A B C D E F G H I K L M N O P Q R S T U V Z

Absolute magnitude	How bright a star would look if it were 32.6 light years away from the Earth. The apparent magnitude that a star would possess it if were placed at a distance of 10 parsecs from the Earth. In this way, absolute magnitude provides a direct comparison of the brightness of stars.
Absolute zero	The lowest possible temperature: -459.6 degrees F.; -273.16 degrees C.
Absorption nebula	A region of dust and gas which has no stars to illuminate it from within. As such, these objects are completely dark and obscure any light from stars or emission nebulae which may lie behind them.
Acceleration	Change in velocity (speed, or direction).
Accretion disc	A disc-like structure of dust and gas which forms around the massive object at the center of a region undergoing gravitational collapse. It appears disc-like because the regions of collapse are not stationary and move through space. Specifically, dust and gas are in orbit around the center of the galaxy.
Active galaxy	A galaxy in which some kind of unusual, perhaps violent, phenomenon is releasing energy. The energy is usually released from the galactic nucleus and is not related to the way in which energy is normally released by stars. Active galaxies comprise ten per cent of the population of galaxies which can be viewed by astronomers from Earth.
Aerolite	A meteorite which is stoney.
Airglow	The natural glow of the night sky due to reactions that take place in the Earths upper atmosphere.
Albedo	The albedo of an object is how much light it reflects. A perfect reflector such as a mirror would have an albedo of 100, the moon has an albedo of 7, and the Earth has an albedo of 36.
Alpha Centauri	Part of a triple star system which has the distinction of being the closest star system to the Sun. It exists at a distance of 4.3 light years. The two other components are Beta Centauri and Proxima Centauri, which is slightly closer than the other two. Alpha Centauri can be seen in the southern sky in the constellation of Crux.
Andromeda galaxy	The nearest spiral galaxy to our own Milky Way. It is 2.2 million light years away and contains in the region of 4 x 1011 stars. The Andromeda galaxy is believed to be a twin of the Milky Way. Their combined gravitational influence dominates the Local Group of galaxies. It is designated M31 in the Messier Catalogue and NGC 224 in the New General Catalogue. Andromeda is surrounded by a number of satellite galaxies and globular clusters.
Angstrom unit	Unit used to measure the wavelength of light, and other electromagnetic radiation.
Antimatter	A particle with precisely opposite properties to those of its matter counterpart. For example, the antimatter counterpart of an electron, known as a positron, has a positive charge, equal in strength to the negative charge of the electron. It also has an opposite spin.
Apastron	When two stars that orbit each other are as far away from each other as they can get.
Aphelion	The point in an objects orbit around the Sun when it is furthest from the Sun.
Apogee	The point in an objects orbit around the earth when it is furthest from the Earth.
Arcsecond	An angular measurement which corresponds to one three hundred and sixtieth of a degree.
Asteroid	A rock, or Minor Planet orbiting the Sun, primarily between the orbits of Mars and Jupiter. The largest asteroid are Ceres, with a diameter of about 578 mi; Pallas, with a diameter of about 343 mi; and Vesta, with a diameter of about 324 mi. About 200 asteroids have diameters of more than 60 mi, and thousands of smaller ones exist.
Astrology	A belief that links the positions of the stars and planets to human destinies. Astrology has no scientific basis.
Astronomical Unit	The distance from the Earth to the Sun; Usually written AU.
Astronomy	The science of celestial bodies, their magnitudes, motions, constitution, etc. (2) A treatise on this science.
Astrophysics	The use of physics and chemistry in the study of Astronomy.
Atmosphere	The gaseous area surrounding a planet or other body.
Atom	The smallest particle of any element. Two or more atoms are needed to make a molecule.
Aurora	Beautiful lights seen over the polar regions which are caused when energized particles from the Sun enter the Earth's atmosphere and react with the magnetic field.
Axis	An imaginary straight line on which an object rotates.
Background radiation	Weak microwave radiation coming from space in all directions, believed to be remnants of the Big Bang.
Barred-spiral galaxy	A form of disc galaxy in which the spiral arms are attached to the galactic nucleus by a straight bar of stars.
Barycentre	The center of gravity of the Earth, and moon.
Big Bang	The beginning: The Big Bang theory is the dominant scientific theory about the origin of the universe. According to the theory, the universe was created about 14 billion years ago from a cosmic explosion that hurled matter in all directions. We now see debris from the blast as the expanding cosmos, including our Solar System (sun, planets, etc.) as fragments.
Binary star	A star which is actually made up of two stars orbiting each other.
Black Hole	A concept in the study of gravitation. A region of space around a very small but extremely dense and heavy object within which the gravitational field is so strong that not even light can escape. [More detail: Black Hole]
Bolide	A brilliant meteor, which may explode during its descent through the Earth's atmosphere.
Bolometer	A sensitive radiation detector.
Celestial sphere	An imaginary sphere surrounding the Earth. It is used to help astronomers explain where objects are found in the sky.
Cepheid	A variable star that scientists can use to determine how distant a galaxy, or star cluster is.
Charge-Coupled Device (CCD)	A sensitive imaging device which is replacing photography in most branches of Astronomy. Simple CCD's are also used in video cameras and desktop scanners.
Chromosphere	Part of the Sun's atmosphere, it is visible during a total solar eclipse.
Circumpolar star	A star which never sets, but can be viewed year round.
Clusters	A group or stars, or galaxies which are held together by their common gravity.
Color index	A measure of a star's color, which tells scientists how hot the stars surface is.
Coma	The hazy-looking patch surrounding the nucleus of a comet.
Comet	A small, nebulous, frozen celestial mass of dust and gas revolving around the sun characterized by a long, luminous tail, but only in the segment of the comet’s orbit when it passes closest to the sun. In ancient times they were called "hairy stars."
Conjunction	When a planet appears to come close to another planet, or star. It only appears to come close because it moves in between the other object, and the Earth.
Constellation	Grouping of stars given names by ancient astronomers because of the way they appear from Earth. ('Stars' in this instance means any bright object in the heavens.) Over ninety such groups -- constellations -- are recognized by astronomers.
Copernican system	The systematic explanation of the movement of the planets around the sun; advanced in 1543 by the Polish astronomer Nicolaus Copernicus. The Copernican system advanced the theories that the earth and the planets are all revolving in orbits around the sun, and that the earth is spinning on its north-south axis from west to east at the rate of one rotation per day. These two hypotheses superceded the Ptolemaic System, which had been the basis of astronomical theory until that time. The Copernican system first described the precession of the equinoxes but did not explain it.
Corona	The outermost part of the Sun's atmosphere.
Coronagraph	A type of telescope designed to view the Sun's Corona.
Cosmic rays	High-speed particles that reach the Earth from Outer Space.
Cosmology	The study of the universe as a whole, including theories about its origin, evolution, large-scale structure, and future. The more specific study of the origin of the universe and of astronomical systems, such as the solar system, is often called cosmogony.
Day	Amount of time it takes the Earth to spin once on its axis.
Density	The compactness of matter.
Direct motion	Objects moving around the Sun in the same direction as the Earth are moving in direct motion, objects moving in the opposite direction are moving in retrograde motion.
Diurnal motion	The apparent motion of the sky from East to West caused by the Earth moving from West to East.
Dwarfs or Brown Dwarfs	Small galaxies that give off dim or brownish light. Dwarf elliptical galaxy is any elliptical galaxy which is substantially smaller than the average. Dwarf irregular galaxy is any irregular galaxy which is substantially smaller than the average. Dwarf star is any star, which lies on the main sequence and is too small to be classified as a giant star or a supergiant star. For instance, the Sun is a yellow dwarf star.
Earthshine	The faint glow of the moon when the side facing Earth is dark. Caused by light reflecting off the Earth.
Eclipse	When our view of one object in the sky is blocked by either another object, or the earth's shadow. In astronomy, the obscuring of one celestial body by another, particularly that of the sun or a planetary satellite. Two kinds of eclipses involve the earth: those of the moon, or lunar eclipses; and those of the sun, or solar eclipses. A lunar eclipse occurs when the earth is between the sun and the moon and its shadow darkens the moon. A solar eclipse occurs when the moon is between the sun and the earth and its shadow moves across the face of the earth. Transits and occultations are similar astronomical phenomena but are not as spectacular as eclipses because of the small size of these bodies as seen from earth.
Ecliptic	The path the Sun, Moon, and planets all follow in the sky.
Ecosphere	The area around a star where it is just the right temperature for life to exist.
Electron	Negative particle which orbits the nucleus of an atom.
Element	Substance which cannot be broken down any further. there are 92 known elements.
Elliptical galaxy	A galaxy in the shape of an ellipse, or oval, as opposed to being round like a wheel. (Also see spiral galaxy).
Envelope	Layer or covering.
Equinox	March 21st, and September 22nd. Twice a year, when the day and night are the same amount of time all around the world.
Escape velocity	The speed an object must have in order to escape from another objects gravity.
Exosphere	The outermost part of the Earth's atmosphere.
Flares (Solar Flares)	Beautiful eruptions in the outer part of the Sun's atmosphere.
Galactic Center	The center of a galaxy's mass or gravatational center of a galaxy. Not necessarily the 'middle' or visual center.
Galaxy	A group of stars, gas and dust held together by gravity.
Gamma ray	Extremely short-wavelength, and energetic electromagnetic radiation.
Geocentric	Simply means the Earth in the Center. People used to believe the Universe was geocentric, or that the Earth was in the center of the Universe.
Geophysics	Study of the Earth using Physics.
Gibbous	When the Moon is more than half full, but less than completely full.
Globular clusters	A spherical collection of old stars found within galactic halos of spiral galaxies and, more commonly, around elliptical galaxies. The globular clusters formed at the same time as the host galaxy.
HI region	Cloud of neutral hydrogen.
HII region	Cloud of ionized hydrogen.
Hertzspurng-Russell Diagram	A diagram which helps scientists understand different kinds of stars.
Hubble Constant	The relationship between the distance of an object, and the speed at which it is traveling away from us. The further away an object is, the faster away from us it is traveling.
Inferior planets	Mercury and Venus which lie closer to the Sun than the Earth are called inferior planets. Planets farther from the Sun than Earth are superior planets.
Ionosphere	Region of the Earths atmosphere.
Kepler's Laws of Planetary Motion	1. The planets move in elliptical orbits, with the Sun at one focus. 2. An imaginary line joining the center of a planet to the center of the Sun sweeps the same amount of space all the time. 3. The time it takes a planet to orbit the Sun is related to how far away from the Sun an object is.
Kirkwood gaps	Regions in the asteroid belt where almost no asteroids can be found. This is due to the fact that the giant planet Jupiter changes the orbits of any object which enters these areas.
Light Year	The distance which a ray of light would travel in one year. This is about 6,000,000,000,000 (6 trillion) miles. The speed of light is 186,300 miles per SECOND.
Limb	The edge of any object in Outer Space. The edge of the Moon for example.
Local Group	A group of around two dozen galaxies. It is the group to which our galaxy, the Milky Way, belongs.
Lunation	Period between new moons. 29 days 12 hours 44 minutes.
Magellanic clouds	The Large and the Small Magellanic clouds are two satellite galaxies of the Milky Way. They are both irregular galaxies and are both forming stars. They are only visible from the Southern hemisphere.
Magnetic star	A star with extraordinarily strong magnetic fields.
Magnetosphere	Region around an object where the influence of the objects magnetic field can be felt.
Mass	How much matter an object contains. It is not the same as weight, although an object's mass does help determine how much it will weigh. Every mass attracts every other mass in the universe according to Newton's law of gravity. Along with length and time, mass is one of the properties which describe anything in the universe.
Matter	Any particle which possesses mass, e.g., proton, electron. Particles which just carry energy, e.g., photons, and possibly neutrinos, do not constitute matter.
Messier catalogue	A catalogue of over one hundred bright galaxies, nebulae and star clusters. It was compiled by the French astronomer Charles Messier and published in 1774.
Meteor	A shooting star, observed when a particle of dust enters the Earth's atmosphere. In astronomy, a meteor is a small solid body entering a planet's atmosphere from outer space and raised to incandescence by the friction resulting from its rapid motion. Brilliant meteors, known as fireballs, occur singly and generally consist of a luminous head, followed by a comet-like train of light that may persist for several minutes; some, called bolides, have been seen to explode with a sound like thunder.
Meteorite	An object from Outer Space such as a rock, that falls into the earth, and lands on its surface. A meteor that reaches the surface of the earth or of another planet before it is entirely consumed.
Meteoroids	Any small object in Outer Space, such as dust, or a rock.
Micrometeorites	An extremely small object. They are so small that when they hit the Earths atmosphere they do not create a shooting star effect.
Milky Way	Our galaxy. (the word "Galaxy" actually means milky way in Greek).
Minor planet	An Asteroid.
Molecule	A group of atoms linked together.
Multiple star	A group of stars that orbit each other.
Nadir	That point on the celestial sphere directly below the observer.
Nebula	A cloud of gas and dust.
Neutrino	A very small particle with no mass or charge.
Neutron star	The remnants of a dead star. They are incredibly compact, and spin very quickly. Some spin 100 times a second.
Nova	A star which suddenly flares up to many times its original brightness before fading again. (Also see supernova).
Occultation	The covering up of one celestial body by another.
Opposition	When a planet is exactly opposite the Sun, so that the Earth is between them.
Orbit	The path one object takes around another.
Ozone	An area in the Earth's upper atmosphere which absorbs many of the lethal radiations coming from space.
Parallax	The shift of an object when it is viewed from two different places. For example if you close one eye, and look at your thumb nail, and then switch eyes, you will see everything in the background move back and forth. Scientists use this to measure the distance to stars.
Parsec	The distance at which a star would have a parallax of one arc second. One parsec is equivalent to 3.26 light years, 206,265 astronomical units or 30.9 x 1012 km.
Penumbra	The lighter part of a shadow found on the shadows edge.
Periastron	When two stars that orbit each other are at their closest point.
Perigee	The point in an objects orbit around the Earth when it is closest to the Earth.
Perihelion	When an object which revolves around the Sun is at the closest point it gets to the Sun.
Perturbations	The disturbances in the orbit of a celestial object caused by the gravitational pull of another object.
Phases	The apparent change in the shape of the Moon, Mercury, and Venus due to how much of the sunlit side is facing the Earth.
Photosphere	The bright surface of the Sun.
Planet	An object moving around a star.
Planetary nebula	A nebula of gas surrounding a star. Called 'planetary' because from Earth it may appear to be a planet. 'Planetary nebula' is now considered a misnomer because they are not associated with planets.
Precession	The Earth behaves like a spinning top. Its poles are spinning in circles causing the poles to point in different directions over time. It takes 25,800 years for the Earth to complete one precession.
Proper motion	The motion of the stars across the sky as seen from Earth. Closer stars have a higher proper motion than more distant ones, just as in a car closer objects such as road signs seem to move faster than distant mountains and trees.
Proton	The center of an Atom is made up of one or more protons. Protons have a positive charge.
Ptolemaic	The theory of the order and action of the heavenly bodies, advanced in the 2d century A.D. by the Greek astronomer Ptolemy. The Ptolemaic theory held that Earth is stationary and at the center of the universe; closest to Earth is the Moon, and beyond it, extending outward, are Mercury, Venus, and the Sun in a straight line, followed successively by Mars, Jupiter, Saturn, and the so-called fixed stars.
Pulsar	A highly magnetized neutron star, with a radius of 10-15 km, having somewhat greater mass than the Sun which has a radius of approximately 1 million km. Radiation is beamed out along the magnetic poles and pulses of radiation are received as the beam crosses the Earth, in the same manner as the beam from a lighthouse causes flashes.(Also see radio galaxy.)
Quasar	An acronym for quasi-stellar radio source. A very distant immensely bright object. Any of the blue, star-like objects that are strong radio emitters and the spectra of which exhibit a strong red shift.
Radiant	The area in the sky where, during a meteor shower, the meteors appear to radiate from.
Radio astronomy	A branch of astronomy in which celestial objects and astrophysical phenomena are studied by examining their emission of electromagnetic radiation in the radio portion of the spectrum.
Radio galaxies	Galaxies which are extremely powerful emitters of radio radiation. (Also see pulsars).
Red shift	When an object is traveling away from Earth, light from the object is stretched out, making it look redder.
Reflection Nebula	A nebula that reflects light as opposed to a nebula that emits light. In other words, a reflection nebula has no inherent light source but is instead reflecting light, just like the Earth's moon reflects sunlight.
Revolve	When something is moving in a circle around another object such as the way the Moon circles the Earth it is said to revolve around that object.
Rotate	When an object spins it is said to be rotating. Rotation: The Milky Way rotates around an axis joining the galactic poles. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise, and the spiral arms trail in the same direction. The period of rotation decreases with the distance from the center of the galactic system. In the neighborhood of the solar system the period of rotation is more than 200 million years. The speed of the solar system due to the galactic rotation is about 270 km/sec (about 170 mi/sec).
Saros cycle	A period of 18 years 11.3 days in which eclipses repeat themselves.
Satellite	A small object orbiting a larger one. There are many electronic objects that orbit the Earth.
Scintillation	Twinkling of stars. Due to the Earth's atmosphere.
Seeing	The condition of the Earth's atmosphere at a particular time. If the sky is clear astronomers say there is good seeing.
Selenography	The study of the Moon's surface.
Seyfert galaxies	Galaxies with small bright centers. A specific class of galaxies which have active galactic nuclei. These can be spiral galaxies or barred spiral galaxies which have bright, compact galactic nuclei. The nuclei far outshine the spiral arms of the host galaxies and are far too bright to be composed only of stars. Many Seyfert galaxies are good sources of radio waves.
Shooting star	A light in the atmosphere caused by a meteor falling towards the Earth.
Solar System	The system of planets and other objects orbiting the star Sol, which happens to be our Sun. A stellar-planetary unit consisting of the Sun; the nine planets and their satellites; the asteroids, comets, and meteoroids; and interplanetary dust and gas. Our Solar System is the only planetary system known to exist, although in the 1980s a number of relatively nearby stars were found to be encircled by swarms of orbiting material of indeterminate size or to be accompanied by suspected brown dwarfs. Many astronomers think it likely that solar systems of some sort are numerous throughout the universe.
Solar mass	A mass equal to the total mass of our Solar System. Two solar masses would be twice the mass of our solar system. A 100 solar mass object would be equal to the mass of our solor system times 100.
Solar wind	A steady flow of particles streaming out from the Sun in all directions.
Solstice	22 June, and 22 December. Time of the year when the day is either shortest, or longest depending on where you are.
Spicules	Jets up to 16,000 kilometers in diameter, in the Sun's atmosphere.
Spiral Galaxy	A round, or spiral, galaxy that looks like a giant wheel. (See also elliptical galaxy).
Stratosphere	Level of the Earths atmosphere from about 11-64 kilometers above sea level.
Star	A self-luminous object that shines through the release of energy produced by nuclear reactions at its core.
Supernova	A super bright explosion of a star. A supernova can produce the same amount of energy in one second, as an entire galaxy. (Also see nova).
Sun	Proper name, Sol. Our small star.
Sundial	Ancient instrument used to tell time.
Sun spots	Dark patches on the Sun's surface.
Superior planets	The planets which lie further from the Sun than the Earth. Those that are closer, Venus and Mercury, are inferior planets.
Synchronous satellite	An artificial satellite which moves around the Earth at the same speed that the Earth rotates, so that it is always above the same part of Earth.
Syzygy	The position of the Moon in its orbit when at new or full phase.
Terminatior	The line between day and night on any celestial object.
Thermocouple	An instrument used for measuring very small quantities of heat.
Time dilation	The idea that as you approach the speed of light, time slows down and mass increases.
Trojan asteroids	Asteroids that circle the Sun following Jupiter's orbit.
Troposphere	The lowest part of the Earth's atmosphere.
Umbra	The dark inner part of a sunspot, or shadow.
Variable stars	Stars which fluctuate in brightness.
Zenith	Point directly above your head in the night sky.
Zodiac	An imaginary belt in the celestial sphere. Zodiac is a term used in Astrology rather than Astronomy. The width of the zodiac was determined originally so as to include the orbits of the Sun and Moon and of the five planets (Mercury, Venus, Mars, Jupiter, and Saturn) that were known to the people of ancient times. The zodiac is divided into 12 sections of 30° each, which are called the signs of the zodiac. Starting with the vernal equinox and then proceeding eastward along the ecliptic, each of the divisions is named for the constellation situated within its limits in the 2d century BC. The names of the zodiacal signs are Aries, the Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricornus, the Goat; Aquarius, the Water Bearer; and Pisces, the Fishes.