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.

Fabry- Perot Etalon – >A nonabsorbing, multireflecting device, similar in design to the Fabry-Perot interferometer, that serves as a multilayer, narrow-bandpass filter.

Fabry-Perot interferometer – >A plane-parallel interferometer that yields extremely high contrast over a wide range of finesse values without significantly reducing transmission.

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