Astronomy Terms and Definitions – Flashcards
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achondrite
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a stone meteorite that contains none of the small glassy spheres commonly found in meteorites.
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albedo
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the reflective property of non-luminous celestial objects.
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albedo feature
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a dark or light marking on the surface of an object that may or may not be a geological or topographical feature.
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altitude
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the angular distance of an object above the horizon.
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antimatter
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matter consisting of particles with charges opposite that of ordinary matter. its protons have a negative charge while electrons have a positive charge.
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antipodal point
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a point that is on the direct opposite side of a planet.
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apastron
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the point of greatest separation of two stars, such as in a binary star system.
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aperture
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the size of the opening through which light passes in an optical instrument such as a camera or telescope. a higher number = a smaller opening, while a lower number = a larger opening.
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aphelion
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the point in the orbit of a planet or other celestial body where it is farthest from the sun.
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apogee
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the point in the orbit of the moon or other satellite where it is farthest from the earth.
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apparent magnitude
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the apparent brightness of an object in the sky as it appears to an observer on earth. bright objects have a lower _____, while dim objects have a higher _____.
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asteroid
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a small planetary body in orbit around the sun, larger than a meteoroid but smaller than a planet.
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astrochemistry
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the branch of science that explores the chemical interactions between dust and gas interspersed between the stars.
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astronomical unit (au)
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a unit of measure equal to the average distance between the earth and the sun, approximately 93 million miles.
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atmosphere
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a layer of gases surrounding a planet, moon, or star.
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aurora
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a glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the sun.
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aurora australis
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southern lights
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aurora borealis
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northern lights
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axis
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an imaginary line through the center of rotation of an object. aka the poles.
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azimuth
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the angular distance of an object around or parallel to the horizon from a predefined zero point (the direction of a celestial object from the observer).
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bar
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a unit of measure of atmospheric pressure.
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big bang
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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.
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binary
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a system of two stars that revolve around a common center of gravity.
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black hole
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the collapsed core of a massive star. its gravitational pull is so strong that not even light can escape.
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blue shift
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indicates that an object is moving toward the observer. the larger the _____, the faster the object is moving.
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bolide
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a term used to describe an exceptionally bright meteor. they typically produce a sonic boom.
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caldera
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a type of volcanic crater that is extremely large, usually formed by the collapse of a volcanic cone or by a violent volcanic explosion.
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catena
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a series or chain of craters.
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cavus
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a hollow, irregular depression.
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celestial equator
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an imaginary line that divides the celestial sphere into a northern and southern hemisphere.
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celestial poles
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the north and south poles of the celestial sphere.
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celestial sphere
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an imaginary sphere around the earth on which the stars and planets appear to be positioned.
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cepheid variable
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a variable star whose light pulsates in a regular cycle. the period of fluctuation is linked to the brightness of the star. brigher ______ will have a longer period.
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chaos
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a distinctive area of broken terrain.
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chasma
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another name used to describe a canyon
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chondrite
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a meteorite that contains the small, glassy spheres commonly found in meteorites.
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chondrule
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small, glassy spheres commonly found in meteorites.
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chromosphere
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the part of the sun's atmosphere between the photosphere and the corona. contains high energy hydrogen atoms.
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corona
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the outer part of the sun's atmosphere. visible from earth during total solar eclipses.
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cosmic ray
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atomic nuclei (mostly protons) that are observed to strike the earth's atmosphere with extremely high amounts of energy.
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cosmic string
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a tube-like configuration of energy that is believed to have existed in the early universe.
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cosmogony
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the study of celestial systems, including the solar system, stars, galaxies, and galactic clusters.
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cosmology
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the study of the universe's origin, structure, and nature.
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crater
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a bowl-shaped depression formed by the impact of an asteroid or meteoroid.
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dark matter
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a term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.
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debris disk
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a ring-shaped circumstellar disk of dust and debris in orbit around a star.
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declination
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the angular distance of an object in the sky from the celestial equator.
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density
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the amount of matter contained within a given volume. measured in grams per cubic centimeter.
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disk
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the surface of the sun or other celestial body projected against the sky.
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double asteroid
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two asteroids that revolve around each other and are held together by the gravity between them. aka binary asteroid.
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doppler effect
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the apparent change in wavelength of sound or light emitted by an object in relation to an observer's position. can be used to estimate an object's speed and direction.
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double star
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a grouping of two stars. this grouping can be apparent, where the stars seem close together, or physical, such as a binary system.
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dwarf planet
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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.
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eccentricity
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the measure of how an object's orbit differs from a perfect circle. eccentricity defines the shape of an object's orbit.
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eclipse
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when one celestial body passes into the shadow of another.
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eclipsing binary
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a binary system where one object passes in front of the other, cutting off some of all of its light.
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ecliptic
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an imaginary line in the sky traced by the sun as it moves in its yearly path through the sky.
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ejecta
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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.
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electromagnetic radiation
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aka light. light waves created by fluctuations of electric and magnetic fields in space.
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electromagnetic spectrum
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the full range of frequencies, from radio waves to gamma waves, that characterizes light.
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ellipse
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an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.
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elliptical galaxy
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a galaxy whose structure is smooth, shaped like an ellipse, and lacks complex structures such as spiral arms.
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elongation
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the angle between the sun and a planet, with earth as the reference point.
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ephemeris
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a table of data arranged by date. ______ tables are typically to list the positions of the Sun, Moon, planets, and other solar system objects.
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equinox
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the two points at which the sun crosses the celestial equator in its yearly path in the sky.
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escape velocity
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the speed required for an object to escape the gravitational pull of a planet or other body.
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event horizon
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the invisible boundary around a black hole past which nothing can escape the gravitational pull- not even light.
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evolved star
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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 lose mass in the form of stellar wind.
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extinction
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the apparent dimming of a star or planet when low on the horizon due to absorption by the earth's atmosphere.
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extragalactic
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a term that means outside of or beyond our own galaxy.
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extraterrestrial
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a term used to describe anything that does not originate on earth.
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eyepiece
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the lens at the viewing end of a telescope.
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faculae
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bright patches that are visible on the sun's surface, or photosphere.
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filament
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a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the sun.
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finder
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a small, wide field telescope attached to a larger telescope. the _____ is used to help point the larger telescope to the desired viewing location.
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fireball
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an extremely bright meteor. also known as bolides, they can be several times brighter than the full moon.
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flare star
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a faint red star that appears to change in brightness due to explosions on its surface.
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galactic halo
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the name given to the spherical region surrounding the center, or nucleus, of a galaxy.
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galactic nucleus
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a tight concentration of stars and gas found at the innermost regions of a galaxy. it is now believed that massive black holes may exist in some of these.
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galaxy
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a large grouping of stars. they are found in a variety of sizes and shapes.
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galilean moons
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the name given to jupiter's four largest moons; io, europa, callisto, and ganymede. discovered by galileo galilei and simon marius.
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gamma ray
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the highest energy, shortest wavelength form of electromagnetic radiation.
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geosynchronous orbit
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an orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. a spacecraft in __________ appears to hang motionless above one position of a planet's surface.
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giant molecular cloud (gmc)
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massive clouds of gas in interstellar space composed primarily of hydrogen molecules. they have enough mass to produce thousands of stars and are frequently the sites of new star formation.
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globular cluster
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a tight, spherical grouping of hundreds of thousands of stars. they are composed of older stars, and are usually found around the central regions of a galaxy.
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granulation
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a pattern of small cells that be seen on the surface of the sun. they are caused by the convective motions of the hot gases inside the sun.
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gravitational lens
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a concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object.
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gravity
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a mutual physical force of nature that causes two bodies to attract each other.
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greenhouse effect
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an increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere.
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heliopause
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the point in space at which the solar wind meets the interstellar medium or solar wind from other stars.
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hydrogen
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an element consisting of one electron and one proton. the building block of the universe. stars form from massive clouds of ______.
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hubble's law
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the law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.
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hydrostatic equilibrium
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a state that occurs when compression due to gravity is balanced by a pressure gradient which creates a force in the opposite direction. it is responsible for keeping stars from imploding and for giving planets their spherical shape.
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hypergalaxy
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a system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. our galaxy and the andromeda galaxy are examples of this.
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inclination
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a measure of the tilt of a planet's orbital plane in relation to that of the earth.
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inferior conjunction
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occurs when an inferior planet is lined up directly between the earth and the sun.
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international astronomical union (iau)
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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.
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interplanetary magnetic field
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the magnetic field carried along with the solar wind.
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interstellar medium
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the gas and dust that exists in open spaces between the stars.qw
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ionosphere
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a region of charged particles in a planet's upper atmosphere.
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iron meteorite
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a meteorite that is composed mainly of iron mixed with smaller amounts of nickel.
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irregular galaxy
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a galaxy with no spiral structure and no symmetric shape. they are usually filamentary or very clumpy in shape.
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irregular satellite
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a satellite that orbits a planet far away with an orbit that is eccentric and inclined. they also tend to have retrograde orbits. believed to have been captured by the planet's gravity rather than being formed along with the planet.
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jansky
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the unit of strength of radio wave emission.
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jet
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a narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.
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kelvin
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a unit of absolute temperature symbolized as K.
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kepler's first law
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a planet orbits the sun in an ellipse with the sun at one focus.
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kepler's second law
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a ray directed from the sun to a planet sweeps out equal areas in equal times.
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kepler's third law
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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.
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kiloparsec
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a distance equal to 1000 parsecs.
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kirkwood gaps
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regions in the main belt of asteroids where few or no asteroids are found
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kuiper belt
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a large ring of icy, primitive objects beyond the orbit of neptune. they are believed to be remnants of the original material that formed the solar system.
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lagrange point
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positions where the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them.
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lenticular galaxy
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a disk shaped galaxy that contains no conspicuous structure within the disk. tend to look more like elliptical galaxies than spiral galaxies.
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libration
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an effect caused by the apparent wobble of the moon as it orbits the earth. the moon always keeps the same side towards the earth, but due to _______, 59% of its surface can be seen over a period of time.
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light year
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an astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.
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limb
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the outer edge or border of a planet or other celestial body.
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local group
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a small group of about 2 dozen galaxies of which our own milky way galaxy is a member.
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luminosity
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the amount of light emitted by a star.
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lunar eclipse
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a phenomenon that occurs when the moon passes into the shadow of the earth.
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lunar month
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aka lunation or synodic month. the average time between successive new or full moons. equal to 29 days, 12 hours, 44 minutes.
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lunation
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aka lunar month or synodic month. the average time between successive new or full moons. equal to 29 days, 12 hours, 44 minutes.
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magellanic clouds
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two small, irregular galaxies found just outside our milky way galaxies. they are visible in the skies of the southern hemisphere.
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magnetic field
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a condition found in the region around a magnet or electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.
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magnetic pole
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either of two limited regions in a magnet at which the magnet's field is most intense.
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magnetosphere
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the area around a planet most affected by its magnetic field. the boundary of this field is set by the solar wind.
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magnitude
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the degree of brightness of a star or other object in the sky according to a scale on which the brightest star has a _____ -1.4 and the faintest visible star has _____ 6. sometimes referred to as apparent ______. each number is 2.5 times the brightness of the previous number.
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main belt
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the asteroid belt between mars and jupiter where most of the solar system's asteroids are found.
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major planet
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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.
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maria (singular: mare)
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a term used to describe a large, circular plane. means "sea". on the moon, the _______ are the smooth, dark colored areas.
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mass
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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.
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matter
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anything that contains mass
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meridian
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an imaginary circle drawn through the north and south poles of the celestial equator.
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metal
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a term used by astronomers to describe all elements except hydrogen and helium.
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meteor
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a small particle of rock or dust that burns away in the earths atmosphere. aka shooting stars.
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meteor shower
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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 take place when the earth passes through the debris left behind by a comet.
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meteorite
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an object, usually a chunk of metal or rock, that survives entry through the atmosphere to reach the earth's surface. meteors become _____s if they hit the ground.
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millibar
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a measure of atmospheric pressure equal to 1/1000th of a bar.
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minor planet
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a term used to describe objects, such as asteroids, that are in orbit around the sun but are not planets or comets. ex. dwarf planets, asteroids, comets, etc.
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molecular cloud
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an interstellar cloud of molecular hydrogen containing trace amounts of other molecules like carbon monoxide or ammonia.
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nadir
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a term used to describe a point directly underneath an object or body.
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nebula
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a cloud of dust and gas in space, usually illuminated by one or more stars. they represent the raw material the stars are made of.
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neutrino
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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.
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neutron star
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a compressed core of an exploded star made up almost entirely of neutrons. they have a strong gravitational field and some, called pulsars, emit pules of energy along their axis.
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newton's first law of motion
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a body continues in its state of constant velocity unless it is acted on by an external force.
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nova
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a star that flares up to several times its original brightness for some time before returning to its original state.
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nuclear fusion
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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. it is the reaction that fuels the sun, where hydrogen nuclei are fused to form helium.
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obliquity
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the angle between a body's equatorial plane and orbital plane.
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oblateness
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a measure of flattening at the poles of a planet or other celestial body.
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occultation
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an event that occurs when one celestial body conceals or obscures another.
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oort cloud
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a theoretical shell of comets that is believed to exist at the outermost regions of our solar system.
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open cluster
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a collection of young stars that formed together.
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opposition
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the position of a planet when it is exactly opposite the sun in the sky as seen from earth. a planet at _____ is at its closest approach to the earth and is best suitable for viewing.
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orbit
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the path of a celestial body as it moves through space.
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parallax
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the apparent change in position of two objects viewed from different locations.
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parsec
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equal to 3.26 light years.
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patera
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a shallow crater with a complex, scalloped edge.
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penumbra
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the area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.
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perigree
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the point in the orbit of the moon or other satellite at which it is closest to the earth.
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perihelion
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the point in the orbit of a planet or other body where it is closest to the sun.
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perturb
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to cause a planet or satellite to deviate from a theoretically regular orbital motion.
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phase
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the apparent change in shape of the moon and inferior planets as seen from earth as they move in their orbits.
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photon
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a particle of light composed of a minute quantity of electromagnetic energy.
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photosphere
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the bright visible surface of the sun.
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planemo
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a large planet or planetary body that does not orbit a star. it is believed that most once orbited their mother star but were ejected from the star system by gravitational interaction with another massive object.
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planet
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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.
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planetary nebula
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a shell of gas surrounding a small, white star. the gas is usually illuminated by the star, producing a variety of colors and shapes.
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planetesimal
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a solid object that is believed to exist in protoplanetary disks and debris disks. they are formed form small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems.
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planitia
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a low plain.
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planum
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a high plain or plateau.
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plasma
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a form of ionized gas in which the temperature is too high for atoms to exist in their natural state. it is composed of free electrons and free atomic nuclei.
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precession
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the apparent shift of the celestial poles caused by a gradual wobble of the earth's axis.
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prominence
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an explosion of hot gas that erupts from the sun's surface.
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prograde orbit
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planets have a _______ if the direction of its orbit is the same as that of the majority of other planets in the system.
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proper motion
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the apparent angular motion across the sky of an object relative to the solar system.
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protoplanetary disk
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a rotating circumstellar disk of dense gas surrounding a young newly formed star. it is thought that planets are eventually formed out of it.
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protostar
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dense regions of molecular clouds where stars are forming.
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pulsar
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a spinning neutron star that emits energy along its gravitational axis. this energy is received as pulses as the star rotates.
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quadrature
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a point in the orbit of a superior planet where it appears at right angles to the sun as seen from earth.
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quasar
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an unusually bright object found in the remote areas of the universe. they release incredible amounts of energy and are among the oldest and farthest objects in the known universe.
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quasi-stellar object
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a star-like object with a large redshift that gives off a strong source of radio waves. highly luminous and presumed to be extragalactic.
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radial velocity
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the movement of an object either towards or away from a stationary observer.
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radiant
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a point in the sky from which meteors in a meteor shower seem to originate.
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radiation
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energy radiated from an object in the form of waves or particles.
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radiation belt
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regions of charged particles in a magnetosphere.
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radio galaxy
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a galaxy that gives off large amounts of energy in the form of radio waves.
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red giant
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a stage in the evolution of a star where the fuel begins to exhaust and the star expands to about 50 times its normal size. the temperature cools, which gives the star a reddish appearance.
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red shift
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indicates that an object is moving away from the observer. the larger the _____, the faster the object is moving.
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regular satellite
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a satellite that orbits close to a planet in a nearly circular, equatorial orbit. believed to have been formed at the same time as the planet.
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resonance
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a state in which an orbiting object is subject to periodic gravitational perturbations by another.
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retrograde motion
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the phenomenon where a celestial body appears to slow down, stop, then move in the opposite direction. cause when the earth overtakes the body in its orbit.
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retrograde orbit
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the orbit of a satellite where the satellite travels in a direction opposite to the direction of the planet's rotation.
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right ascension
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the amount of time that passes between the rising of aries and another celestial object. right ascension is one unit of measure for locating an object in the sky.
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ring galaxy
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a galaxy with a ring-like appearance. the ring usually contains luminous blue stars. they are believed to have been formed by collisions with other galaxies.
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roche limit
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the smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primart.
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rotation
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the spin of a body about its axis.
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satellite
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a natural or artificial body in orbit around a planet.
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seyfert galaxy
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a main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell.
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shell star
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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.
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shepherd satellite
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a satellite that constrains the extent of a planetary ring through gravitational forces. also known as a shepherd moon.
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sidereal
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of, relating to, or concerned with the stars.
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sidereal month
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the average period of revolution of the moon around the earth in reference to a fixed star, equal to about 27 days.
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sidereal period
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the period of revolution of a planet around the sun or a satellite around its primary.
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singularity
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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 ______.
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small solar system body
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objects in the solar system that are neither planets nor dwarf planets (asteroids, comets, etc.)
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solar cycle
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the approximately 11-year variation in frequency or number of solar active events.
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solar eclipse
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a phenomenon that occurs when the earth passes into the shadow of the moon.
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solar flare
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a bright eruption of hot gas in the sun's photosphere.
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solar nebula
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the cloud of dust and gas out of which the solar system was believed to have formed about 5 billion years ago.
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solar wind
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a flow of charged particles that travels from the sun out into the solar system.
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solstice
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the time of year when the sun appears furthest north or south of the celestial equator.
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spectrometer
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the instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.
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spectroscopy
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the technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed.
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spectrum
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the range of colors that make up visible white light. it is produced when visible light passes through a prism.
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spicules
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grass-like patterns of gas seen in the atmosphere of the sun.
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spiral galaxy
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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.
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star
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a giant ball of hot gas that creates and emits its own radiation through nuclear fusion.
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star cluster
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a large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.
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steady state theory
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the theory that suggests the universe is expanding but exists in a constant, unchanging state in the large scale. abandoned by most astronomers in favor of the big bang theory.
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stellar wind
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the ejection of gas from the surface of a star. aka solar wind. it is strongest near the end of a star's life.
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stone meteorite
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a meteorite that resembles a terrestrial rock and is composed of similar materials.
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stony iron
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a meteorite that contains regions resembling both a stone meteorite and an iron meteorite.
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sunspot
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areas of the sun's surface that are cooler than surrounding areas.
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supergiant
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the stage in a star's evolution where the core contracts and the star swells to about 500 times its original size.
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superior conjunction
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a conjunction that occurs when a superior planet passes behind the sun and is on the opposite side of the sun from the earth.
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superior planet
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a planet that exists outside the orbit of the earth.
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supernova
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a cataclysmic explosion caused when a star exhausts its fuel and ends its life. these are the most powerful forces in the universe. all of the heavy elements were created in these.
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supernova remnant
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an expanding shell of gas ejected at high speeds by a supernova explosion.
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synchronous rotation
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a period of rotation of a satellite about its axis that is the same as the period of its orbit around its primary, causing the satellite to always keep the same face to the primary. our moon is in _______ with the earth.
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synodic period
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the interval between points of opposition of a superior planet.
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tektite
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a small, glassy material formed by the impact of a large body, usually a meteor or asteroid.
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telescope
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an instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography.
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terminator
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the boundary between the light side and dark side of a planet or other body.
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terrestrial
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anything originating on Earth.
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terrestrial planet
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a planet composed mainly of rock and iron, similar to that of earth.
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tidal force
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the differential gravitational pull exerted on any extend a force that occurs when the gravity pulling on one side of an object is larger than that on the other side, causing the object to stretch.
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tidal heating
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Frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and/or other neighboring satellites.
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transit
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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.
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trans-neptunian object (tno)
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any one of a number of celestial objects that orbit the sun at a distance beyond the orbit of the planet neptune.
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trojan
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an object orbiting in the lagrange points of another larger object.
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ultraviolet
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electromagnetic radiation at wavelengths shorter than the violet end of visible light. it can be deadly but earth's atmosphere effectively blocks most of it.
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umbra
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the area of total darkness in the shadow caused by an eclipse.
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universal time (ut)
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also known as greenwich mean time, this is the local time on the greenwich meridian. it is used by astronomers as a standard measure of time.
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van allen belts
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radiation zones of charged particles that surround the earth. the shape of them is determined by the earth's magnetic field.
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variable star
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a star that fluctuates in brightness
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visible light
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wavelengths of electromagnetic radiation that are visible to the human eye.
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virgo cluster
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a giant cluster of over 2000 galaxies that is located mainly within the constellation of virgo. it is located about 60 million light-years from earth.
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visual magnitude
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a scale used by astronomers to measure the visible brightness of a star or other celestial object.
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wavelength
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the distance between consecutive crests of a wave. serves as a unit of measure of electromagnetic radiation.
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white dwarf
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a very small, white star formed when an average sized star uses up its fuel supply and collapses. often produces a planetary nebula with the ______ at its center.
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x-ray
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electromagnetic radiation of a very short wavelength and very high energy. they have shorter wavelength than ultraviolet light.
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x-ray astronomy
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the field of astronomy that studies celestial objects by the x-rays they emit.
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x-ray star
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a bright celestial object that gives off x-rays as a major portion of its radiation.
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yellow dwarf
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an ordinary star such as the sun at a stable point in its evolution.
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zenith
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a point directly overhead from an observer.
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zodiac
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an imaginary belt across the sky in which the sun, moon, and all of the planets can always be found.
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zodiacal light
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a faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise. it is caused by sunlight reflecting off small particles of material in the plane of the solar system.
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