereof.
white dwarf The remnant core of a red giant after it has lost its outer layers as a planetary nebula. Since fusion has now halted, the carbon-oxygen core is supported against further collapse only by the pressure supplied by densely packed electrons.
winter solstice On or about December 21; in the Northern Hemisphere, the shortest day and the start of winter.
zonal flow The prevailing east-west wind pattern that is found on Jupiter.
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