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# The Astronomy Dictionary - A Complete List!

 A Definition Absolute Magnitude A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light-years) away from Earth. On this scale, the Sun has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it is so close. Absolute Zero The temperature at which the motion of all atoms and molecules stops and no heat is given off. Absolute zero is reached at 0 degrees Kelvin or -273.16 degrees Celsius. Ablation A process by where the atmosphere melts away and removes the surface material of an incoming meteorite. Accretion The process by where dust and gas accumulated into larger bodies such as stars and planets. Accretion Disk A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation. Achondrite A stone meteorite that contains no chondrules. Albedo The reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have an albedo of 0%. Albedo Feature A dark or light marking on the surface of an object that may or may not be a geological or topographical feature. Altitude The angular distance of an object above the horizon. Antimatter Matter consisting of particles with charges opposite that of ordinary matter. In antimatter, protons have a negative charge while electrons have a positive charge. Antipodal Point A point that is on the direct opposite side of a planet. Apastron The point of greatest separation of two stars, such as in a binary star system. Aperture The size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number represents a smaller opening while a lower number represents a larger opening. Aphelion The point in the orbit of a planet or other celestial body where it is farthest from the Sun. Apogee The point in the orbit of the Moon or other satellite where it is farthest from the Earth. Apparent Magnitude The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude. Asteroid A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets. Astrochemistry The branch of science that explores the chemical interactions between dust and gas interspersed between the stars. Astronomical Unit (AU) A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles. Atmosphere A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases. Aurora A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth's northern hemisphere and the Aurora Australis in the Earth's Southern Hemisphere. Aurora Australis Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the northern hemisphere. Aurora Borealis Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Australis in the southern hemisphere. Axis Also known as the poles, this is an imaginary line through the center of rotation of an object. Azimuth The angular distance of an object around or parallel to the horizon from a predefined zero point.

 B Definition Bar A unit of measure of atmospheric pressure. One bar is equal to 0.987 atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square inch. Big Bang The theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 13.7 billion years ago. This is the current accepted theory for the origin of the universe and is supported by measurements of background radiation and the observed expansion of space. Binary A system of two stars that revolve around a common center of gravity. Black Hole The collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The gravitational pull is so strong that not even light can escape. Blueshift A shift in the lines of an object's spectrum toward the blue end. Blueshift indicates that an object is moving toward the observer. The larger the blueshift, the faster the object is moving. Bolide A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom.

 C Definition Caldera A type of volcanic crater that is extremely large, usually formed by the collapse of a volcanic cone or by a violent volcanic explosion. Crater Lake is one example of a caldera on Earth. Catena A series or chain of craters. Cavus A hollow, irregular depression. Celestial Equator An imaginary line that divides the celestial sphere into a northern and southern hemisphere. Celestial Poles The North and South poles of the celestial sphere. Celestial Sphere An imaginary sphere around the Earth on which the stars and planets appear to be positioned. Cepheid Variable This is a variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter Cepheids will have a longer period. Chaos A distinctive area of broken terrain. Chasma Another name used to describe a canyon. Chondrite A meteorite that contains chondrules. Chondrule Small, glassy spheres commonly found in meteorites. Chromosphere The part of the Sun's atmosphere just above the surface. Circumpolar Star A star that never sets but always stays above the horizon. This depends on the location of the observer. The further South you go the fewer stars will be circumpolar. Polaris, the North Star, is circumpolar in most of the northern hemisphere. Circumstellar Disk A torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle. Coma An area of dust or gas surrounding the nucleus of a comet. Comet A gigantic ball of ice and rock that orbit the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun's rays. Conjunction An event that occurs when two or more celestial objects appear close close together in the sky. Constellation A grouping of stars that make an imaginary picture in the sky. Corona The outer part of the Sun's atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen in most solar eclipse photos. Cosmic Ray Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy. Cosmic String A tube-like configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other. Cosmogony The study of celestial systems, including the Solar System, stars, galaxies, and galactic clusters. Cosmology A branch of science that deals with studying the origin, structure, and nature of the universe. Crater A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.

 D Definition Dark Matter A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies. Debris Disk A ring-shaped circumstellar disk of dust and debris in orbit around a star. Debris disks can be created as the next phase in planetary system development following the protoplanetary disk phase. They can also be formed by collisions between planetesimals. Declination The angular distance of an object in the sky from the celestial equator. Density The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3. Disk The surface of the Sun or other celestial body projected against the sky. Double Asteroid Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid. Doppler Effect The apparent change in wavelength of sound or light emitted by an object in relation to an observer's position. An object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. The Doppler effect can be used to estimate an object's speed and direction. Double Star A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system. Dwarf Planet A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. Pluto is considered to be a dwarf planet.

 E Definition Eccentricity The measure of how an object's orbit differs from a perfect circle. Eccentricity defines the shape of an object's orbit. Eclipse The total or partial blocking of one celestial body by another. Eclipsing Binary A binary system where one object passes in front of the other, cutting off some or all of its light. Ecliptic An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky. Ejecta Material from beneath the surface of a body such as a moon or planet that is ejected by an impact such as a meteor and distributed around the surface. Ejecta usually appear as a lighter color than the surrounding surface. Electromagnetic Radiation Another term for light. Light waves created by fluctuations of electric and magnetic fields in space. Electromagnetic Spectrum The full range of frequencies, from radio waves to gamma waves, that characterizes light. Ellipse An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular. Elliptical Galaxy A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms. Elongation The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen in the evening sky and a planet at greatest western elongation will be seen in the morning sky. Ephemeris A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects. Equinox The two points at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21 and September 22. The equinoxes signal the start of the Spring and Autumn seasons. Escape Velocity The speed required for an object to escape the gravitational pull of a planet or other body. Event Horizon The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light. Evolved Star A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in the form of stellar wind. Extinction The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere. Extragalactic A term that means outside of or beyond our own galaxy. Extraterrestrial A term used to describe anything that does not originate on Earth. Eyepiece The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument. Eyepieces are available in different powers, yielding differing amounts of magnification.

 F Definition Faculae Bright patches that are visible on the Sun's surface, or photosphere. Filament A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun. Finder A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location. Fireball An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom. Flare Star A faint red star that appears to change in brightness due to explosions on its surface.

 G Definition Galactic Halo The name given to the spherical region surrounding the center, or nucleus of a galaxy. Galactic Nucleus A tight concentration of stars and gas found at the innermost regions of a galaxy. Astronomers now believe that massive black holes may exist in the center of many galaxies. Galaxy A large grouping of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth. Galilean Moons The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius. Gamma-ray The highest energy, shortest wavelength form of electromagnetic radiation. Geosynchronous Orbit An orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet's surface. Giant Molecular Cloud (GMC) Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation. Globular Cluster A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found around the central regions of a galaxy. Granulation A pattern of small cells that can be seen on the surface of the Sun. They are caused by the convective motions of the hot gases inside the Sun. Gravitational Lens A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. Gravitational lensing results in duplicate images of distant objects. Gravity A mutual physical force of nature that causes two bodies to attract each other. Greenhouse Effect An increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere. Carbon dioxide and water vapor are two of the major gases responsible for this effect.

 H Definition Heliopause The point in space 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 the Solar System. Hydrogen An element consisting of one electron and one proton. Hydrogen is the lightest of the elements and is the building block of the universe. Stars form from massive clouds of hydrogen gas. Hubble's Law The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us. Hydrostatic equilibrium A state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Hydrostatic equillibrium is responsible for keeping stars from imploding and for giving planets their spherical shape. Hypergalaxy A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.

 I Definition Ice A term used to describe water or a number of gases such as methane or ammonia when in a solid state. Inclination A measure of the tilt of a planet's orbital plane in relation to that of the Earth. Inferior Conjunction A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun. Inferior Planet A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system. International Astronomical Union (IAU) An international organization that unites national astronomical societies from around the world and acts as the internationally recognized authority for assigning designations to celestial bodies and their surface features. Interplanetary Magnetic Field The magnetic field carried along with the solar wind. Interstellar Medium The gas and dust that exists in open space between the stars. Ionosphere A region of charged particles in a planet's upper atmosphere. In Earth's atmosphere, the ionosphere begins at an altitude of about 25 miles and extends outward about 250. Iron Meteorite A meteorite that is composed mainly of iron mixed with smaller amounts of nickel. Irregular Galaxy A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape. Irregular Satellite A satellite that orbits a planet far away with an orbit that is eccentric and inclined. They also tend to have retrograde orbits. Irregular satellites are believed to have been captured by the planet's gravity rather than being formed along with the planet.

 J Definition Jansky A unit used in radio astronomy to indicate the flux density (the rate of flow of radio waves) of electromagnetic radiation received from outer space. A typical radio source has a spectral flux density of roughly 1 Jy. The jansky was named to honor Karl Gothe Jansky who developed radio astronomy in 1932. Jet A narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.

 K Definition Kelvin A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius. Kepler's First Law A planet orbits the Sun in an ellipse with the Sun at one focus. Kepler's Second Law A ray directed from the Sun to a planet sweeps out equal areas in equal times. Kepler's Third Law The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of proportionality is the same for all planets. Kiloparsec A distance equal to 1000 parsecs. Kirkwood Gaps Regions in the main belt of asteroids where few or no asteroids are found. They were named after the scientist who first noticed them. Kuiper Belt A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the Solar System. Some astronomers believe Pluto and Charon are Kuiper Belt objects.

 L Definition Lagrange Point French mathematician and astronomer Joseph Louis Lagrange showed that three bodies could 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. Lenticular Galaxy A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies. Libration An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time. Light Year An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles. Limb The outer edge or border of a planet or other celestial body. Local Group A small group of about two dozen galaxies of which our own Milky Way galaxy is a member. Luminosity The amount of light emitted by a star. Lunar Eclipse A phenomenon that occurs when the Moon passes into the shadow of the Earth. A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow. In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow. Lunar Month The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month. Lunation The interval of a complete lunar cycle, between one new Moon and the next. A lunation is equal to 29 days, 12 hours, and 44 minutes.

 M Definition Magellanic Clouds Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic Clouds are visible in the skies of the southern hemisphere. Magnetic Field A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles. Magnetic Pole Either of two limited regions in a magnet at which the magnet's field is most intense. Magnetosphere The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind. Magnitude The degree of brightness of a star or other object in the sky according to a scale on which the brightest star has a magnitude -1.4 and the faintest visible star has magnitude 6. Sometimes referred to as apparent magnitude. In this scale, each number is 2.5 times the brightness of the previous number. Thus a star with a magnitude of 1 is 100 times brighter than on with a visual magnitude of 6. Main Belt The area between Mars and Jupiter where most of the asteroids in our solar system are found. Major Planet A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major planets. Mare A term used to describe a large, circular plain. The word mare means "sea". On the Moon, the maria are the smooth, dark-colored areas. Mass A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies. Matter A word used to describe anything that contains mass. Meridian An imaginary circle drawn through the North and South poles of the celestial equator. Metal A term used by astronomers to describe all elements except hydrogen and helium, as in "the universe is composed of hydrogen, helium and traces of metals". This astronomical definition is quite different from the traditional chemistry definition of a metal. Meteor A small particle of rock or dust that burns away in the Earth's atmosphere. Meteors are also referred to as shooting stars. Meteor Shower An event where a large number of meteors enter the Earth's atmosphere from the same direction in space at nearly the same time. Most meteor showers take place when the Earth passes through the debris left behind by a comet. Meteorite An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth's surface. Meteors become meteorites if they reach the ground. Meteoroid A small, rocky object in orbit around the Sun, smaller than an asteroid. Millibar A measure of atmospheric pressure equal to 1/1000 of a bar. Standard sea-level pressure on Earth is about 1013 millibars. Minor Planet A term used since the 19th century to describe objects, such as asteroids, that are in orbit around the Sun but are not planets or comets. In 2006, the International Astronomical Union reclassified minor planets as either dwarf planets or small solar system bodies. Molecular Cloud An interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide and ammonia.

 N Definition Nadir A term used to describe a point directly underneath an object or body. Nebula A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of. Neutrino A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of them pass completely through the Earth without interacting. Neutron Star A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars. Newton's First Law of Motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force. Newton's Second Law of Motion For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body. Newton's Third Law of Motion In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction. Nova A star that flares up to several times its original brightness for some time before returning to its original state. Nuclear Fusion The nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. Nuclear fusion is the reaction that fuels the Sun, where hydrogen nuclei are fused to form helium.

 O Definition Obliquity The angle between a body's equatorial plane and orbital plane. Oblateness A measure of flattening at the poles of a planet or other celestial body. Occultation An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon. Oort Cloud A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. The Oort cloud was named after the Dutch astronomer who first proposed it. Open Cluster A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed. Opposition The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing. Orbit The path of a celestial body as it moves through space.

 P Definition Parallax The apparent change in position of two objects viewed from different locations. Parsec A large distance often used in astronomy. A parsec is equal to 3.26 light-years. Patera A shallow crater with a complex, scalloped edge. Penumbra The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse. Perigee The point in the orbit of the Moon or other satellite at which it is closest to the Earth. Perihelion The point in the orbit of a planet or other body where it is closest to the Sun. Perturb To cause a planet or satellite to deviate from a theoretically regular orbital motion. Phase The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits. Photon A particle of light composed of a minute quantity of electromagnetic energy. Photosphere The bright visible surface of the Sun. Planemo A large planet or planetary body that does not orbit a star. Planemos instead wander cold and alone through the cosmos. It is believed that most planemos once orbited their mother star but were ejected from the star system by gravitational interaction with another massive object. Planet A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals. Planetary Nebula A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes. Planetesimal A solid object that is believed to exist in protoplanetary disks and in debris disks. Planetesimals are formed from small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems. Planitia A low plain. Planum A high plain or plateau. Plasma A form of ionized gas in which the temperature is too high for atoms to exist in their natural state. Plasma is composed of free electrons and free atomic nuclei. Precession The apparent shift of the celestial poles caused by a gradual wobble of the Earth's axis. Prominence An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere. Prograde Orbit In reference to a satellite, a prograde orbit means that the satellite orbits the planet in the same direction as the planet's rotation. A planet is said to have a prograde orbit if the direction of its orbit is the same as that of the majority of other planets in the system. Proper Motion The apparent angular motion across the sky of an object relative to the Solar System. Protoplanetary Disk A rotating circumstellar disk of dense gas surrounding a young newly formed star. It is thought that planets are eventually formed from the gas and dust within the protoplanetary disk. Protostar Dense regions of molecular clouds where stars are forming. Pulsar A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates.

 Q Definition Quadrature A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth. Quasar An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active galaxies. Quasi-Stellar Object Sometimes also called quasi-stellar source, this is a star-like object with a large redshift that gives off a strong source of radio waves. They are highly luminous and presumed to be extragalactic.

 S Definition Satellite A natural or artificial body in orbit around a planet. Scarp A line of cliffs produced erosion or by the action of faults. Seyfert Galaxy A main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell. Shell Star A type of star which is believed to be surrounded by a thin envelope of gas, which is often indicated by bright emission lines in its spectrum. Shepherd Satellite A satellite that constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon. Sidereal Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite. Sidereal Month The average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time. Sidereal Period The period of revolution of a planet around the Sun or a satellite around its primary. Singularity The center of a black hole, where the curvature of space time is maximal. At the singularity, the gravitational tides diverge. Theoretically, no solid object can survive hitting the singularity. Small Solar System Body A term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets or dwarf planets. These include most of the asteroids, comets, and other small bodies in the Solar System. Solar Cycle The approximately 11-year quasi-periodic variation in frequency or number of solar active events. Solar Eclipse A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is close enough to completely block the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is not able to completely block the light. This results in a ring of light around the Moon. Solar Flare A bright eruption of hot gas in the Sun's photosphere. Solar prominences are usually only detectable by specialized instruments but can be visible during a total solar eclipse. Solar Nebula The cloud of dust and gas out of which the Solar System was believed to have formed about 5 billion years ago. Solar Wind A flow of charged particles that travels from the Sun out into the Solar System. Solstice The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the Summer and Winter seasons. Spectrometer The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum. Spectroscopy The technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed. Spectrum The range of colors that make up visible white light. A spectrum is produced when visible light passes through a prism. Spicules Grass-like patterns of gas seen in the atmosphere of the Sun. Spiral Galaxy A galaxy that contains a prominent central bulge and luminous arms of gas, dust, and young stars that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy. Star A giant ball of hot gas that creates and emits its own radiation through nuclear fusion. Star Cluster A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction. Steady State Theory The theory that suggests the universe is expanding but exists in a constant, unchanging state in the large scale. The theory states that new matter is being continually being created to fill the gaps left by expansion. This theory has been abandoned by most astronomers in favor of the big bang theory. Stellar Wind The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A star's stellar wind is strongest near the end of its life when it has consumed most of its fuel. Stone Meteorite A meteorite that resembles a terrestrial rock and is composed of similar materials. Stony Iron A meteorite that contains regions resembling both a stone meteorite and an iron meteorite. Sunspot Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field. Supergiant The stage in a star's evolution where the core contracts and the star swells to about five hundreds times its original size. The star's temperature drops, giving it a red color. Superior Conjunction A conjunction that occurs when a superior planet passes behind the Sun and is on the opposite side of the Sun from the Earth. Superior Planet A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and Venus. These two planets are inferior planets. Supernova A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life. Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions. Supernova Remnant An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble "bubbles" in space. Synchronous Rotation A period of rotation of a satellite about its axis that is the same as the period of its orbit around its primary. This causes the satellite to always keep the same face to the primary. Our Moon is in synchronous rotation around the Earth. Synodic Period The interval between points of opposition of a superior planet.

 T Definition Tektite A small, glassy material formed by the impact of a large body, usually a meteor or asteroid. Tektites are commonly found at the sites of meteor craters. Telescope An instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography. A Telescope can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves. Terminator The boundary between the light side and the dark side of a planet or other body. Terrestrial A term used to describe anything originating on the planet Earth. Terrestrial Planet A name given to a planet composed mainly of rock and iron, similar to that of Earth. Tidal Force The differential gravitational pull exerted on any extended body within the gravitational field of another body. Tidal Heating Frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and/or other neighboring satellites. Transit The passage of a celestial body across an observer's meridian; also the passage of a celestial body across the disk of a larger one. Trans-Neptunian Object (TNO) Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune. Trojan An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points. Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans.

 U Definition Ultraviolet Electromagnetic radiation at wavelengths shorter than the violet end of visible light. The atmosphere of the Earth effectively blocks the transmission of most ultraviolet light, which can be deadly to many forms of life. Umbra The area of total darkness in the shadow caused by an eclipse. Universal Time (UT) Also known as Greenwich Mean Time, this is local time on the Greenwich meridian. Universal time is used by astronomers as a standard measure of time.

 V Definition Van Allen Belts Radiation zones of charged particles that surround the Earth. The shape of the Van Allen belts is determined by the Earth's magnetic field. Variable Star A star that fluctuates in brightness. These include eclipsing binaries. Visible Light Wavelengths of electromagnetic radiation that are visible to the human eye. Virgo Cluster A gigantic cluster of over 2000 galaxies that is located mainly within the constellation of Virgo. This cluster is located about 60 million light-years from Earth. Visual Magnitude A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the visible light from the object. On this scale, bright objects have a lower number than dim objects.

 W Definition Wavelength The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation. White Dwarf A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its center.

 X Definition X-ray Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays. X-ray Astronomy The field of astronomy that studies celestial objects by the x-rays they emit. X-ray Star A bright celestial object that gives off x-rays as a major portion of its radiation.

 Y Definition Yellow Dwarf An ordinary star such as the Sun at a stable point in its evolution.

 Z Definition Zenith A point directly overhead from an observer. Zodiac An imaginary belt across the sky in which the Sun, moon, and all of the planets can always be found. Zodiacal Light A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise. Zodiacal light is caused by sunlight reflecting off small particles of material in the plane of the Solar System.

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