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Absolute magnitude- the brightness a star would be as seen from a distance of 10 parsecs

Absolute Zero- the lowest possible temperature, at which substances contain no heat energy, and atomic movement has stopped

Accretion- accumulation of dust and gas into larger bodies such as stars, planets, and moons

Accretion disk- a disk of hot, glowing matter spiraling into a black hole

Active galaxy- a galaxy under-going a violent outburst in its central regions

Adaptive optics- the technology that allows, based on a laser beam aimed through the atmosphere, a computer to make very slight modifications to a telescope's mirror, which will correct for atmospheric distortions

Albedo- the reflecting power of a non-luminous body; a perfect reflector would have an albedo of 100 per cent

Albedo feature- a dark or light marking on the surface of an object that may not be a geological or topographical feature

Altazimuth mounting- a telescope mounting that swings from side to side parallel to the horizon, and up and down

Angstrom unit- the hundred-millionth part of a centimeter (10-10 m)

Annihilation- total destruction of matter in a burst of energy

Anthropic principle- we see the universe the way it is because if it were different we would not be here to observe it

Antigravity- a gravitational field that repels, rather than attracts, matter and light rays

Antimatter- the exact opposite of matter; when matter meets the tiny amount of antimatter in the universe, the two annihilate each other (see antiparticle)

Antiparticle- an atomic particle that has exactly the opposite properties of its counter-part (e.g. a positron and an electron)

Antipodal point- the point that is directly on the opposite side of the planet

Roman Numeral Description
I Perfect seeing, without a quiver
II Slight undulations, with some moments of calm lasting several seconds
III Moderate seeing, with larger air tremors
IV Poor seeing, with constant troublesome undulations
V Very poor seeing, scarcely allowing the making of a rough sketch

Antoniadi Scale- a roman numeral indicates the quality of seeing according to the following scale:

Aperture- the diameter of an opening through which light passes in an optical instrument

Aphelion- the position of a planet (or other body) when it is at its furthest from the sun

Apparent magnitude- the brightness of an object as seen from Earth

Arcuate- having the form of a bow; curved; arc-shaped

Asteroid- a small rocky object orbiting the sun, less than 1,000 kilometers in diameter

Astronomical Unit (A.U.)- the average distance between the Earth and the sun, roughly 150 million km (93 million miles)

Atmosphere- the layer of gases enveloping a celestial object

Atom- the smallest part of an element that can take part in a chemical reaction; most of the mass of the an atom is concentrated in its nucleus, which is about .000000000001 meters (.01 angstroms) across

Aurora- curtains and arks of light in the sky visible over middle and high latitudes; they are caused by particles from the sun hitting the Earth's atmosphere and causing some of its gases to glow

Axis- the imaginary line through the center of a planet, star, or galaxy around which it rotates; also, a similar line through a telescope mounting

Bar- equals 0.987 atmosphere = 1.02 kg/cm2= 100 kilopascal = 14.5 lbs/inch2

Baryon- baryons are composed of three quarks; they include protons and neutrons

Big Bang- the violent event that gave birth to our universe

Big Crunch- the ultimate collapse of the universe that may take place in the future if the universe starts to contract

Binary system- system of two stars that orbit around each other

Black dwarf- a dead star with a maximum possible mass of 1.4 solar masses that has cooled to a point where it no longer glows with residual heat

Black hole- a collapsed object with such strong gravity that nothing can escape it; as a result, the object is black, and it is a hole because nothing can escape from it

Blazar- a type of active galaxy that is angled in such a way to us that we look almost directly at its accretion disk and jet

Blue shift- a shift in the wave-length of radiation emitted by an object when it is approaching us; the Doppler shift makes the wave fronts bunch closer together, causing the light to appear of a shorter wavelength, and hence bluer

Bolide- a fireball that produces a sonic boom

Brown dwarf- a "failed star" in the sense that when it was finished forming, it did not have enough mass to begin fusion; it does not shine as a star does, but can generate heat through very slow gravitational contraction -- this works because when a gas is compressed, it gains temperature

Caldera- a volcanic crater

Carbonate- a compound containing carbon and oxygen (e.g. calcium carbonate AKA limestone)

Casimir effect- the attractive pressure between two flat, parallel metal plates placed very near to each other in a vacuum; the pressure is due to a reduction in the usual number of virtual particles in the space between the plates

Catadioptric telescope- a telescope that uses both mirrors and lenses to form and image

Catena- a chain of craters

Cavus- a hollow, irregular depression

Charge-coupled device (CCD)- a computer-controlled electronic detector that can record an image

Celestial equator- the imaginary line encircling the sky midway between the two celestial poles

Celestial poles- the imaginary points on the sky where Earth's rotation axis, extended infinitely, would touch the imaginary celestial sphere

Celestial sphere- the imaginary sphere enveloping the Earth upon which the stars, galaxies, and other celestial objects all appear to lie

Cepheid variable- a variable star of short period; the fluctuations are regular and are linked with its real luminosity; the longer the period, the more luminous the star

Chandrasekhar limit- the maximum possible mass of a stable cold star, above which it must collapse into a black hole

Chaos- a distinctive area of broken terrain

Chasma- a canyon

Chromosphere- that part of the sun's atmosphere that lies just above its visible surface, or photosphere

Circumpolar stars- stars that never set when seen from a given location

Colles- small hills or knobs

Collimation- the procedure of aligning a telescope's optics

Coma- the dust and gas surrounding an active comet's nucleus

Comet- a small body composed of ices and dust which orbits the sun on an elongated path

Conjunction- the moment when two celestial objects lie closest together in the sky

Conservation of angular momentum- the law of science that states that momentum must be conserved within a system

Conservation of energy- the law of science that states that energy (or its equivalent in mass) can neither be created nor destroyed

Constellation- one of the eighty-eight official patterns of stars into which the night sky is divided

Convection- fluid circulation driven by large temperature gradients; the transfer of heat by this automatic circulation

Corona- the high-temperature outermost atmosphere of the sun, visible from Earth only during a total solar eclipse

Coronagraph- a special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere

Cosmic censor- mythical being who dictates that singularities must be surrounded by an event horizon

Cosmic ray- an extremely energetic (relativistic) charged particle

Cosmological Constant- a mathematical device used by Einstein to give space-time an inbuilt tendency to expand

Cosmology- the study of the universe as a whole

Core- in a star, the central region that is undergoing nuclear fusion; in a galaxy, the innermost few light-years

Crater- bowl-shaped depression formed by the impact of a meteoroid; depression around the orifice of a volcano

Culmination- the maximum altitude of a celestial body from the celestial equator

Dark adaptation- the process by which the human eye increases sensitivity under conditions of low, or none, illumination

Dark matter- invisible matter that is believed to make up 99% of the mass of the universe

Declination- the angular distance of a celestial object north or south of the celestial equator; it corresponds to latitude on the Earth

Density- degree of "solidity" of a body: its mass divided by its volume

Direct motion- the movement of a celestial body from west to east - that is, in the same direction as that of the Earth around the sun

Disk- the visible surface of the Sun (or any heavenly body) projected against the sky

Doppler effect- change in the observed frequency of sound or radiation that takes place when the observer and the source are moving relative to each other

Dorsum- a ridge

Double-star system- a system of two stars in orbit around each other

Duality- a correspondence between apparently different theories that lead to the same physical results

Dust- microscopic grains in space that absorb starlight; the grains are "soot" left by dying stars, and they will sometimes clump together in huge dark clouds

Dwarf star- a star, such as the sun, that lies on the Main Sequence

Eccentricity- a measure of how closely a planet's orbit approximates to a perfect circle

Eclipse- when one celestial body passes in front of another, dimming or obscuring its light (e.g. solar, lunar, and eclipsing binaries)

Ecliptic- the apparent yearly path of the sun against the stars

Einstein-Rosen bridge- the "throat" of a black hole in one universe connecting up with one in a different universe; in theory, it is a bridge from one universe to another

Electromagnetic radiation- radiation made up of magnetic and electrical fields that move at the speed of light

Electron- tiny particle with a negative charge, often in orbit around the nucleus of an atom

Electroweak unification energy- the energy (around 100 GeV) above which the distinction between the electromagnetic force and the weak force disappears

Elementary particle- a particle that, it is believed, cannot be subdivided

Ellipse- the oval, closed path followed by a celestial object moving under gravity (e.g. a planet around the sun)

Elongation- the angular distance of a planet from the sun or a satellite from its primary planet

Equatorial mounting- a telescope mounting which has one axis parallel to the Earth's rotational axis, so the motion of the heavens can be followed with a single movement

Equinox- the two points at which the sun crosses the celestial equator; the spring equinox is about March 21, and the autumnal equinox is about September 22

Ergosphere- region surrounding a spinning black hole, between the static limit and the outer event horizon, in which it is impossible to be at rest

Escape velocity- speed a body needs to travel in order to escape the surface gravity of a star or planet

Event- a point in space-time, specified by its time and place

Event horizon- the "edge" of a black hole: and imaginary surface where the escape velocity reaches the speed of light

Exclusion Principle- the idea that two identical spin-1/2 particles cannot have (within the limits set by the uncertainty principle) both the same position and the same velocity

Extinction- the apparent reduction in the brightness of a star or planet when low over the horizon because more of its light is absorbed by the Earth's atmosphere

Eyepiece- a set of lenses used to magnify the image produced by a telescope's objective

Faculae- the bright patches on the sun's photosphere

Farrum- a pancake-like structure (like those on Venus)

Field- something that exists throughout space and time, as opposed to a particle that exists at only one point at a time

Filament- a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun; a filament on the limb of the Sun seen in emission against the dark sky is called a prominence

Finder- a small, low-power telescope attached to and aligned with a larger one; its wider field of view makes it useful for locating celestial objects

Fireball- any meteor brighter than Venus, about magnitude -4

Flare, solar- brilliant outbreaks in the solar atmosphere, normally detectable only by spectroscopic methods

Flare star- a faint red star that has short-lived explosions on its surface; these explosions cause the star to appear temporarily brighter

Fluctus- flow terrain

Fossa- long, narrow, shallow depression

Fraunhofer Lines- the dark lines in the spectrum of the sun

Frequency- for a wave, the number of complete cycles per second

Gaia Hypothesis- named for the Greek Earth goddess Gaea, holds that the Earth as a whole should be regarded as a living organism and that biological processes stabilize the environment. First advanced by British biologist James Lovelock in 1969

Galactic cluster- (see open cluster)

Galaxy- a huge gathering of stars, gas, and dust, bound by gravity and having a mass ranging from 100,000 to 10 trillion times that of the sun; there are spiral, elliptical, and irregular types of galaxies

Gamma rays- the highest energy, shortest wavelength electromagnetic radiation of all

Gegenschein- a round or elongated spot of light in the sky at a point 180 degrees from the sun; also called counter glow

General relativity- the theory of relativity that describes how matter behaves in the presence of strong gravitational fields

Geodesic- the shortest (or longest) path between two points

Geosynchronous orbit- a direct, circular, low inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface

Globular cluster- a spherical cluster that may contain up to a million stars

Grand unification energy- the energy above which, it is believed, the electromagnetic force, weak force, and strong force become indistinguishable from each other

Grand Unified Theory (GUT)- a theory that unifies the electromagnetic, strong, and weak energy forces

Granulation- a pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas

Gravitational lens- distortion of an image - or the production of many images - by a powerful gravitational field

Gravitational waves- ripples in space that travel at the speed of light, produced by the movement of very massive bodies

Gravity- force of attraction that is felt between two masses, such as the pull between the Earth and the Moon

Hadron- particles made of quarks that are influenced by the Strong Nuclear Force; includes mesons and bosons, but excludes leptons

Heliocentric- sun-centered solar system theories

Heliopause- the point at which the solar wind meets the interstellar medium or solar wind from other stars

Heliosphere- the space within the boundary of the heliopause containing the Sun and solar system

Hertzsprung-Russel (HR) Diagram- a graph whose horizontal axis plots star color (or temperature) against a vertical axis plotting stellar luminosity (or absolute magnitude)

Ice- used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system

Imaginary time- time measured using imaginary numbers (i.e. the square root of negative one)

Inclination- measure of the tilt of a planet's orbital plane, in relation to that of the Earth

Inferior planet- either of the two planets, Mercury and Venus, that orbit between the sun and the Earth

Inferometry- if one combines two or more telescope's light, the resulting image would have the resolution of a telescope the size of the distance separating them

Infrared- heat radiation, inter../mediate in wavelength between light and radio waves

Interplanetary Magnetic Field (IMF)- the magnetic field carried with the solar wind

Kelvin (K)- 0 Kelvin is absolute zero; water melts at 273 K; water boils at 373 K; developed by William Thomson

Lacus- lake

Lagrange points- Lagrange showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane; if one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable (such bodies are sometimes referred to as Trojans); the leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5

Last stable orbit- the closest an object can circle a black hole without being pulled in

Lepton- one of the fundamental types of particles, with quarks being the other; common examples are electrons and neutrinos

Libration- an effect caused by the apparent slight "wobbling" of the Moon from side to side, as seen from Earth; as a result, a total of fifty-nine per cent of the Moon can be observed from Earth, though no more than fifty per cent at one time

Lidar- an instrument similar to radar that operates at visible wavelengths

Light-year- distance covered by a ray of light traveling at 300,000 kps (186,000 mps) in a year; it is about 9,460,528,404,880 km (5,878,499,814,140 miles)

Light cone- a surface in space-time that marks out the possible directions for light rays passing through a given event

Limb- an edge or border, as of the sun, Moon, or any planet

Local Group- a gathering of roughly thirty galaxies to which the Milky Way belongs

Luminosity- the total intrinsic brightness of a star or galaxy

Lunar month- The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes

Lunation- the interval between one new moon and the next: that is, 29 days, 12 hours, and 44 minutes

MACHO- an acronym for "Massive Compact Halo Object" - supposed dark, massive objects surrounding our galaxy

Magnetosphere- the are around a planet in which its magnetic field is dominant

Magnetotail- the portion of a planetary magnetosphere which is pushed away from the sun by the solar wind

Magnetic field- the field responsible for magnetic forces, now incorporated along with the electric field, into the electromagnetic field

Magnitude- a logarithmic unit used to measure the optical brightness of celestial objects; numerically lower magnitudes are brighter than numerically larger ones; a five-magnitude difference represents a 100-fold change in brightness

Main Sequence- the band on the HR Diagram where stars lie for much of their life

Mare- literally "sea" (a very bad misnomer, still in use for historical reasons); really a large circular plain

Mass- amount of matter making up a body

Matter- what everything that we know of is made of; the opposite of antimatter

Meridian- an imaginary line on the sky that runs due north and south and passes through your zenith

Meson- matter that is composed of two quarks

Metal- used by astrophysicists to refer to all elements except hydrogen and helium, as in: "The universe is composed of hydrogen, helium and traces of metals;" (note: this is quite different from the usual chemistry definition)

Meteor- the bright, transient streak of light produced by a piece of space debris burning up as it enters the atmosphere at high speed

Meteorite- any piece of space debris that reaches the Earth's surface intact

Milky Way- a soft, glowing band of light encircling the sky, it is the disk of the spiral galaxy in which the sun lies, seen from the inside

Microwave background radiation- the radiation from the glowing of the hot early universe, now so greatly red-shifted that it appears not as light but as microwaves (radio waves with a wavelength of a few centimeters)

Mini black hole- one of many tiny black holes with the mass of a mountain but the size of an atom that are believed to have been created in the Big Bang

Minor planets- the official term used for asteroids

Missing mass- several seperate experiments show that there is more matter in the universe than we can see; more than that, this so-called "dark matter" accounts for about 25% of the universe's total mass-energy, but there is a "dark energy" which appears to be moving everything apart from everything else (acting only at very large scales) that seems to account for 70% of the universe's mass-energy content

Mons- mountain (plural is montes)

Naked singularity- a singularity that is not surrounded by an event horizon

Nebula- a cloud of dust and gas in space, from which new stars are created

Neutrino- miniscule particle with little or no mass and no charge that travels at the speed of light

Neutron- electrically neutral particle that makes up part of the nucleus of an atom

Neutron star- collapsed star composed mainly of neutrons; pulsars are young, fast-spinning neutron stars

No boundary condition- the idea that the universe is finite but has no boundary (in imaginary time)

Nova- a white dwarf star in a binary system that brightens suddenly by several magnitudes as gas pulled away from its companion star explodes in a thermonuclear reaction

Nuclear fusion- nuclear reaction in which one kind of atom, under extreme heat and pressure, is combined with another and forms a different one


  1. the central part of an atom, consisting only of protons and neutrons, held together by the strong force
  2. the central part of a comet, whose existence is not a function of its distance from the sun -- the only "permenant" feature of a comet

Objective- the main light gathering optical element in a telescope; it may be a lens or a mirror

Oblateness- the degree of flattening at the poles of a celestial body

Occulation- the covering up of one celestial object by another, such as the Moon passing in front of a star or planet as seen from Earth

Open cluster- a group of some few hundred stars bound by gravity and moving through space together

Opposition- the position of a superior planet when exactly opposite the sun in the sky, as seen from Earth; the planet is then best placed for observation

Orbit- the path followed by any celestial object moving under the control of another's gravity

Orbital family- a group of asteroids that follows the same relative orbital path, velocity, and is usually seen close together; they are thought to have once been one asteroid that was broken apart due to a collision in the past

Parallax- the apparent change in position of a nearby star due to Earth's orbital motion around the sun

Parsec- a unit of distance equal to 3.26 light-years; it is the distance at which a star would have a parallax of one second of arc

Particle accelerator- a machine that, using electromagnets, can accelerate moving charged particles, giving them more energy

Patera- a shallow crater with a scalloped, complex edge

Penumbra- the outer part of an eclipse shadow; also, the lighter area surrounding the center of a sunspot

Perihelion- a planet or comet's closest approach to the sun

Periodic time- (see sidereal period)

Perturb- to cause a planet or satellite to deviate from a theoretically regular orbital motion

Phase- the varying illuminated part of the Moon and planets caused by the relative locations of the object, the Earth, and the sun

Photon- a quantum of light

Photosphere- the visible surface of the sun or a star

Plage- bright regions seen in the solar chromosphere

Planetary nebula- a shell of gas puffed off by a star late in its life; their often round appearance led to the name

Planitia- plateau or high plain

Plank's Quantum Principle- the idea that light (or any other classical waves) can be emitted or absorbed only in discrete quanta, whose energy is proportional to their wavelength

Positron- the positively charged antiparticle of the electron

Precession- a slow periodic wobble in the Earth's axis caused by the gravitational pull of the sun and Moon

Prominence, solar- a loop of cooler gas seen above the sun's surface, which sometimes erupts outwards into space

Proton- a positively charged particle that forms part of the nucleus of an atom

Protoplanet- a stage in the formation of a planet which implies the body is nearly full-size

Protostar- a stage in the formation of a star which implies the body is nearly full-size; the star is still within its parent nebula, and does not yet produce energy through nuclear fusion

Pulsar- collapsed star composed mainly of neutrons; pulsars are young, fast-spinning neutron stars

Quantum- the indivisible unit in which waves may be emitted or absorbed

Quantum chronodynamics (QCD)- the theory that describes the interactions of quarks and gluons

Quantum mechanics- the theory developed from Plank's Quantum Principle and Heisenberg's Uncertainty Principle

Quasar- the brilliant core of a distant young active galaxy with outer regions that are often too faint to be visible

Quark- a (charged) elementary particle that feels the strong force; protons and neutrons are composed of three quarks each

Radar- a system using pulsed radio waves to detect the position of objects by measuring the time it takes a single pulse to reach the object and be reflected back

Radial velocity- the movement of a celestial body toward or away from an observer

Radiant- the point on the sky from where a shower of meteors appears to come

Radiation- made up of magnetic and electrical fields that move at the speed of light

Radio galaxy- active galaxy that gives out as much energy in radio waves as it does in light

Radio telescope- a telescope that picks up radio waves from objects in space

Radioactivity- the spontaneous breakdown of one type of atomic nucleus into another

Red giant- an old star whose outer layers have billowed out and cooled down

Red shift- shift in the light of a retreating object toward red wavelengths, caused by the Doppler effect

Reflector- a telescope that forms an image with mirrors

Refractor- a telescope that forms an image with a lens

Relativity- (see general relativity and special relativity)

Resolving power- the ability of a telescope to image two closely spaced objects as separate

Resonance- a state in which one orbiting object is subject to periodic gravitational perturbations by another


  1. an apparent westward motion of a planet, asteroid, or comet relative to the stars
  2. the motion of a moon that orbits contrarty to the spin of its planet -- a strong indication that the moon was captured and did not form in the vicinity of the planet

Rift valley- an elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike

Right ascension- the celestial coordinate analogous to longitude on Earth

Roche Limit- the closest one celestial object can get to another before the weaker object is broken apart by tidal effects

Satellite- any small object orbiting a larger one, although the term is most often used for rocky or man-made objects orbiting a planet

Scarp- line of cliffs produced by faulting or erosion

Schwarzchild Radius- radius of the event horizon around a black hole

Seeing- a measure of the steadiness of the atmosphere; good seeing is essential to using high magnification

Semimajor axis- the semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru the foci of the ellipse with endpoints on the ellipse itself; the semimajor axis of a planetary orbit is also the average distance from the planet to its primary

Seyfert galaxy- a galaxy that has a small, bright nucleus and faint spiral arms; it is often a strong radio source

Shepherd moon- a satellite which constrains the extent of a planetary ring through gravitational forces

Sidereal period- the time, relative to the stars, needed for a planet or moon to make one rotation or revolution around its primary body

Singularity- the center of a black hole; a point (or ring) of infinite density that occupies zero space

Singularity theorem- a theorem that shows that a singularity must exist under certain circumstances - in particular, that the universe must have started with a singularity

Solar cycle- the approximately 11-year quasi-periodic variation in frequency or number of solar active events

Solar filter- a filter that reduces the sun's light to a level where you can view it with a telescope; only those filters which fit over a telescope's objective are safe to use

Solar mass- the mass of the sun; it is used as a standard weight against which other celestial objects can be compared

Solar nebula- the cloud of gas and dust that began to collapse about 5 billion years ago to form the Solar System

Solar System- our sun with everything that orbits it: nine planets (plus their satellites), thousands of asteroids, and countless comets, meteors, and other debris

Solar wind- charged particles from the sun that travel into the Solar System at about 1.5 million kph (932,000 mph)

Solstice- the time when the sun reaches its greatest northern or southern declination

Space-Time- the four-dimensional description of the universe in which length, breadth, and height make up the first three spatial dimensions, while time makes up the fourth dimension

Spaghettification- gravitational stretching of a body falling into a black hole

Spatial dimension- any of the three dimensions that are space-like - that is, any except the time dimension (i.e. length, width, depth)

Special relativity- a branch of relativity dealing with the behavior of objects traveling close to the speed of light

Spectrograph- an instrument that breaks the light from a celestial object into its component colors for analysis

Spectroscopic binary- a very close double that is recognizable only by the periodic splitting of lines in the combined spectrum of the two stars, owing to the opposite Doppler effects resulting from their motions

Spectrum- the range of color produced when light is split up by a prism of diffraction grating

Spicules- grass-like patterns of gas seen in the solar atmosphere

Static limit- a limit close to a black hole inside of which is impossible to remain at rest

Stationary state- one that is not changing with time; a sphere spinning at a constant rate is stationary because it looks identical at any given instant

Stellar mass black hole- a black hole produced by the explosion of a massive star as a supernova; most weigh about ten solar masses

String Theory- a theory of physics in which particles are described as waves on strings; strings have length but no other dimension

Strong force- the strongest of the four fundamental forces, with the shortest range of all; it holds the quarks together within protons and neutrons, and holds the protons and neutrons together to form atoms

Sunspot- a highly magnetized dark spot on the sun's surface, cooler than surrounding area

Supergiant- the stage in the evolution of a massive star when its core contracts, its surface expands to about 500 solar radii, and its temperature drops, giving the star its red color

Superior planet- any planet beyond the orbit of the Earth in the Solar System

Supermassive black hole- a black hole located at the center of a galaxy; these holes, formed by material falling onto the galaxy's core, may weight billions of solar masses

Supernova- an explosion of a massive star at the end of its life; the star may briefly equal an entire galaxy in brightness

Supernova remnant- the gaseous debris, rich in heavy elements, thrown off by a supernova

Synchronous orbit radius- the orbital radius at which the satellite's orbital period is equal to the rotational period of the planet; a synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface (such orbits are commonly used for communications satellites; it is also seen in the cases of Pluto and Charon)

Synchronous rotation- said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary; this implies that the satellite always keeps the same hemisphere facing its primary (e.g. the moon); it also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward; most of the satellites in the solar system rotate synchronously

Synodic month- see lunar month

Synodic period- the interval between the successive oppositions, conjunctions, etc., of a celestial body

Telrad- a sighting device for telescopes which projects a bull's-eye on the sky

Terminator- the boundary between the illuminated and dark portions of a planet or satellite

Terra- extensive land mass

Terrestrial- refers to either Mercury, Venus, Earth, or Mars, due to them all having a solid surface, and they are within the inner solar system

Tholus- small dome-shaped mountain or hill

Tidal heating- frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites (e.g. Jupiter's moon Io)

Tidally locked- two bodies are tidally locked when their rotations are such that they always present the same side to the other -- their day is as long as their year; examples include Earth's moon, and the Pluto-Charon system


  1. the instant when a celestial object crosses the meridian
  2. when an object passes in front of another object

Ultraviolet (UV)- the portion of the spectrum with wavelengths just shorter than the bluest light visible

Umbra- the dark inner part of an eclipse shadow; also, the dark central part of a sunspot

Uncertainty Principle- the principle, formulated by Heisenberg, that one can never be exactly sure of both the position and the velocity of a particle; the more accurately one knows the one, the less accurately one can know the other

Undae- dunes (literally 'waves')

Vallis- sinuous valley (plural: valles)

Van Allen Belts- radiation zones of charged particles surrounding the Earth

Variable star- any star, the brightness of which appears to change, with periods ranging from minutes to years

Virtual particle- in quantum mechanics, a particle that can never be directly detected, but whose existence does have measurable effects

Visible spectrum- the wavelengths of light to which the human eye is sensitive

Vastitas- widespread lowlands

Volatile- As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes Hydrogen (H), Helium (He), water (H20), ammonia (NH3), carbon dioxide (CO2), and methane (CH4)

Wave/particle duality- the concept in quantum mechanics that there is no distinction between waves and particles; particles may sometimes behave like waves, and waves like particles

Wavelength- the distance between eave crests on any train of electromagnetic radiation; short ones are more energetic than long ones

Weak force- the second weakest of the four fundamental forces, with a very short range; it affects all matter particles, but not force-carrying particles

Weight- the force exerted on a body by a gravitational field; it is proportional to, but not the same as, its mass

White dwarf- a collapsed core of a normal star such as the sun after it has lost its outer layers

White hole- the exact opposite of a black hole; an object that spews out matter and energy

Wormhole- an object with two mouths in different parts of our universe connected by a tunnel that allows two-way traffic; they may be safe shortcuts through space

X-ray source- a region of extremely hot gas; matter torn away from a normal star by a black hole or a neutron star becomes violently heated and emits x rays

Yellow dwarf- an ordinary star such as the sun at a comparatively stable and long-lived stage of evolution

Zenith- the point on the celestial sphere directly overhead

Zodiac- the twelve constellations straddling the ecliptic through which the sun, Moon, and planets appear to move during the year

Zodiacal light- a faint cone of light rising from the horizon after sunset or before sunrise; it is caused by sunlight reflected from thinly spread interplanetary material lying in the main plane of the Solar System

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