Aberration of Starlight - The angular shift in the apparent direction of a star caused by the orbital motion of the Earth Absolute Magnitude - The apparent magnitude a star would have if it were at a distance of 10 parsecs (pc) Absorption Line - A dark line superimposed on a continuous spectrum when a gas absorbs light from a continuous source that is hotter than the absorbing gas Acceleration - The rate of change of velocity. An acceleration may involve a change of speed, direction of motion, or both Acceleration of Gravity - The acceleration of a body, equal to 9.8 meters per second per second (m/s2), caused by the force of gravity near the surface of the Earth Accretion - The growth in the mass of a body by the infall of matter gravitationally attracted to the body Accretion Disk - A disk of gas and dust spiraling inward toward a star or toward the nucleus of a galaxy Accretional Heating - The heating of a body by the impacts that occur as it grows by adding infalling material Achondrite - A stony meteorite lacking chondrules Active Galactic Nucleus - The nucleus of an active galaxy Active Galaxy - A galaxy whose nucleus is unusually bright and small. Seyfert galaxies, BL Lacertae objects, and quasars are examples of active galaxies Active Region - A region of the Sun's surface layers that has a large magnetic field and in which sunspots, flares, and prominences preferentially occur Adaptive Optics - A system for modifying the shape of the mirror of a telescope to compensate for atmospheric seeing and to produce sharp images Ae and Be Stars - Pre-main sequence stars more massive than 3 solar masses Aerosol - Liquid droplets and solids suspended in the atmosphere of a planet or satellite Aesthenosphere - A layer of plastic, deformable rock located in the upper mantle of a planet directly below the lithosphere Albedo - The ratio of the light reflected in all directions by a surface to the light incident on it. A perfectly reflecting surface has an albedo of 1, a perfectly absorbing surface has an albedo of 0 Alpha Particle - The nucleus of a helium atom, consisting of two protons and two neutrons Altitude - The angular distance between the direction to an object and the horizon. Altitude ranges from 0 degrees for an object on the horizon to 90 degrees for an object directly overhead Amino Acid - A carbon-based molecule from which protein molecules are assembled Amor Asteroid - A member of a class of asteroids having orbits that cross the orbital distance of the Earth Angular Momentum - The momentum of a body associated with its rotation or revolution. For a body in a circular orbit, angular momentum is the product of orbital distance, orbital speed, and mass. When two bodies collide or interact, angular momentum is conserved Annihilation - The mutual destruction of a matter-antimatter pair of particles. The charges on the two particles cancel and the mass of the particles is entirely converted to energy Annular eclipse - A solar eclipse in which the Moon is too far from the Earth to block the entire Sun from view and a thin ring of sunlight appears around the Moon Antapex - The direction in the sky away from which the Sun is moving. Because of the Sun’s motion, nearby stars appear to converge toward the antapex Antimatter - A type of matter which annihilates ordinary matter on contact. For every particle, there is a corresponding antimatter particle. For example, the antimatter counterpart of the proton is the antiproton Apex - The direction in the sky toward which the Sun is moving. Because of the Sun’s motion, nearby stars appear to diverge from the apex Aphelion - The point in the orbit of a solar system body where it is farthest from the Sun Apollo Asteroid - A member of a class of asteroids having orbits that cross the orbital distance of the Earth Apparent Brightness - The observed brightness of a celestial body Apparent Magnitude - The observed magnitude of a celestial body Apparent Solar Day - The amount of time that passes between successive appearances of the Sun on the meridian. The apparent solar day varies in length throughout the year Apparent Solar Time - Time kept according to the actual position of the Sun in the sky. Apparent solar noon occurs when the Sun crosses an observer’s meridian Arachnoid - A circular feature on the surface of Venus connected to other similar features by a web of fractures Ascending Node - The point in the Moon’s orbit where it crosses the ecliptic from south to north Association - A group of stars whose gravity is insufficient to hold it together but has not yet had time to disperse Asteroid - A small, planet-like solar system body. Most asteroids are rocky in makeup and have orbits of low eccentricity and inclination Asteroid Belt - The region of the solar system lying between 2.1 and 3.3 astronomical units (AU) from the Sun. The great majority of asteroids are found in the asteroid belt Astrology - A pseudoscience that holds that people and events are influenced by the configurations of the Sun, Moon, and planets with respect to each other and the stars Astronomical Unit (AU) - The average distance between the Earth and the Sun Asymptotic Giant Branch (AGB) - The portion of the H-R diagram occupied by enormous, cool stars with helium-burning shells Aten Asteroid - An asteroid having an orbit with semi-major axis smaller than 1 AU Atom - A particle consisting of a nucleus and one or more surrounding electrons Atomic Number - The number of protons in the nucleus of an atom. Unless the atom is ionized, the atomic number is also the number of electrons orbiting the nucleus of the atom Aurora Australis - Light emitted by atoms and ions in the upper atmosphere near the south magnetic pole. The emission occurs when atoms and ions are struck by energetic particles from the Sun Aurora Borealis - Light emitted by atoms and ions in the upper atmosphere near the north magnetic pole. The emission occurs when atoms and ions are struck by energetic particles from the Sun Autumnal Equinox - The point in the sky where the Sun appears to cross the celestial equator moving from north to south. This happens on approximately September 22 Azimuth - The angular distance between the north point on the horizon eastward around the horizon to the point on the horizon nearest to the direction to a celestial body Baily's Beads - Points of light around the limb of the Moon just before and just after a total eclipse of the Sun. Baily’s beads are caused by sunlight shining through valleys on the Moon’s limb Balmer Series - A series of absorption or emission lines of hydrogen seen in the visible part of the spectrum Barred Spiral Galaxy - A spiral galaxy in which the nucleus is crossed by a bar. The spiral arms start at the ends of the bar Basalt - An igneous rock often produced in volcanic eruptions Big Bang - The explosive event at the beginning of the universe. The expansion produced the Big Bang that continues today Binary Accretion Theory - A theory of the origin of the Moon that holds that the Moon and the Earth formed at about the same time out of the same swarm or cloud of material Binary Star System - A pair of stars that orbit each other under their mutual gravitational attraction Bipolar Outflow - Relatively narrow beams of matter ejected in opposite directions by a protostar Black Hole - A region of space from which no matter or radiation can escape. A black hole is a result of the extreme curvature of space by a massive compact body Blackbody - An object that is a perfect absorber of radiation falling on it Blackbody Radiation - The electromagnetic radiation emitted by a blackbody. The spectrum and intensity of blackbody radiation are controlled by the temperature of the blackbody. Many stars and other celestial bodies approximate blackbodies Blazar - A type of active galaxy named for BL Lacertae, the first of the type discovered. Blazars show rapid, unpredictable variations in brightness Bow Shock - The region where the solar wind is slowed as it impinges on the Earth’s magnetosphere Broad Line Region - The high-density region in a quasar where broad emission lines are formed Brown Dwarf - A star with too low a mass for nuclear fusion to begin in its core C-type Asteroid - One of a class of very dark asteroids whose reflectance spectra show no absorption features due to the presence of minerals Capture Theory - The theory of the origin of the Moon that holds that the Moon formed elsewhere in the solar system and then was captured into orbit about the Earth Carbonaceous Chondrite - A stony meteorite that contains carbon-rich material. Carbonaceous chondrites are thought to be primitive samples of material from the early solar system Cassini's Division - A conspicuous 1800 kilometer (km) wide gap between the outermost rings of Saturn Celestial Equator - The circle where the Earth’s equator, if extended outward into space, would intersect the celestial sphere Celestial Horizon - The circle on the celestial sphere which is 90 degrees from the zenith. The celestial horizon is approximately the boundary between the Earth and sky Celestial Mechanics - The part of physics and astronomy that deals with the motions of celestial bodies under the influence of their mutual gravitational attraction Celestial Sphere - An imaginary sphere surrounding the Earth. The celestial bodies appear to carry out their motions on the celestial sphere Cell - The smallest structural unit of an organism that is capable of independent functioning, consisting of one or more nuclei, cytoplasm, and various organelles, all surrounded by a semipermeable cell membrane Central Force - A force directed at the center of motion of a body. Gravity is the central force that accounts for the orbital motion of solar system bodies Centripetal Acceleration - The acceleration toward the center of motion, that causes the path of an orbiting body to continually bend away from a straight line path Centripetal Force - The central force that produces centripetal acceleration Cepheid Variable - A member of a class of yellow pulsating stars that vary in brightness as they expand and contract. The period of a Cepheid is related to its luminosity Chandrasekhar Limit - The maximum mass, about 1.4 solar masses, that a white dwarf star can have. Charge Coupled Device (CCD) - An array of photosensitive electronic elements that can be used to record an image falling on it Chondrite - A meteorite containing chondrules Chondrule - A small, spherical body embedded in a meteorite. Chondrules are composed of iron, aluminum, and magnesium silicate rock Chromosphere - The part of the Sun’s atmosphere between the photosphere and the corona Circle - A curve on which all points are equidistant from the center Circular Speed - The speed that causes an orbiting body to have a circular orbit rather than an elliptic one Circumpolar - A body is close enough to a celestial pole that its diurnal circle is always above the horizon. Circumpolar stars neither rise nor set Close Pair - A binary system in which the two stars are close enough together that they transfer matter to one another during some stages of their evolution Cloud Core - The dense part of molecular cloud where star formation takes place Cluster of Galaxies - A group of galaxies held together by their mutual gravitational attraction Cluster of Stars - A group of stars held together by their mutual gravitational attraction CNO Cycle - The series of reactions by means of which massive stars fuse hydrogen into helium Collision Fragment - A satellite which probably is a fragment of a larger satellite broken apart by a collision with a meteoroid Coma - A spherical gaseous region that surrounds the nucleus of a comet. The coma of a comet may be 100,000 kilometers (km) or more in diameter Comet - A small, icy body in orbit about the Sun. When a comet is near the Sun, it displays a coma and a tail Common Envelope - A stage in the evolution of a close pair of stars in which matter shed by one of the stars fills the region just outside the Roche lobes of the two stars Conduction - The transfer of heat by means of direct collisions between adjacent atoms, molecules, or ions Conic Section - One of four kinds of curves (circle, ellipse, hyperbola, and parabola) that can be formed by slicing a right circular cone with a plane Conjunction - The appearance of two celestial bodies, often a planet and the Sun, in approximately the same direction Conucleation - A possible explanation for the origin of a wide binary pair of stars in which the two cloud fragments that become the stars are already in orbit about one another when they form Constellation - One of 88 regions into which the celestial sphere is divided Continuous Spectrum - A spectrum containing neither emission nor absorption lines Convection - The process of energy transport in which heat is carried by hot, rising and cool, falling currents or bubbles of liquid or gas Convection Zone - The outer part of the Sun’s interior in which convection occurs Coordinate System - A system in which numbers are used to give the location of a body or event. The longitude-latitude system is an example of a coordinate system used to locate things on the Earth’s surface Coordinates - The numbers used in a coordinate system. Longitude and latitude are examples of coordinates Core - The innermost region of the interior of the Earth or another planet Coriolis Effect - The acceleration which a body experiences when it moves across the surface of a rotating body. The acceleration results in a westward deflection of projectiles and currents of air or water when they move toward the Earth’s equator and an eastward deflection when they move away from the equator Corona - The outermost layer of the Sun's atmosphere. Gases in the corona are tenuous and hot Corona - A circular feature on the surface of Venus. Coronae appear to be collapsed volcanic domes and can be as much as several hundred kilometers across Corona - A type of surface feature of Uranus's satellite Miranda. Coronae consist of parallel ridges and troughs producing a striped appearance. Coronae have sharp boundaries. Coronal Hole - A low density, dim region in the Sun’s corona. Coronal holes occur in regions of open magnetic field lines where gases can flow freely away from the Sun to form the solar wind Coronal Mass Ejection - A blast of gas moving outward through the Sun’s corona and into interplanetary space following the eruption of a prominence Cosmic Background Radiation (CBR) - Radiation observed to have almost perfectly uniform brightness in all directions in the sky. The CBR is highly redshifted radiation produced about a million years after the universe began to expand Cosmic Ray - Extremely energetic ions and electrons that travel through space almost at the speed of light. Most cosmic rays come from great distances and may be produced in supernovas and pulsars Cosmic Ray Exposure Age - The length of time that has passed since a meteorite broke off from a larger body and became exposed to radiation damage from cosmic rays Cosmological Principle - The assumption that all observers in the Universe at a given time would observe the Universe to have the same essential features and large-scale structure Cosmology - The study of the Universe as a whole Crater - A roughly circular feature on the surface of a solar system body caused by the impact of an asteroid or comet Crater Density - The number of craters of a given size per unit area of the surface of a solar system body Crater Saturation - The maximum crater density a solar system body can have. Once saturation is reached, new craters can only be produced by eradicating old ones Crescent Phase - The phase of the moon at which only a small, crescent-shaped portion of the near side of the Moon is illuminated by sunlight. Crescent phase occurs just before and after new moon Critical Density - The value that the average density of the Universe must equal or exceed if the universe is closed. If the density of the Universe is less than the critical density, the Universe will continue to expand forever Crust - The outermost layer of the interior of a planet or satellite Dark Matter - Matter that cannot be detected or has not yet been detected by the radiation it emits. The presence of dark matter can be deduced from its gravitational interaction with other bodies Dark Nebula - A dense, interstellar cloud containing enough gas and dust to block the light of background stars. The dimming of background stars gives the appearance of a region with no stars Declination - The angular distance of a celestial body north or south of the celestial equator. Declination is analogous to latitude in the terrestrial coordinate system Decoupling Epoch - The time about a million years after the expansion of the universe began when the universe became transparent and light could, for the first time, travel great distances before being absorbed or scattered. The cosmic background radiation was produced at the decoupling epoch Deferent - One of the circles on which a planet moved according to the Ptolemaic model of the solar system Degenerate Gas - A gas in which a type of particle (electrons or neutrons) are as tightly packed as permitted by the Pauli exclusion principle. In a degenerate gas, temperature has essentially no influence on pressure Degree - A unit used to measured angles. There are 360 degrees in a circle Density - The mass of a body divided by its volume Density Wave Theory - A theory to account for the spiral arms of spiral galaxies. According to the density wave theory, spiral arms are the crests of waves moving through a galaxy like water waves move through water Descending Node - The point in the Moon’s orbit where it crosses the ecliptic from north to south Detector - A device used to measure light once it has been brought into focus by a telescope Deuterium - An isotope of hydrogen. The nucleus of a deuterium atom is a deuteron Deuteron - A nucleus of deuterium, an isotope of hydrogen. A deuteron contains one proton and one neutron Diamond Ring - The last of Baily's beads, which seems to shine with special brilliance just before a solar eclipse becomes total Differential Rotation - Rotation in which the rotation period of a body varies with latitude. Differential rotation occurs for gaseous bodies like the Sun or for planets with thick atmospheres Differentiation - The gravitational separation of the interior of a planet into layers according to density. When differentiation occurs inside a molten body, the heavier materials sink to the center and the light materials rise to the surface Direct Motion - The eastward apparent motion of a solar system body with respect to the stars. Direct motion is interrupted by regular episodes of retrograde (westward) motion Disk Instability - A possible explanation for the origin of a close binary pair of stars in which one star forms within the disk of gas and dust orbiting another, newly formed star Dispersion - The separation of white light according to wavelength. Dispersion produces a rainbow-like spectrum Diurnal - Daily Diurnal Circle - The circular path that a celestial body traces out as it appears to move across the sky during an entire day. Diurnal circles are centered on the north and south celestial poles Doppler Effect - The change in the frequency of a wave (such as electromagnetic radiation) caused by the motion of the source and observer toward or away from each other Dust Tail - A comet tail that is luminous because it contains dust that reflects sunlight. The dust in a comet tail is expelled from the nucleus of the comet Dwarf - A main sequence star Dynamo - A process in which electric currents within a rotating, convective body produce a magnetic field Eccentricity - A measure of the extent to which an orbit departs from circularity. Eccentricity ranges from 0.0 for a circle to 1.0 for a parabola Eclipse - The obscuration of the light from the Sun when the observer enters the Moon’s shadow or the Moon when it enters the Earth's shadow. Also, the obscuration of a star when it passes behind its binary companion Eclipse Seasons - The times, separated by about 5 1/2 months, when eclipses of the Sun and Moon are possible Eclipse Track - The path of the Moon’s shadow across the Earth during a solar eclipse Eclipse Year - The interval of time (346.6 days) from one passage of the Sun through a node of the Moon’s orbit to the next passage through the same node Eclipsing Binary - Binary star systems for which the orbital plane of the stars lies so nearly in the line of sight that two stars alternately pass in front of one another, causing eclipses Ecliptic - The plane of the Earth’s orbit about the Sun. As a result of the Earth’s motion, the Sun appears to move among the stars, following a path that is also called the ecliptic Eddington Luminosity - The maximum luminosity that a body could emit without driving away surrounding material Einstein Ring - The ring or near ring into which the image of a distant quasar is distorted if the quasar lies directly behind a galaxy or cluster of galaxies producing a gravitational lens Electromagnetic Wave - A periodic electrical and magnetic disturbance that propagates through space and transparent materials at the speed of light. Light is an example of an electromagnetic wave Electron - A low-mass, negatively charged particle that can either orbit a nucleus as part of an atom, or exist independently as part of a plasma Element - A substance that cannot be broken down into a simpler chemical substance. Oxygen, nitrogen, and silicon are examples of the approximately 100 known elements Ellipse - A closed, elongated curve describing the shape of the orbit that one body follows about another Elliptical Galaxy - A galaxy having an ellipsoidal shape and lacking spiral arms Emission Line - A narrow, bright region of the spectrum. Emission lines are produced when electrons in atoms jump from one energy level to lower energy level Energy Flux - The rate at which a wave carries energy through a given area Energy Level - Any of the many energy states that an atom may have. Different energy levels correspond to different distances of the electron from the nucleus Epicycle - One of the circles upon which a planet moved according to the Ptolemaic (geocentric) model of the solar system. The center of the epicycle moved on a larger circle, called the deferent Equant - In the Ptolemaic system, the point from which the motion of the epicycle around the deferent is uniform Equation of State - The relationship among pressure, density, and temperature for a gas or fluid. The ideal gas law, for which pressure is proportional to the product of temperature and density, is an example of an equation of state Equator - The line around the surface of a rotating body that is midway between the rotational poles. The equator divides the body into northern and southern hemispheres Equatorial Jet - The high-speed, eastward, zonal wind in the equatorial region of Jupiter's atmosphere Equatorial System - A coordinate system, using right ascension and declination as coordinates, used to describe the angular location of bodies in the sky Equipotential - A line or surface of equal potential energy. On the Earth, a line of equal elevation is approximately an equipotential Escape Velocity - The speed that an object must have to achieve a parabolic trajectory and escape from its parent body Event Horizon - The boundary of a black hole. No matter or radiation can escape from within the event horizon Evolutionary Track - The path in an H-R diagram followed by the point representing the changing luminosity and temperature of a star as it evolves Exosphere - The outer part of the thermosphere. Atoms and ions can escape from the exosphere directly into space Explosion Model - A model for the formation of clusters of galaxies in which the clusters form at the intersections of expanding shells of matter driven outward by gigantic explosions Extinction - The dimming of starlight due to absorption and scattering by interstellar dust particles. Filament - A dark line on the Sun’s surface when a prominence is seen projected against the solar disk Fireball - An especially bright streak of light in the sky produced when an interplanetary dust particle enters the Earth’s atmosphere, vaporizing the particle and heating the atmosphere Fission - A nuclear reaction in which a nucleus splits to produce two less massive nuclei Fission - A possible explanation for the origin of a close binary pair of stars in which a star splits into two pieces, each of which becomes a star Fission Theory - A theory for the origin of the Moon in which the Moon consists of matter that was flung from the primitive Earth because of the Earth’s rapid rotation Flare - A brief, sudden brightening of a region of the Sun’s atmosphere, probably caused by the abrupt release of magnetic energy Focal Length - The distance between a mirror or lens and the point at which the lens or mirror brings light to a focus Focal Plane - The surface where the objective lens or mirror of a telescope forms the image of an extended object Focal Point - The spot where parallel beams of light striking a lens or mirror are brought to a focus Focus - One of two points from which an ellipse is generated. For all points on the ellipse, the sum of the distances to the two foci is the same Force - A push or a pull Fragmentation - A possible explanation for the origin of a close binary pair of stars in which a collapsing cloud breaks into several pieces, each of which becomes a star Frequency - The number of oscillations per second of a wave Full Phase - The phase of the moon at which the bright side of the Moon is the face turned toward the Earth Fusion - A nuclear reaction in which two nuclei merge to form a more massive nucleus Galactic Bulge - A somewhat flattened distribution of stars, about 6 kiloparsecs (kpc) in diameter, surrounding the nucleus of the Milky Way Galactic Cannibalism - The capture and disruption of one galaxy by another Galactic Disk - A disk of matter, about 30 kiloparsecs (kpc) in diameter and 2 kiloparsecs thick, containing most of the stars and interstellar matter in the Milky Way Galactic Equator - The great circle around the sky that corresponds approximately to the center of the glowing band of the Milky Way Galactic Halo - The roughly spherical outermost component of the Milky Way, reaching to at least 30 to 40 kiloparsecs (kpc) from the center Galactic Latitude - The angular distance of a body above or below the galactic equator Galactic Longitude - The angular distance, measured eastward around the galactic equator, from the galactic center to the point on the equator nearest the direction to a body Galactic Nucleus - The central region of the Milky Way Galaxy - A massive system of stars, gas, and dark matter held together by its own gravity Gamma Ray - The part of the electromagnetic spectrum having the shortest wavelengths Geocentric - Centered on the Earth. In a geocentric model of the solar system, the planets moved about the Earth Geodesic - The path in spacetime followed by a light beam or a freely moving object Giant - A star larger and more luminous than a main sequence star (dwarf) of the same temperature and spectral type Giant Impact Theory - The theory of the origin of the Moon that holds that the Moon formed from debris blasted into orbit when the Earth was struck by a Mars-size body Giant Molecular Cloud - An unusually large molecular cloud that may contain as much as 1 million solar masses Gibbous Phase - The phase of the moon at which the near side of the Moon is more than half illuminated by sunlight. Gibbous phase occurs just before and after full moon Globular Cluster - A tightly packed, spherically shaped group of thousands to millions of old stars Granule - A bright convective cell or current of gas in the Sun’s photosphere. Granules appear bright because they are hotter than the descending gas that separates them Gravitational Lens - A massive body that bends light passing near it. A gravitational lens can distort or focus the light of background sources of electromagnetic radiation Gravitational Potential Energy - The energy stored in a body subject to the gravitational attraction of another body. As the body falls, its gravitational potential energy decreases and is converted into kinetic energy Gravitational Redshift - The increase in the wavelength of electromagnetic radiation that occurs when the radiation travels outward through the gravitational field of a body Gravity - The force of attraction between two bodies generated by their masses Great Attractor - A great concentration of mass toward which everything in our part of the universe apparently is being pulled Great Circle - A circle that bisects a sphere. The celestial equator and ecliptic are examples of great circles Great Red Spot - A reddish elliptical spot about 40,000 km by 15,000 km in size in the southern hemisphere of the atmosphere of Jupiter. The Red Spot has existed for at least 3 1/2 centuries Greatest Elongation - The position of Mercury or Venus when it has the greatest angular distance from the Sun Greenhouse Effect - The blocking of infrared radiation by a planet’s atmospheric gases. Because its atmosphere blocks the outward passage of infrared radiation emitted by the ground and lower atmosphere, the planet cannot cool itself effectively and becomes hotter than it would be without an atmosphere Ground State - The lowest energy level of an atom HII Region - A region of ionized hydrogen surrounding a hot star. Ultraviolet radiation from the star keeps the gas in the HII region ionized Habitable Zone - The range of distances from a star within which liquid water can exist on the surface of an Earth-like planet Half-life - The time required for half of the atoms of a radioactive substance to disintegrate Heliocentric - Centered on the Sun. In the heliocentric model of the solar system, the planets move about the Sun Heliopause - The boundary of the heliosphere, where the solar wind merges into the interstellar gas Helioseismology - A technique used to study the internal structure of the Sun by measuring and analyzing oscillations of the Sun’s surface layers Heliosphere - The region of space dominated by the solar wind and the Sun’s magnetic field Helium Flash - The explosive consumption of helium in the core of a star when helium fusion begins in a degenerate gas in which pressure doesn’t rise as energy is produced and temperature increases Herbig-Haro Object - A clump of gas illuminated by a jet of matter streaming away from a young star Hertzsprung-Russell Diagram (H-R diagram) - A plot of luminosities of stars against their temperatures. Magnitude may be used in place of luminosity and spectral type in place of temperature Hierarchical Clustering Model - A model for the formation of clusters of galaxies in which individual galaxies form and then begin to collect into clusters Horizon System - A coordinate system, using altitude and azimuth as coordinates, used to locate the positions of objects in the sky Horizontal Branch Star - A star which is undergoing helium fusion in its core and hydrogen fusion in a shell surrounding the core Hubble Time - An estimate of the age of the universe obtained by taking the inverse of Hubble’s constant. The estimate is only valid if there has been no acceleration or deceleration of the expansion of the universe Hubble's Constant (H) - The rate at which the recession speeds of galaxies increase with distance. Current estimates of Hubble's constant range from 50 to 100 kilometers per second per megaparsec (km/s per Mpc) Hubble's Law - The linear relationship between the recession speeds of galaxies and their distances. The slope of Hubble's law is Hubble's constant Hydrostatic Equilibrium - The balance between the inward directed gravitational force and the outward directed pressure force within a celestial body Hyperbola - A curved path that does not close on itself. A body moving with a speed greater than escape velocity follows a hyperbola Ideal Gas Law - The equation of state for a low-density gas in which pressure is proportional to the product of density and temperature Igneous Rock - A rock formed by solidification of molten material Impetus - A theory of motion, developed in the fourteenth and fifteenth centuries, that motion could continue only so long as a force was at work Inclination - The tilt of the rotation axis or orbital plane of a body Index of Refraction - The ratio of the speed of light in a vacuum to the speed of light in a particular substance. The index of refraction, which always has a value greater than 1.0, describes how much a beam of light is bent on entering or emerging from the substance Inertia - The tendency of a body at rest to remain at rest and a body in motion to remain in motion at a constant speed and in constant direction Inertial Motion - Motion in a straight line at constant speed followed by a body when there are no unbalanced forces acting on it Inferior Planet - A planet whose orbit lies inside the Earth's orbit Inflation - A brief period of extremely rapid and enormous expansion that may have occurred very early in the history of the universe Infrared - The part of the electromagnetic spectrum having wavelengths longer than visible light but shorter than radio waves Instability Strip - A region of the H-R diagram occupied by pulsating stars, including Cepheid variables and RR Lyrae stars Intercrater Plain - Smooth portions of the surface of Mercury that lie between and around clusters of large craters Interferometry - The use of two or more telescopes connected together to operate as a single instrument. Interferometers can achieve high angular resolution if the individual telescopes of which they are made are widely separated Interstellar Matter - Gas and dust in the space between the stars Interstellar Reddening - The obscuration, by interstellar dust particles, of blue starlight more strongly than red starlight Ion - An atom from which one or more electrons has been removed Ionization - The removal of one or more electrons from an atom Ionosphere - The lower part of the thermosphere of a planet in which many atoms have been ionized by ultraviolet solar photons Iron Meteorite - A meteorite composed primarily of iron and nickel Irregular Cluster - A cluster of galaxies that lacks a symmetrical shape and structure Irregular Galaxy - A galaxy having an amorphous shape and lacking symmetry Isochrone - Lines in an H-R diagram occupied by stars of different masses but the same age Isotopes - Nuclei with the same number of protons but different numbers of neutrons Isotropic - Looking the same in all directions Jet - A narrow beam of gas ejected from a star or the nucleus of an active galaxy Kelvin-Helmholtz Time - The time it would take a star to contract from infinite diameter down to the main sequence while radiating away the gravitational energy released during contraction Kepler's Laws of Planetary Motion - Three laws, discovered by Kepler, that describe the motions of the planets around the Sun Kiloparsec (kpc) - A unit of distance, equal to 1000 parsecs (pc), often used to describe distances within the Milky Way or the Local Group of galaxies Kinetic Energy - Energy of motion. Kinetic energy is given by one half the product of a body’s mass and the square of its speed Kirchhoff's Laws - Three "laws" that describe how continuous, bright line, and dark line spectra are produced Kuiper Belt - A region beyond Neptune within which a large number of comets are believed to orbit the Sun. Short period comets are thought to originated in the Kuiper belt L1 - The point between two stars in a binary system where matter may flow from one star to the other Latitude - The angular distance of a point north or south of the equator of a body as measured by a hypothetical observer at the center of a body Lava - Molten rock at the surface of a planet or satellite Leap year - A year in which there are 366 days Light - The visible form of electromagnetic radiation Light Curve - A plot of the brightness of a body versus time Light-Gathering Power - A number, proportional to the area of the principal lens or mirror of a telescope, that describes the amount of light that is collected and focused by the telescope Light Year - The distance that light travels in a year Limb - The apparent edge of the disk of a celestial body Limb Darkening - The relative faintness of the edge of the Sun's disk (limb) compared with the center of the Sun's disk Line of Nodes - The line connecting the two nodes of the Moon’s orbit around the Earth Lithosphere - The rigid outer layer of a planet or satellite, composed of the crust and upper mantle Local Group - The small cluster of galaxies of which the Milky Way is a member Local Hour Angle - The angle, measured westward around the celestial equator, between the meridian and the point on the equator nearest a particular celestial object Long-period Comet - A comet with an orbital period of 200 years or longer Longitude - The angular distance around the equator of a body from a zero point to the place on the equator nearest a particular point as measured by a hypothetical observer at the center of a body Lookback Time - The length of time that has elapsed since the light we are now receiving from a distant object was emitted Luminosity - The rate of total radiant energy output of a body Luminosity Class - The classification of a star’s spectrum according to luminosity for a given spectral type. Luminosity class ranges from I for a supergiant to V for a dwarf (main sequence star) Luminosity Function - The distribution of stars or galaxies according to their luminosities. A luminosity function is often expressed as the number of objects per unit volume of space that are brighter than a given absolute magnitude or luminosity. Lunar Eclipse - The darkening of the Moon that occurs when the Moon enters the Earth’s shadow. Lyman A Forest - The large number of absorption lines seen at wavelengths just longer than the wavelength of the Lyman a line of hydrogen in the spectrum of a quasar. The Lyman a forest is caused by absorption by gas clouds lying between the quasar and the Earth Lyman Series - A series of absorption or emission lines of hydrogen lying in the ultraviolet part of the spectrum M-type Asteroid - One of a class of asteroids that have reflectance spectra like those of metallic iron and nickel Magellanic Clouds - Two irregular galaxies that are among the nearest neighbors of the Milky Way Magma - Molten rock within a planet or satellite Magnetopause - The outer boundary of the magnetosphere of planet Magnetosphere - The outermost part of the atmosphere of a planet, within which a very thin plasma is dominated by the planet’s magnetic field Magnetotail - The part of the magnetosphere of a planet stretched behind the planet by the force of the solar wind Magnitude - A number, based on a logarithmic scale, used to describe the brightness of a star or other luminous body. Apparent magnitude describes the brightness of a star as we see it. Absolute magnitude describes the intrinsic brightness of a star Main Sequence - The region in an H-R diagram occupied by stars that are fusing hydrogen into helium in their cores. The main sequence runs from hot, luminous stars to cool, dim stars Main Sequence Lifetime - The length of time that a star spends as a main sequence star Major Axis - The axis of an ellipse that passes through both foci. The major axis is the longest straight line that can be drawn inside an ellipse Mantle - The part of a planet lying between its crust and its core Maria - A dark, smooth region on the Moon formed by flows of basaltic lava Mascon - A concentration of mass below the surface of the Moon that slightly alters the orbit of a spacecraft orbiting the Moon Mass - A measure of the amount of matter a body contains. Mass is also a measure of the inertia of a body Mass Number - A measure of the mass of a nucleus given by the total number of protons and neutrons in the nucleus Mass-Luminosity Relation - The relationship between luminosity and mass for stars. More massive stars have greater luminosities Maunder Minimum - A period of few sunspots and low solar activity that occurred between 1640 and 1700 Mean Solar Time - Time kept according to the average length of the solar day Megaparsec (Mpc) - A unit of distance, equal to 1 million parsecs, often used to describe the distances of objects beyond the Local Group Meridian - The great circle passing through an observer’s zenith and the north and south celestial poles Mesopause - The upper boundary of the mesosphere layer of the atmosphere of a planet Mesosphere - The layer of a planet’s atmosphere above the stratosphere. The mesosphere is heated by absorbing solar radiation Messier Objects - Deep sky objects list by Charles Messier (1730-1817). Charles Messier was a French Astronomer whose work on the discovery of comets led to the compilation of the Messier Catalogue of nebulae and star clusters. The reason Messier compiled this catalogue was to save time while comet hunting. It takes time for a comet hunter to check each suspected comet (by checking for motion). With his small instrument (2 or 3 inch diameter refractor), even star clusters would look fuzzy like comets. (Check this by observing some of the M objects using binoculars.) By using his catalogue, Messier could see whether a comet suspect was actually a nebulae that he had previously observed. In all Messier has his name on 12 comets between 1760 and 1798. Actually, he independently discovered at least 15 comets, but did not get credit for all of his discoveries - in some cases they had been previously discovered. (Remember that communications were very slow in those days.) Louis XV gave Messier the nickname "Comet Ferret." Messier compiled an initial list of 103 objects. Of the seven other objects, M104 was added in 1921 by Camilille Flammarion who found it on Messier's copy of his 1781 catalogue; M105 through M107 were observed by Messier's chief comet hunting rival, Pierre Mechain (1744-1804) and were added in 1947; M108 and M109 were mentioned by Messier in his description of M97 and were added in 1960; and M110 was on Messier's map of M31. M110 was added in 1966. Note that not all the objects are real - some are clearly mistakes. Metallic Hydrogen - A form of hydrogen in which the atoms have been forced into a lattice structure typical of metals. In the solar system, the pressures and temperatures required for metallic hydrogen to exist only occur in the cores of Jupiter and Saturn Metamorphic Rock - A rock that has been altered by heat and pressure Meteor - A streak of light produced by meteoroid moving rapidly through the Earth’s atmosphere. Friction vaporizes the meteoroid and heats atmospheric gases along the path of the meteoroid Meteor Shower - A temporary increase in the normal rate at which meteors occur. Meteor showers last for a few hours or days and occur on about the same date each year Meteorite - The portion of a meteoroid that reaches the Earth’s surface Meteoroid - A solid interplanetary particle passing through the Earth’s atmosphere Microlensing event - The temporary brightening of a distant object that occurs because its light is focused on the Earth by the gravitational lensing of a nearer body Micrometeorite - A meteoritic particle less than a 50 millionths of a meter in diameter. Micrometeorites are slowed by atmospheric gas before they can be vaporized, so they drift slowly to the ground Milky Way - The galaxy to which the Sun and Earth belong. Seen as a pale, glowing band across the sky Mineral - A solid chemical compound Minimum - The time of minimum light in a light curve Minor Planet - Another name for asteroid Minute of Arc - A unit of angular measurement equal to 1/60 of a degree Mode of Oscillation - A particular pattern of vibration of the Sun Molecular Cloud - A relatively dense, cool interstellar cloud in which molecules are common Momentum - A quantity, equal to the product of a body’s mass and velocity, used to describe the motion of the body. When two bodies collide or otherwise interact, the sum of their momenta is conserved Narrow Line Region - The low density region in a quasar where narrow emission lines are formed Neap Tide - An unusually low high tide and unusually high low tide that occur when the tidal forces of the Sun and Moon act at right angles to one another Neutral Gas - A gas containing atoms and molecules but essentially no ions or free electrons Neutrino - A particle with no charge and probably no mass that is produced in nuclear reactions. Neutrinos pass freely through matter and travel at or near the speed of light Neutron - A nuclear particle with no electric charge Neutron Star - A star composed primarily of neutrons and supported by the degenerate pressure of the neutrons Neutronization - A process by which, during the collapse of the core of a star, protons and electrons are forced together to make neutrons New Comet - A comet that has entered the inner solar system for the first time New Phase - The phase of the moon in which none or almost none of the near side of the Moon is illuminated by sunlight, so the near side appears dark Nodes - The points in the orbit of the Moon where the Moon crosses the ecliptic plane Normal Spiral Galaxy - A galaxy in which the spiral arms emerge from the nucleus North Celestial Pole - The point above the Earth’s north pole where the Earth’s polar axis, if extended outward into space, would intersect the celestial sphere. The diurnal circles of stars in the northern hemisphere are centered on the north celestial pole North Circumpolar Region - The region of the northern sky within which the diurnal circles of stars do not dip below the horizon. The size of the north circumpolar region varies with the latitude of the observer Nova - An explosion on the surface of a white dwarf star in which hydrogen is abruptly converted into helium Nucleic Acid - A long chain of nucleotides. DNA and RNA are nucleic acids Nucleosynthesis - The building up of more massive elements from less massive elements through nuclear reactions in stars Nucleoitide - The class of organic molecules of which nucleic acids are composed Nucleus - The massive, positively charged core of an atom. The nucleus of an atom is surrounded by one or more electrons. A nucleus missing one or more accompanying electrons is called an ion Nucleus - An irregularly shaped, loosely packed lump of dirty ice several kilometers across that is the permanent part of a comet Number Density - The number of particles in a given volume of space Objective - The main lens or mirror of a telescope Oblateness - A departure from spherical shape of a body in which the body’s polar diameter is smaller than its equatorial diameter Oort Cloud - The region beyond the planetary system, extending to 100,000 AU or more, within which a vast number of comets orbit the Sun. When comets from the Oort cloud enter the inner solar system, they become new comets Opacity - The ability of a substance to absorb radiation. The higher the opacity, the less transparent the substance is Opposition - The configuration of a planet or other body when it appears opposite the Sun in the sky Orbit - The elliptical or circular path followed by a body that is bound to another body by their mutual gravitational attraction Organic molecule - A molecule containing carbon Outflow channel - A Martian valley with few tributaries probably formed by the sudden melting and runoff of sub-surface water Outgassing - The release of gas from the interior of a planet or satellite Ozone - A molecule consisting of three oxygen atoms. Ozone molecules are responsible for the absorption of solar ultraviolet radiation in the Earth’s atmosphere Pair Production - A process in which gamma rays are transformed into a particle and its antiparticle (such as an electron and a positron) Pancake Model - A model for the formation of clusters of galaxies in which protoclusters form first and then fragment into individual galaxies Parabola - A geometric curve followed by a body that moves with a speed exactly equal to escape velocity Parallax - The shift in the direction of a star caused by the change in the position of the Earth as it moves about the Sun Parsec - The distance at which a star has a parallax of 1 second of arc. At a distance of 1 parsec (pc), an AU fills an angle of one second of arc Patera - A type of Martian volcano that resembles shield volcanos, but has even more gentle slopes Pauli Exclusion Principle - A physical law that limits the number of particles of a particular kind that can be placed in a given volume. A gas in which that limit is reached is degenerate Penumbra - The outer part of the shadow of a body where sunlight is partially blocked by the body Perihelion - The point in the orbit of a body when it is closest to the Sun Period - The time it takes for a regularly repeated process to repeat itself Period-luminosity Relationship - The relationship between the period of brightness variation and the luminosity of a Cepheid variable star. The longer the period of a Cepheid is, the more luminous the Cepheid Perturbation - A deviation of the orbit of a solar system body from a perfect ellipse due to the gravitational attraction of one of the planets Phase Change - A change in the physical state of a substance. The boiling, freezing, and melting of water are examples of phase changes Photon - A massless particle of electromagnetic energy Photosphere - The visible region of the atmosphere of the Sun or another star Pixel - A "picture element," consisting of an individual detector in an array of detectors used to capture an image Planet - One of the nine major bodies in orbit around the Sun Planetary Nebula - A luminous shell surrounding a hot star. The gas in a planetary nebula was ejected from the star while it was a red giant Planetesimal - A primordial solar system body of intermediate size that accreted with other planetesimals to form planets and satellites Planetology - The comparative study of the properties of planets Plasma - A fully or partially ionized gas Plasma Tail - A narrow, ionized comet tail pointing directly away from the Sun Plate - A section of the Earth’s lithosphere pushed about by convective currents within the mantle Plate Tectonics - The hypothesis that the features of the Earth’s crust such as mountains and trenches are caused by the slow movement of crustal plates Plerion - A supernova remnant, like the Crab Nebula, which has a filled center rather than being a shell Plume - A rising column of gas over a hot region in the interior or atmosphere of a body Polarity - The property of a magnet that causes it to have north and south magnetic regions Precession - The slow, periodic conical motion of the rotation axis of the Earth or another rotating body Pressure - The force exerted per unit area Primary Distance Indicator - A type of object, such a Cepheid variable, for which we know the size or brightness by observing them in the Milky Way Prime Meridian - The circle on the Earth’s surface that runs from pole to pole through Greenwich, England. The zero point of longitude occurs where the prime meridian intersects the Earth’s equator Primeval Atmosphere - The original atmosphere of a planet Prograde Motion - The eastward (normal) revolution of a solar system body. Prograde Rotation - The eastward rotation of a solar system body Prominence - A region of cool gas embedded in the corona. Prominences are bright when seen above the Sun’s limb, but appear as dark filaments when seen against the Sun’s disk Proper Motion - The rate at which a star appears to move across the celestial sphere with respect to very distant objects Protein - A large molecule, consisting of a chain of amino acids, that makes up the bodies of organisms Proton - A positively charged nuclear particle Proton-proton Cycle - A series of nuclear reactions through which stars like the Sun produce energy by converting hydrogen to helium. Named because the first reaction in the series is the reaction of one proton with another Protostar - A star in the process of formation Pulsar - A rotating neutron star with beams of radiation emerging from its magnetic poles. When the beams sweep past the Earth, we see "pulses" of radiation Quarter phase - The phase of the moon in which half of the near side of the Moon is illuminated by the Sun Quasar - A distant galaxy, seen as it was in the remote past, with a very small, luminous nucleus R-process - The process of building up massive nuclei in which neutrons are captured at a rate faster than the newly produced nuclei can undergo radioactive decay Radial Velocity - The part of the velocity of a body that is directed toward or away from an observer. The radial velocity of a body can be determined by the Doppler shift of its spectral lines Radiant - The point in the sky from which the meteors in a meteor shower seem to originate Radiation Era - The period of time, before about 1 million years after the expansion of the universe began, when radiation rather than matter was the dominant constituent of the universe Radiative Transfer - The transport of energy by electromagnetic radiation Radio Galaxy - A galaxy that is a strong source of radio radiation Radioactivity - The spontaneous disintegration of an unstable nucleus of an atom Rays - Long, narrow light streaks on the Moon and other bodies that radiate from relatively young craters. Rays consist of material ejected from a crater at the time it was formed by an impact Recession Speed - The rate of movement of a galaxy away from the Milky Way caused by the expansion of the universe Recombination Epoch - The time, about 1 million years after the expansion of the universe began, when most of the ions and electrons in the universe combined to form atoms Recurrent Nova - A binary system in which the white dwarf star undergoes repeated nova outbursts Reflectance Spectrum - The reflectivity of a body as a function of wavelength Reflection - The bouncing of a wave from a surface Reflection Nebulae - A cloud of interstellar gas and dust that is luminous because the dust it contains reflects the light of a nearby star Reflectivity - The ability of a surface to reflect electromagnetic waves. The reflectivity of a surface ranges from 0% for a surface that reflects no light to 100% for a surface that reflects all the light falling on it Reflector - A telescope in which the objective is a mirror Refraction - The bending of light when it passes from a material having one index of refraction to another material having a different index of refraction Refractor - A telescope in which the objective is a lens Regolith - The surface layer of dust and fragmented rock, caused by meteoritic impacts, on a planet, satellite, or asteroid Regular Cluster - A cluster of galaxies that has roughly spherical symmetry Regular Satellites - Regularly spaced satellites with nearly circular orbits that form miniature "solar systems" about their parent planets Resolution - The ability of a telescope to distinguish fine details of an image Resonance - The repetitive gravitational tug of one body on another when the orbital period of one is a multiple of the orbital period of the other Retrograde Motion - The westward revolution of a solar system body around the Sun Retrograde Rotation - The westward rotation of a solar system body Richness - A measure of the number of galaxies in a cluster. The more galaxies there are, the greater the richness Right Ascension - Angular distance of a body along the celestial equator from the vernal equinox eastward to the point on the equator nearest the body. Right ascension is analogous to longitude in the terrestrial coordinate system Rille - A lunar valley, probably the result of volcanic activity Roche distance - The distance from a planet or other celestial body within which tidal forces from the body would disintegrate a smaller object Roche lobe - The region around a star in a binary system in which the gravity of that star dominates Rock - A solid aggregation of grains of one or more minerals Rotation Curve - A plot of the speed of revolution of the stars and gas in a galaxy versus distance from the center of the galaxy RR Lyrae Star - A member of a class of giant pulsating stars, all of which have pulsation periods of about 1 day Runoff Channel - One of a network of Martian valleys that probably were formed by the collection of widespread rainfall S-process - The process of building up massive nuclei in which neutrons are captured at a rate slower than the newly produced nuclei can undergo radioactive decay S-type Asteroid - One of a class of asteroids whose reflectance spectra show an absorption feature due to the mineral olivine Saros - The length of time between one member of a series of similar eclipses and the next (6585 1/3 days) Scarp - A cliff produced by vertical movement of a section of the crust of a planet or satellite Scattering - The redirection of light in random directions when it strikes atoms, molecules, or solid particles Schwarzschild Radius - The radius of the event horizon of a black hole Sea Floor Spreading - The splitting of the oceanic crust where magma forces the existing crust apart, creating new ocean floor Second of Arc - A unit of angular measurement equal to 1/60 of a minute of arc or 1/3600 of a degree Secondary Atmosphere - The atmosphere that forms after a planet has lost any original atmosphere it had Secondary Distance Indicator - A type of object for which we know the size or brightness because objects of that type have been found in nearby galaxies Sedimentary Rock - A rock formed by the accumulation of small mineral grains carried by wind, water, or ice to the spot where they were deposited Seeing - A measure of the blurring of the image of an astronomical object caused by turbulence in the Earth’s atmosphere Seismic Wave - Waves that travel through the interior of a planet or satellite and are produced by earthquakes or their equivalent Seismometers - Sensitive devices used to measure the strengths and arrival times of seismic waves Semi-major Axis - Half of the major axis of an ellipse. Also equal to the average distance from the focus of a body moving on an elliptical orbit Seyfert Galaxy - A barred or normal spiral galaxy with a small, very bright nucleus SETI - The search for extraterrestrial intelligence Sgr A* - A small, bright source of radio emission, possibly the accretion disk of a black hole, that probably marks the exact center of the Milky Way Shield Volcano - A broad, gently sloped volcano built up by the repeated eruption of very fluid lava Short-period Comet - A comet with an orbital period shorter than 200 years Sidereal Clock - A clock that marks the local hour angle of the vernal equinox Sidereal Day - The length of time (23 hours, 56 minutes, 4.091 seconds) between successive appearances of a star on the meridian Sidereal Month - The length of time required for the Moon to return to the same apparent position among the stars Sidereal Period - The time it takes for a planet or satellite to complete one full orbit about the Sun or its parent planet Silicate - A mineral whose crystalline structure is dominated by silicon and oxygen atoms Sinuous Rille - A winding lunar valley possibly caused by the collapse of a lava tube Smooth Plains - Widespread sparsely cratered regions of the surface of Mercury possibly having a volcanic origin Solar Constant - The solar energy received by a square meter of surface oriented at right angles to the direction to the Sun at the Earth’s average distance (1 AU) from the Sun. The value of the solar constant is 1,372 watts per square meter Solar Flare - An explosive release of solar magnetic energy Solar Motion - The motion of the Sun with respect to the nearby stars Solar Nebula - The rotating disk of gas and dust, surrounding the newly formed Sun, from which planets and smaller solar system bodies formed Solar Wind - The hot plasma that flows outward from the Sun Solidification Age - The amount of time that has passed since a meteorite solidified from the molten state South Celestial Pole - The point above the Earth’s South Pole where the Earth's polar axis, if extended outward into space, would intersect the celestial sphere. The diurnal circles of stars in the southern hemisphere are centered on the south celestial pole Spacelike Trip - A path in spacetime that would require motion at a speed faster than the speed of light Spacetime - The combination of three spatial coordinates and one time coordinate that we use to locate an event Spacetime Diagram - A diagram showing one spatial coordinate against time, in which the paths of bodies and beams of light can be plotted Spectral Class - A categorization, based on the pattern of spectral lines of stars, that groups stars according to their surface temperatures Spectrograph - A device used to produce and record a spectrum Spectroscopic Binary - A pair of stars whose binary nature can be detected by observing the periodic Doppler shifts of their spectral lines as they move about one another Spectroscopy - The recording and analysis of spectra Spicule - A hot jet of gas moving outward through the Sun's chromosphere Spiral Arm - A long narrow feature of a spiral galaxy in which interstellar gas, young stars, and other young objects are found Spiral Galaxy - A flattened galaxy in which hot stars, interstellar clouds, and other young objects form a spiral pattern Spokes - Dark, short-lived radial streaks in Saturn's rings Spring Tide - An unusually high, high tide and unusually low, low tide that occur when the tidal forces of the Sun and Moon are aligned. This occurs at full moon and new moon Star - A massive gaseous body that has used, is using, or will use nuclear fusion to produce the bulk of the energy it radiates into space Starburst Galaxy - A galaxy in which a very large number of stars have recently formed Steady State Theory - A cosmological theory in which the universe always remains the same in its essential features, such as average density. In order to maintain constant density while expanding, the steady state theory required the continual creation of new matter Stefan-Boltzmann Law - The relationship between the temperature of a blackbody and the rate at which it emits radiant energy Stellar Occultation - The obstruction of the light from a star when a solar system body passes between the star and the observer Stellar Parallax - The shift in the direction of a star caused by the change in the position of the Earth as it moves about the Sun Stellar Population - A group of stars that are similar in spatial distribution, chemical composition, and age Stony Meteorite - A meteorite made of silicate rock Stony-iron Meteorite - A meteorite made partially of stone and partially of iron and other metals Stratosphere - The region of the atmosphere of a planet immediately above the troposphere Subduction - The process through which lithospheric plates of a planet or satellite are forced downward into the mantle Summer Solstice - The point on the ecliptic where the Sun’s declination is most northerly. The time when the Sun is at the summer solstice, around June 21, marks the beginning of summer Sunspot - A region of the Sun’s photosphere that appears darker than its surroundings because it is cooler Sunspot Cycle - The regular waxing and waning of the number of spots on the Sun. The amount of time between one sunspot maximum and the next is about 11 years Sunspot Group - A cluster of sunspots Supergiant - An extremely luminous star of large size and mass Supergranulation - The pattern of very large (15,000 to 30,000 km in diameter) convective cells in the Sun’s photosphere Superior Planet - A planet whose orbit lies outside the Earth's orbit Superluminal Motion - The apparent separation of components of a quasar at speeds faster than the speed of light Supernova - An explosion in which a star’s brightness temporarily increases by as much as 1 billion times. Type I supernovas are caused by the rapid fusion of carbon and oxygen within a white dwarf. Type II supernovas are produced by the collapse of the core of a star Supernova Remnant - The luminous, expanding region of gas driven outward by a supernova explosion Synchronous Rotation - Rotation for which the period of rotation is equal to the period of revolution. An example of synchronous rotation is the Moon, for which the period of rotation and the period of revolution about the Earth are both 1 month Synchrotron Emission - Electromagnetic radiation, usually observed in the radio region of the spectrum, produced by energetic electrons spiraling about magnetic field lines Synodic Month - The length of time (29.53 days) between successive occurrences of the same phase of the Moon Synodic Period - The length of time it takes a solar system body to return to the same configuration (opposition to opposition, for example) with respect to the Earth and the Sun T Tauri Star - A pre-main sequence star, less massive than about 3 solar masses, showing intense emission lines Terminal Velocity - The speed with which a body falls through the atmosphere of a planet when the force of gravity pulling it downward is balanced by the force of air resistance Terrae - The light-colored, ancient, heavily cratered portions of the surface of the Moon Terrestrial Planet - A rocky planet located in the inner solar system Thermal Equilibrium - The condition in which a body or a portion of a body gains energy (by generating it or absorbing it) at the same rate at which energy is transported away from it Thermal Pulse - The rapid consumption of helium in a shell within an asymptotic giant branch star Thermosphere - The layer of the atmosphere of a planet lying above the mesosphere. The lower thermosphere is the ionosphere. The upper thermosphere is the exosphere Tidal Capture - A possible explanation for the origin of a wide binary pair of stars in which two cloud fragments tidally interact with and capture one another Tidal Force - The differences in gravity in a body being attracted by another body Tidal Heating - The frictional heating of the interior of a satellite as it is flexed and released by a variable tidal force due to its parent planet Tides - Distortions in a body’s shape resulting from tidal forces Timelike Trip - A path in spacetime that can be followed by a body moving slower than the speed of light Transform Fault - The boundary between two of the Earth’s crustal plates that are sliding past each other Transverse Velocity - The part of the orbital speed of a body perpendicular to the Sun between the body and the Sun Triple A Process - A pair of nuclear reactions through which three helium nuclei (alpha particles) are transformed into a carbon nucleus Trojan Asteroid - One of a group of asteroids that orbit the Sun at Jupiter’s distance and lie 60 degrees ahead of or behind Jupiter in its orbit Tropical Year - The interval of time, equal to 365.242 solar days, between successive appearances of the Sun at the vernal equinox Tropopause - The upper boundary of the troposphere of the atmosphere of a planet Troposphere - The lowest layer of the atmosphere of a planet, within which convection produces weather Type Ia Supernova - An extremely energetic explosion produced by the abrupt fusion of carbon and oxygen in the interior of a collapsing white dwarf star Type II Supernova - An extremely energetic explosion that occurs when the core of a massive star collapses, probably producing a neutron star or black hole Ultraviolet - The part of the electromagnetic spectrum with wavelengths longer than X rays, but shorter than visible light Umbra - The inner portion of the shadow of a body, within which sunlight is completely blocked Umbra - The dark central portion of a sunspot Universe - All the matter and space there is V-type Asteroid - The asteroid Vesta, which is unique in having a reflectance spectra resembling those of basaltic lava flows V/Vmax Test - A statistical method used to determine whether quasars have changed over time Van Allen Belts - Two doughnut-shaped regions in the Earth’s magnetosphere within which many energetic ions and electrons are trapped Velocity - A physical quantity that gives the speed of a body and the direction in which it is moving Vernal Equinox - The point in the sky where the Sun appears to cross the celestial equator moving from south to north. This happens approximately on March 21 Visual Binary Star - A pair of stars orbiting a common center of mass in which the images of the components can be distinguished using a telescope and which have detectable orbital motion Vogt-Russell Theorem - The concept that the original mass and chemical composition of an isolated star completely determine the course of its evolution Voids - Immense volumes of space in which few galaxies, and clusters of galaxies can be found Volatile - Element or compound that vaporizes at low temperature. Water and carbon dioxide are examples of volatiles Waning Crescent - The Moon's crescent phase that occurs just before new moon Wave - A regular series of disturbances that moves through a material medium or through empty space Wavelength - The distance between crests of a wave. For visible light, wavelength determines color Waxing Crescent - The Moon's crescent phase that occurs just after new moon Weight - The gravitational force exerted on a body by the Earth (or another astronomical object) White Dwarf - A small, dense star that is supported against gravity by the degenerate pressure of its electrons Wide Pair - A binary star system in which the components are so distant from one another that they evolve independently Wien's Law - The relationship between the temperature of a blackbody and the wavelength at which its emission is brightest Winter Solstice - The point on the ecliptic where the Sun has the most southerly declination. The time when the Sun is at the winter solstice, around December 22, marks the beginning of winter X Ray - The part of the electromagnetic spectrum with wavelengths longer than gamma rays but shorter than ultraviolet X-ray burst - Sporadic burst of X rays originating in the rapid consumption of nuclear fuels on the surface of the neutron star in a binary system X-ray pulsar - A neutron star from which periodic bursts of X rays are observed Year - The length of time required for the Earth to orbit the Sun Zeeman Effect - The splitting of a spectral line into two or more components when the atoms or molecules emitting the line are located in a magnetic field Zenith - The point on the celestial sphere directly above an observer Zero Point - The point from which the coordinates in a coordinate system are measured. For example, the vernal equinox is the zero point of right ascension and declination in the celestial coordinate system Zodiacal Constellations - The band of constellations along the ecliptic. The Sun appears to move through the 12 zodiacal constellations during a year Zodiacal Light - The faint glow extending away from the Sun caused by the scattering of sunlight by interplanetary dust particles lying in and near the ecliptic Zonal Winds - The pattern of winds in the atmosphere of a planet in which the pattern of wind speeds varies with latitude Zone of Convergence - According to plate tectonics, a plate boundary at which the crustal plates of a planet are moving toward one another. Crust is destroyed in zones of convergence Zone of Divergence - According to plate tectonics, a plate boundary at which the crustal plates of a planet are moving away from one another. Crust is created in zones of divergence