[A] [B] [C] [D] [E]


A

Absorbtion Line:
A dark line or band at a particular wavelenth on a spectrum, formed when a substance between a radiating source and an observer absorbs electromagnetic radiation of that wavelength. Different substances produce characteristic patterns of absorption li nes.
Acceleration:
a change in velocity. The term includes changes of direction and decreases as well as increases in speed.
Accretion Disk:
A diskformed from gases and toehr materials drawn in by a compact body, such as a black hole or a neutron star, at the disk's center.
Active Galaxy:
A galaxy with a highly energetic nucleus.
Alpha Particle:
The nucleus of one form of heliuim, consisting of two portons and two neurons.
Albedo:
The reflectivity of a body; in planetary astronomy, the fraction of sunlight reflected by a planet or moon.
Ammonia:
A molecule made up of one atom of nitrogen and three of hydrogen. Astronomers use ammonia to gauge the densit and temperature of interstellar clouds because its unique energy state transitions are readily observable at microwave wavelengths.
Angstrom:
A unit of lenth equal to one ten-billionth of a meter (about four-billionths of an inch); used in astronomy as a measure of wavelength.
Angular diameter:
An object's width on the sky, measured in units of arc distance. The Moon's angular diameter is just over half a degree; Mars's is 3.7 seconds of arc at the planet's nearest approach to Earth, 17.9 seconds of act at conjunction.
Angular momentum:
A measure of an object's inertia, or state of motion, about an axis of rotation.
Annihilation:
The disappearance of a particle and matching antiparticle as a result of their collision. the collision converts the masses of the particle and antiparticle into energy.
Antenna:
A device that beams and collects radio waves; in radio astronomy, the component of a radio telescope that converts received radio waves into electrical signals.
Antilepton:
An antiparticle to any of several types of leptons.
Antileptoquark:
In theory, a boson that is the antiparticle to the leptoquark. Like its counterpart, it would have existed only for an instant in the early universe.
Antimatter:
Matter made up of antiparticles. Most antimatter would have been annihilated in the first second after the Big Bang.
Antiparticle:
A particle identical in mass to a matter particle but opposite to it in properties such a s electrical charge. For example, a positron is the antiparticle to an electron.
Antiproton:
an antiparticle to a proton
Antiquark:
An antiparticle to a quark. Antiquarks have anticolor instead of color.
Aphelion:
The pint in the orbit of a planet or a comet where it is farthest from the Sun.
Apparent day:
The period between one high noon and the next, as observed from a particular point on a planet's surface; also called solar day, or sol.
Arc distance:
Units used in measuring the apparent position and size of celestial bodies. Arc distance is expressed in degrees, minutes (sixty per degrees), and seconds (sixty per minute)
Arc second:
a sixtieth of an arc minute, which is in turn a sixtieth of degree of arc; there are 360 degrees in a circle. Arc seconds, minutes, and degrees measure an object's apparent size and postion on the sky.
Arrow-Breaking:
Using the air around the ship to build up friction to decellerate.
Asteroid:
A small, rocky, airless body that orbits a star.
Astronomical unit (AU)
A unit of mesure, often used to express distances within the Solar System, that is equal to the mean distance between Earth and the Sun, or 92,960,116 miles.
Astrophysics:
The study of the physics of astronomical objects and processes, made possible by the development of spectroscopy and photography in the nineteenth century.
Atmosphere:
The gaseous outer layer of a planet, star, or other body with sufficient gravity to maintain it.
Atom:
the smallest component of a chemical element that retains the properties associated withthat element. Atoms are composed of protons, neutrons, and electrons; the number of protons determines the identity of the element.
Atomic weight:
the mass of an atom, roughly equal to the number of protons and neutrons in its nucleus.
Attitude:
The orientation of an object with respect to its direction of motion.
Aurora:
Light fiven off by collisons between charged particles trapped in a planet's magnetic field and atoms of atmospheric gases near a planet's magnetic poles.
Axis:
The imaginary line, drawn through the poles of a celestial body, around which the body rotates; also, one of two perpendicular lines (the major axis and the minor axis) passing through the center of an ellipse.

B

Background limit:
the distance beyond which objects form a continuous screen; if objects do not extend to the limit, gaps will be observed between them. In an infinite universe, stars would extend past the background limit, filling the night sky with continuous starlig ht.
Background Radiation:
a detectable, steady emission of electromagnetic radiation from all directions of the sky. Cosmic Background radiation in microwave frequencies is commonly attributed to the Big Bang.
Barred Spiral galxy:
A disk galaxy in which the spiral arms begin at the ends of a central bar rather than at the nucleus.
Basalt:
A dark, close-grained igneous rock, formed by the hardening of lava. Most volcanic rocks are basalts.
Bending Wave:
In a planetary ring, a corrugated wave pattern caused by the gravitational pull of a moon alternatiely orbiting above and below the ring plane.
Big Bang:
According to a widely accepted theory, the primeval moment, 15 to 20 billion years ago, when the universe began expanding from a state of infinite density.
Binary Stars:
A gravitational bound pair of stars in orbit around their mutual center of gravity. Binary stars are extremely common, as are systems of three or more stars.
Binary Star System:
A gravitationally bound pair of stars in orbit around their mutual center of gravity. Binary stars are extremely common, as are systems of three or more stars.
Bit:
The smallest unit of computerized information, represented by a single zero or one(the term is an abbreviation of "binary digit"). Streams of bits are used to transmit images and other data between unmanned spacecraft and controllers on Earth. see< /i> Byte.
Black body:
a hypothetical object that absorbs and reemits all radiation reaching it.
Black Dwarf:
the hypothetical remnant of a dwarf star.
Black Hole:
In theory, an extremely compact body with such great gravitational force that no radiation can escape from it.
BL Laceratae object:
An N-galaxy with a particularly bright nucleus that changes rapidly in luminosity; named for BL Lacertae, the first such galaxy to be identified.
Blue Giant:
A young, luminous, high-mass star of spectral class O or B.
Blueshift:
A Doppler effect seen when a radiating source approaches the observer. The received wavelengths shorten so that any absorption or emission lines move from their expected frequencies toward the short-wavelength, blue end of the spectrum. see Re dshift.
Boson:
A particle that carries force; one of two types of elementary particles.
Bow shock:
In planetary science, the boundary region where the solar wind is first deflected by a planet's magnetic field.
Bridge:
A filament of stars stretching between two galaxies that may result from the galaxies' interaction.
Brightness:
The amount of light received from an object; a combined result of its actual luminosity, its distance, and any light absorption by intervening dust or gas.
Brown Dwarf:
A dim body of less than .1 solar mass with not enough self-gravity to fuse hydrogen to helium.
B stars:
A spectral class of young, massive, blue stars, the second brightest type, found mainly in the galaxy's spiral arms. Because they emit high levels of radiation, B stars ionize the hydrogen in surrounding clouds.
Bubble Chamber:
an experimental device that maintains a quantity of liquid just at the point of boiling, so that passing high-energy particles will leave trails of bubbles.
Bulge:
A large sphere of stars at the center of a spiral galaxy.
Byte:
A common unit of computerized information, usually consisting of eight bits. see bits.

C

Caldera:
A crater formed by the collapse or subsidence of the central part of a volcano.
Calcium: A chemical element common in its solid state on Earth and present as a gas in interstellar space.
Carbon cycle:
A fusion reaction that occurs in high-mass stars, in which carbon, oxygen and nitrogen nuclei provide sites for hydrogen nuclei to fuse to helium.
Carbon monoxide:
A diatomic compound of carbon and oxygen found on Earth as a toxic by-product of combustion and present in large quantities in interstellar space
Cassini Division:
The seeming gap, now known to include several narrow rings, between Saturn's two brightest rings; named for the Italian astronomer Giovanni cassini, who discovered it in 1675. .
Catalog:
A published list of astronomical objects and their precise positoins.
Celestial coordinates:
A pair of numbers designating an object's location on the celestial sphere. One coordinate, declination, is a north-south value similar to latitude; the other, right ascension, is similar to latitude.
Celestial mechanics:
The study of the motions and interactions of astronomical objects.
Celestial sphere:
The apparent sphere of sky that surrounds the Earth; used by astronomers as a convention for specifying the location of a celestial object.
Celsius:
A scientific temperature scale in which 0 degrees is the freezing point and 100 degrees is the boiling point of water.
Cepheid variable:
star that changes regularly in luminosity over a set period of days or weeks.
Chandrasakhar Limit:
The maximum core mass for a stable white dwarf star, about 1.44 solar masses; named for Subrahmanyan Chandrasekhar, who proposed it. Stars that exceed the limit may become neutron stars or black holes.
Charge-coupled Device:
An electronic array of detectors, usually positioned at a telescope's focus, for registering electromagnetic radiation.
Closed Universe:
A universe in which the average density of mass exceeds a critical value, so that gravity will eventually reverse the expansion of space. According to relativity theory, the geometry of such a universe is spherical.
Cluster (of galaxies):
A gravitationally bound system of galaxies, ranging in number from a few dozen to several thousand.
Collision:
The gravitational interaction of galaxies at very close range, sometimes resulting in a merger.
Color:
An abstract property, not related to ordinary visible color, that governs the interactions of some elementary particles. Particle color may be assigned the value blue, red, or green.
Comet:
An asteroid-size body of dusty ice that travels an elongated orbit around the Sun.
Compression wave:
A seismic wave that expands and then contracts the medium through which it passes.
Conjunction:
An apparent meeting or close approach of two celestial bodies aas viewed from Earth. In planetary astronomy, if only one planet is mentioned, the second body in the conjunction is the Sun. Inferior conjunction occurs when Venus or Mercury passes betwe en the Sun and Earth; superior conjunction occurs when a planet is directly behind the Sun.
Constellation:
Originally a pattern of stars named for an object, animal, or person, but now more commonly the area of sky assigned to that pattern. Every astronomical object is located in a specific constellation.
Continuous Spectrum:
A spectrum consisting of all wavelengths in a given range, without absorbtion or emission lines.
Convection:
The transfer of heat in a fluid or gas by the movement of currents from hotter to cooler regions.
Core:
The central component of a celestial body. In planets, it is usually composed of dense, hot material, frequently solid.
Corona:
The outer layer of the Sun's atmosphere, composed of diffused, ionized gas.
Coronae:
see Ovoids.
Coriolis effect:
The apparent deflection of an object's trajectory over the surface of a rotating body's rotation. The effect is seen, for example, in the spiral shapes taken by storms on Earth and in the atmospheres of other planets.
Cosmic Ray:
An atomic nucleus or other charged particle moving at close to the speed of light; thought to originate in supernovae and other violent celestial phenomena.
Cosmic singularity:
In theory, the state of the universe before the Big Bang, when all matter was compressed into a state of infinite density.
Cosmic string:
According to theory, a type of massive, one-dimensional object that formed during the early expanse of the universe.
Cosmological constant:
A mathematical factor introduced by Einstein into the field equations of general relativity to accomodate his belief in a static universe. Today the cosmological constant is predicted by some theories but is normally set at zero and thus disregarded.
Cosomological principle:
the assumption that the universe is homogeneous at the very largest scale, appearing essentially the same to observers in all locations.
Cosmological red shift:
see Red Shift
Cosmology:
The study of the universe as a whole, including its large-scale structure and movements, origin, evolution, and ultimate fate. A specialist in this field is called a cosmologist.
Cosmos:
The universe; also, a mathematical or scientific model of the universe.
Critical density:
A crucial value for the density of matter in the universe, about 4.5 times 10^-29 gram per cubic centimeter. The relation between the actual mass density of the universe and the critical density determines whether the universe is closed, flat, or ope n. See closed universe, flat universe, open universe.
Crater:
A bowl-shaped depression on a planet's surface, formed by the impact of a meteorite or comet.
Crust:
The solid surface layer of a planet.
Curved Space:
A distortion in the goemetry of space, normally occuring near massive bodies, that can be detected by observing the curved path followed by light in that region.

D

Dark Matter:
A form of matter that has not yet been directly observed but whose existence is deduced from its gravitational effects.
Data compression:
Any of several methods used to reduce the number of bits required to transmit computerized information in the lest amount of time.
Decay:
The spontaneous transformation of a particle into one or more other particle into one or more other particles, which may then decay as well.
Degeneracy:
A highly compressed state of matter characteristic of the core of certain dwarf stars. In degenerate matter, the ideal gas law relating density to pressure does not apply.
Density:
The nucleus of an atom of deuterium, an isotope of hydrogen. A deuteron contains one proton and one neutron.
Density Wave:
A moving pattern of compression and rarefraction. Density waves are thought to produce the grooved structure of Saturn's rings.
Deuteron:
A particle consisting of one neutron and one proton; equivalent ot the nucleus of an atom of deuterium.
Deuterium:
An isotope of hydrogen, sometimes referred to as heavy hydrogen. A hydrogen nucleus contains a single proton; a deuterium nucleus contains one proton and one neutron.
Diatomic:
Having two atoms in a molecule.
Differential Rotation:
Rotation in which components at different distances from the center orbit at different rates rather than as a rigid body.
Dish
A colloquial term for a radio telescope design in which a large bowl-shaped structure focuses received radio waves on a central point; may also refer to a large optical mirror.
Disk:
The pancake-shaped component of a spiral galxy, made up of stars, gas, and dust; often incorporates bright spiral arms.
Disk Galaxy:
Any galaxy with a disk, including spiral and barred spiral galaxies. Astronomers using computer simulations to study galactic structure and interactions often employ disk galaxies with no spiral features.
Doppler Effect:
A phenomenon in which waves appear to compress as their source approaches the observer or stretch out as the source recedes from the observer. See red shift.
Doppler Shift:
A change in the wavelength and frequency of sound or electromagnetic radiation, caused by the motion of the emitter , the observer, or both.
Double radio source:
An active galaxy consisting of two radio lobes flanking an elliptical galaxy.
Dust:
Small grains of matter, largely graphite and silicates, that pervade interstrellar space. Although extremely diffuse, dust is responsible for scattering, absorbing, and polarizing starlight. Dust grains are also the building blocks of most celestial m atter and the site of chemical reactions that produce interstellar molecules.
Dynamical friction:
A dragging force created by the combined gravitational effects of several bodies.

E

Eccentricity:
The amount of separation between the two foci of an ellipse and, hence, the degree to which the ellipse deviates from a circular shape. Mercury's elongated orbit, for example, is more eccentric than Earth's.
Eclipse:
The obscuration of light from a celestial body as it passes through the shadow of another body.
Ejecta:
The matter that is thrown from a depression on the surface of a planet, either through a meteorite or comet collisoin or through a volcanic eruption or other tectonic process.
Electromagnetic Radiation:
Radiation consisting of periodically varying electric and magnetic fields that vibrate perpendicularly to each other and travel through space at the speed of light.
Electromagnetic spectrum:
The array of electromagnetic radiation, ranging in order of frequency or wavelength from low-frequency , long-wavelength radio waves through infrred radiation, visible light, and ultraviolet radiation to high-frequency, short-wavelength gamma rays.
Electromagnetism:
The force that attractsd oppositely charged particles and repels similarly charged particles. Electro magnetism affects all charged particles but not neutral particles such as neutrinos.
Electron:
a negatively charged particle that normally orbits an atom's nucleus but may exist in isolation.
Electronuclear force:
According to grand unification theories (GUTs), a force in which electromagnetism, the weak force, and the strong force are combined and indistinguishable. The electronuclear force could exist only at the extremely high energies of the very early u niverse or in particle accelerators.
Electroweak force:
A force in which electromagnetism and the weak force are combined and indistinguishable. The electroweak force can exist only at very high energies.
Element:
One of just over 100 substances that cannot be reduced by chemical means to simpler substances.
Elementary Particle:
A fundamental, irreducible component of the physical universe. The two currently recognized classes of elementary particles are bosons and fermions.
Ellipse:
A closed, symmetrical curve with tow focal points and with vertical and horizontal axex of unequal length.
Elliptical galaxy:
A galaxy shaped like a flattened sphere, with no discernible features and no disk.
Elongation:
The separation of two celestial bodies as seen from a third, expressed in units of arc distance. The elongation of a planet refers to its distance from the Sun, as measured from Earth.
Emission Line:
A bright band at a particular wavelength on a spectrum, emitted directly by the source and indicating by its wavelength a chemical constituent of that source.
Energy:
A closed, symmetrical curve with two focal points and with vertical and horizontal axes of unequal length.
Energy level:
A discrete quantity of energy associated with a particle within an atom or a nucleus . An increase in energy will shift electrons to higher energy levels within the atom.
Epicycle:
A small circle centered on the rim of a larger one, used by early astronomers to explain the motions of the planets in what was thought to be an Earth-centered universe.
Equator:
The imaginary line around a celestial object that lies in a plane passing through the center of the object perpendicular to its spin axis.
Equivalence principle:
The rule, derived from general relativity, that in a small region of space-time, the effects of a gravitational field are indistinguishable from tose produced by an acceleration of the frame of reference.
Ergosphere:
THe region around a black hole, inside the proton sphere and outside the event horizon, where only objects that remain in motion can avoid entering the singularity.
Escape Velocity:
The speed and direction of motion that are required for an object to move beyond a closed orbit around a planet or other body.
Ether:
A hypothetical substance capable of carrying light waves, once thought to permeate all of space.
Euclidean geometry:
seeGeometry.
Event Horizon:
The boundary around a black hole's singularity, within which gravitational forces prevent anything, including light, from escaping.
Exclusion principle:
The rule of quantum mechanics stating that no two electrons, neutrons, or protons with the same energy, angular momentum, and spin can exist simultaneously in the same atom. Also called the Pauli exclusion principle after its formulator, Wolfgang Paul i.
Expanding universe:
The expansion of space itself as a result of the Big Bang.
Extinction:
In astronomy, t he blocking of light from stars or other celestial bodies caused by a combination of absorption and scattering.
Extragalactic:
Located outside the Milky Way galaxy.