B-type shell star A star of spectral type B that occasionally blows off a cloud of gas, forming a gaseous shell around the star.
Balmer alpha line A brilliant red spectral line at wavelength 656.3 nm, caused by transition of the electron in the hydrogen atom from the third-level orbit to the second-level orbit.
barred spiral galaxy A spiral galaxy whose spiral arms attach to a barlike feature containing the nucleus.
baryonic particles Particles such as protons, neutrons, electrons, and neutrinos out of which normal matter is made.
basalt A type of igneous rock, often formed in lava flows, common on the Moon and terrestrial planets.
basaltic rock Igneous rock (including basalt) with a composition resembling basalt and a relatively low content of quartz (SiO₂).
basin Large impact crater on a planet, usually several hundred kilometers across, flooded with basaltic lava and surrounded by concentric rings of faulted cliffs.
belts Dark cloud bands on giant planets.
big bang model The theory that the universe started from an extremely hot and dense state, about 12-16 billion years ago. Supported by observations of the universal recession of galaxies, the abundance of light elements, and the cosmic background radiation.
big crunch The idea that if the mean density of the universe exceeds the critical density, the universe will eventually recollapse into a final state of infinite density and temperature.
binaries, classes of Three categories of binary stars, depending on whether neither, one, or both fill their Lagrangian lobes.
binary star system A pair of coorbiting stars.
bipolar jets A phenomenon in which narrow streams of gas are ejected at very high speed in opposite directions from the centers of some disks of gas, perpendicular to the disk; bipolar jetting is seen both in accretion disks around individual stars and in galactic disks. The mechanism is uncertain.
black hole An object whose surface gravity is so great that no radiation or matter can escape from it. Some black holes discussed in astronomy are collapsed stars, but much smaller ones are theoretically possible.
blueshift A doppler shift of spectral features toward shorter wavelengths, indicating approach of the source.
Bode's rule A convenient memory aid for listing the planets' distances from the Sun.
body tide A tidal bulge raised in the solid body of a planet.
Bok globule A relatively small, dense, dark cloud of interstellar gas and dust, usually silhouetted against bright clouds.
bolometric correction Correction that must be made to the brightness or luminosity measured through any filter to yield the total emission of a star at all wavelengths. For visual magnitudes, the bolometric correction is largest for very hot or very cool stars.
bolometric luminosity The total energy radiated by an object at all wavelengths, usually given in joules per second (identical to watts).
Tycho Brahe (1546-1601): Danish astronomer who recorded planetary positions, ultimately enabling Kepler to deduce laws of planetary orbits.
breccia A rock made from angular fragments cemented together.
brecciated meteorite A meteorite formed from cemented fragments of one or more meteorite types.
broken symmetry A situation in which the clear relationship between different forces that is apparent at high energies is masked at low energies.
brown dwarf A starlike object too small to achieve nuclear reactions in its center; any stellar object smaller than about 0.08 M.
bubble A roughly spheroidal shell of interstellar gas blown outward from a star by a stellar explosion or strong stellar wind.
bulk flow Motion of a large group of galaxies that displays a systematic departure from the smooth Hubble flow.

Callisto Outermost of the four large Moons of Jupiter, and most heavily cratered of the four.
Cambrian period A period from 570 to 500 million years ago during which fossilproducing species of plants and animals first proliferated.
canals Alleged straight-line markings on Mars found not to exist by spacecraft visits to Mars.
captured moons Satellites that did not originate in orbit around a planet but were captured into orbit from interplanetary space.
capture theory A theory of origin of a planet-satellite or binary star system in which one body captures another body by gravity.
carbonaceous chondrite A type of carbon-rich and volatile-rich meteorite, believed to be a nearly unaltered example of some of the earliest-formed matter in the solar system.
carbonaceous material Black material rich in carbon and carbon compounds, found in carbonaceous chondrites and believed to color many black comets and asteroids of the outer solar system.
carbon cycle A series of nuclear reactions in which hydrogen is converted to helium, releasing energy in stars more massive than about 1.5 M. Carbon is used as a catalyst.
Cassini's division The most prominent gap in Saturn's rings.
catastrophic theory A theory invoking sudden or very short (cosmically or geologically speaking) energetic events to explain observed phenomena.
catastrophism An early scientific school which held that most features of nature formed in sudden events, or catastrophes, instead of by slow processes.
cause of eclipses The falling of a shadow of one body onto another body.
cause of the seasons The tilt (obliquity) of the Earth's axis to its orbit plane causes first the North Pole and later the South Pole to be tipped toward the Sun during the course of a year.
CCD A charge-coupled device, which is an electronic instrument for detecting light or recording an image--and is much more sensitive than film.
celestial equator The projection of the Earth's equator onto the sky.
celestial poles The projections of the two poles of the Earth's rotation onto the sky.
cell The unit of structure in living matter.
center of mass The imaginary point of any system or body at which all the mass could be concentrated without affecting the motion of the system as a whole; the balance point.
Cepheid variable Any of a group of luminous variable stars with periods of 5 to 30 d (depending on their population). The periods are correlated with luminosity, allowing distance estimates out to about 3 Mpc.
cgs system Metric system of measurement using centimeters, grams, and seconds as the fundamental units.
Chandrasekhar limit A mass of about 1.4 M, the maximum for white dwarfs; stars of greater mass have too great a central pressure, causing formation of a star type denser than a white dwarf.
channel One of the riverbed-like valleys on Mars, which are possible sites of ancient Martian rivers.
chaotic rotation A form of rotation in which dynamical forces cause the rotation period to change irregularly from one rotation to the next. Applies to Saturn's moon Hyperion.
Charon Pluto's satellite.
chemical reaction Reaction between elements or compounds in which electron structures are altered; atoms may be moved from one molecule to another, but nuclei are not changed and thus no element is changed to another.
chondrite Stony meteorite containing chondrules, believed to be little altered since their formation 4.6 billion years ago.
chondrule BB-sized spherule in certain stony meteorites, believed among the earliest-formed solid materials in the solar system.
chromosphere A reddish-colored layer in the solar atmosphere, just above the photosphere.
circular velocity Velocity of an object in circular orbit:

V	=	sqrt(GM/R)
where   G	=	Newton's gravitational constant. In SI units it is 6.67×10-11 N·m2/kg2
M	=	mass of central body
R	=	distance of orbiter from center of central body

circumpolar zone The zone of stars, centered on a celestial pole, that never sets, as seen from a given latitude.
circumstellar nebula Gas and dust surrounding a star.
cirrus Diffuse clouds of dust grains in the disk of the Milky Way, which are cold and so emit thermal radiation at far infrared wavelengths. (Derived from Earth's wispy, high-altitude cirrus clouds.)
classes of binaries See binaries, classes of.
closed universe A universe with positive space curvature and more than the critical mass density. It will eventually recollapse.
cluster of galaxies A relatively close grouping of galaxies, often with some members coorbiting or interacting with each other.
coacervate Cell-sized, nonliving globule of proteins and complex organic molecules that forms spontaneously in a water solution.
Coal Sack Nebula A prominent dark nebula about 170 kpc away silhouetted against the Milky Way.
cocoon nebula A dust-rich nebula enclosing and obscuring a star during its formation, but later shed.
cold dark matter Unseen material in the universe; candidates include weakly interacting particles that were traveling slowly when the first structures formed in the universe. They may include exotic particles not yet seen in the laboratory.
collapse Rapid contraction, especially of a cloud of gas and dust during star formation.
colliding galaxies Galaxies undergoing interpenetration or close enough to cause major gravitational distortion of each other.
coma (1) The diffuse part of the head of a comet surrounding the nucleus; (2) a type of distortion in some telescopes and optical systems.
comet An ice-rich interplanetary body that, when heated by the Sun in the inner solar system, releases gases that form a bright head and diffuse tail. (See also coma.)
comet head The coma and nucleus regions of a comet.
comet nucleus The brightest starlike object near the center of a comet's head; the physical body (believed to be icy and a few kilometers across) within a comet.
comet tail Diffuse streamers of gas and dust released from a comet and blown in the direction away from the Sun by the solar wind.
comparative planetology An interdisciplinary field of astronomy and geology attempting to discover and explain differences between planets in properties such as climate and interior structure.
composition of Population II stars About ¾ hydrogen and ¼ helium, by mass, with virtually no heavier elements. compound telescope A telescope that combines lenses and mirrors in the light-gathering system.
condensation sequence The sequence in which chemical compounds condense to form solid grains in a cooling, dense nebula.
conduction One of three processes that transfers heat from hot to cold regions; occurs as fast-moving molecules in the hot region agitate adjacent molecules.
conjunction The period when a planet lies at zero or minimum angular distance from the Sun, as seen from the Earth.
conservation of angular momentum A useful physical rule which states that the total angular momentum in an isolated system remains constant.
constellation Imaginary pattern found among the stars, resembling animals, mythical heroes, and the like; different cultures map different constellations.
contact binary A coorbiting pair of stars whose inner atmospheres or surfaces touch.
continental drift The motion of continents due to (convective?) motion of underlying material in the Earth's mantle.
continental shield A stable, ancient region, usually flat and oval-shaped, in a continent.
contingency The idea that evolution consists of many branching points and that an organism's future evolution depends on previous branching points. This theory would predict that evolution is unlikely to produce similar species on different planets.
continuous spectrum A spectrum made up of all wavelengths, without emission or absorption lines or bands.
continuum In a spectrum with absorption or emission lines, the background continuous spectrum.
convection One of three modes of transmission of heat (energy) from hot regions to cold regions; involves motions of masses of material.
Copernican revolution The intellectual revolution associated with adopting Copernicus' model of the solar system, which displaced the Earth from the center of the universe.
core The densest inner region of the Earth, probably of nickel-iron composition; in other planets, similar high-density central regions; in the Sun or stars, a dense central region where nuclear reactions occur; in galaxies, the densest, brightest central regions.
core collapse Occurs when a star loses its pressure support because no more energy can be released by nuclear reactions. The core collapse drives the explosion and heavy-element creation of a supernova.
Coriolis drift A departure from a straight-line trajectory, perceived by an observer in a rotating system; Coriolis effects in clouds were early evidence of the Earth's rotation.
corona The outermost atmosphere of the Sun, having a temperature of about 1 to 2 million kelvins.
coronograph An instrument permitting direct observation of the solar corona without an eclipse.
cosmic background radiation The nearly uniform thermal radiation believed to be a relic of the hot big bang. It is observed in microwaves with a temperature of 2.7 K.
cosmic fuels Nonfossil energy sources provided by cosmic processes; for example, solar and geothermal energy.
cosmic nucleosynthesis The fusion of light elements during the early hot phase of the big bang, to produce heavier elements. It resulted in nearly a quarter of the mass of the universe being turned from hydrogen into helium.
cosmic rays High-energy atomic particles (85% protons) that enter the Earth's atmosphere from space. Many may originate in supernovae and pulsars.
cosmological constant Term that can be added to the equations of general relativity to give a static solution.
cosmological principle The assumption that the universe is homogeneous and isotropic. This assumption is crucial to modern cosmology, and it has been shown to be a good approximation to the observed state of the universe.
cosmological redshift Any redward Doppler shift attributed to the mutual recession of galaxies or the expanding universe.
cosmology The study of the structure of the universe. The term is often broadened to include the origin of the universe as well.
Cretaceous-Tertiary impact The impact of one or more asteroids or comets, up to about 10 km in size, about 65 million years ago, which apparently led to extinction of most species of plants and animals living at that time, including dinosaurs.
critical density The mass density needed to just halt the universal expansion.
crust The outermost, solid layer of a planet, with composition distinct from the mantle and differentiated by a seismic discontinuity.
cultural hurdle The hypothetical survival requirement for a planetary culture between the time it achieves technology capable of quickly altering its planetary environment and the time it can establish viable bases off its planet; the uncertainty of the probability of survival affects our estimates of the probability of intelligent life elsewhere in space.

dark matter Mass that makes its presence felt by gravitational forces but does not emit light. It is usually detected by anomalously high orbital velocities in gravitational systems. Dark matter makes up about 90% of the mass of the universe, and its nature is still unknown.
daughter isotope An isotope resulting from radioactive disintegration of a parent isotope.
deceleration parameter The rate of deceleration of the universal expansion, measured at the current epoch it is given the symbol q₀.
declination Angular distance north or south of the celestial equator. (Abbreviation: Dec.)
decoupling The point in the history of the universe when the big bang radiation is cool enough for stable atoms to form. At this point, radiation and matter decouple, and photons travel freely through the universe.
degenerate matter Matter in a very high-density state in which electrons are freed from atoms and pressure is a function of density but not temperature.
degree An angle equaling 1/360 of a circle.
density fluctuations The seeds for galaxy formation.
density parameter The ratio of the mean mass density of the local universe to the density required to just halt the universal expansion, given the symbol V₀.
density-wave theory The leading theory for explaining the formation of spiral arms in galaxies, which posits periodicities in star, dust, and gas motions.
deposition The accumulation of eroded materials in one place.
desert Any of the brighter regions of Mars.
differential rotation The differences in speed for stars at different distances from the center of the galaxy. Orbital velocities are actually slower at 5000 pc from the center than at the Sun's distance, which is 8000 to 10,000 pc from the center.
differentiation Any process that tends to separate different chemicals from their original mixed state and concentrate them in different regions.
diffraction The slight bending of light rays as they pass edges, producing spurious rays and rings in telescopic images of stars.
dimensions of the galaxy As early as 1935 astronomers agreed that the Sun is about 8000 to 10,000 pc from the center, and the overall diameter of our galaxy is about 30,000 pc.
Dione (See Tethys, Dione, and Rhea.)
dirty iceberg model A theoretical description of a comet nucleus as a large icy body with bits of silicate ``dirt'' embedded in it.
disruption of open clusters Gradual dispersion of stars from an open cluster as the cluster is sheared by differential galactic rotation, and as high-speed stars escape. Disruption time is usually a few hundred million years.
distance limit for reliable parallaxes 100 parsecs.
distance scale An overlapping set of techniques that are used to measure distances in the universe, starting with direct geometric methods such as parallax for nearby stars, and ending with global measures of galaxies. The errors in the distance scale increase with increasing distance from the Earth.
DNA (deoxyribonucleic acid) A long, replicating molecule, shaped like a twisted ladder, that is the basis of the genetic code. Information for all of life's functions is coded in the sequence of base pairs that join the two strands. DNA is found in all living organisms.
Doppler effect The shift in wavelength of light or sound as perceived by the observer of an approaching or receding body. For speeds well below that of light, the shift is given by the equation

Original wavelength

×

radial velocity  velocity of light

Drake equation The statement that the fraction of stars harboring intelligent life equals the number of all stars times a sequence of fractions, such as the fraction of all stars having planets, the fraction of planets that are habitable, and so on. Named after radio as tronomer Frank Drake.
dust scattering Scattering of light by particles much bigger than the light's wavelength. Causes extinction and reddening of starlight.
dust trails Toroidal lanes of dust stretching along elliptical orbits around the solar system; probably due to asteroid collisions or comet dust ejection.
dwarf elliptical galaxy An ellipsoidal galaxy resembling a globular cluster but usually at least a few times larger.
early dense atmosphere The atmosphere of a planet (if any) just after planet formation. (See also secondary atmosphere.)
early intense bombardment The intense bombardment of planets and moons by interplanetary debris during their first 500 million years, from 4.5 to 4.0 billion years ago.
Earth The third planet from the Sun.
earthquake Vibration or rolling motion of the Earth's surface accompanying the fracture of underground rock.
eclipse An event in which the shadow of one body falls on another body.
eclipsing binary A binary star system seen virtually edge-on so that the stars eclipse each other during each revolution.
eclipsing-spectroscopic binary An eclipsing binary whose motions are measurable from spectral Doppler shifts. The most informative type of binary star.
ecliptic (1) The plane of the Earth's orbit and its projection in the sky as seen from Earth; (2) approximately, the plane of the solar system.
effective temperature Temperature of an object as calculated from the properties of the radiation it emits.
ejecta blanket A layer of debris thrown out of a crater onto a planet's surface.
electromagnetic radiation Light, radio waves, X rays, and other forms of radiation that propagate as disturbances in electric and magnetic fields, travel at the speed of light, and combine to make up the electromagnetic spectrum.
electromagnetism The force of nature that governs the structure of atoms and molecules and accounts for electromagnetic radiation.
electron Negatively charged particle orbiting around the atomic nucleus, with mass 9.1×10^-31 kg.
electroweak theory Well-established theory stating that the electromagnetic and weak nuclear forces are unified at temperatures around 10¹⁵ K.
element A chemical material with a specified number of protons in the nucleus of each atom. Atoms with one proton are hydrogen; with two protons, helium; and so on.
ellipse A closed, oval-shaped curve (generated by passing a plane through a cone) describing the shape of the orbit of one body around another.
emission Release of electromagnetic radiation from matter.
emission band Narrow wavelength interval in which molecules emit light.
emission line Very narrow wavelength intervals in which atoms emit light.
emission-line clouds Small, dense clouds of gas that orbit the center of an active galaxy. Clouds near the nucleus orbit faster and so produce broader lines than clouds far from the nucleus.
Enceladus One of the inner moons of Saturn, notable for its very bright, fissured surface of water ice, with several young, sparsely cratered areas.
energy In physics, a specific quality equal to work or the ability to do work. Energy may appear in many forms, including electromagnetic radiation, heat, motion, and even mass (according to the theory of relativity).
energy level The orbit of an electron around the nucleus of an atom.
English system A nondecimal system of units using pounds, inches, and seconds. In scientific use, and in most countries, it is replaced by the more convenient metric system.
ephemeris A table of predicted positions of a planet, asteroid, or other celestial body.
ephemeris time A timekeeping system based on the motions of planets; more regular than conventional systems based on the Earth's rotation.
epicycle A small circular motion superimposed on a larger circular motion.
epicycle theory An early theory by Ptolemy that the planets move around the Earth in epicycles.
equatorial zone On Jupiter, Saturn, and possibly other great planets, a bright cloud zone near the equator.
equinox The date when the Sun passes through the Earth's equatorial plane (occurs twice annually).
erg The unit of energy in the cgs metric system.
erosion Removal of rock and soil by any natural process.
escape velocity The minimum speed needed to allow a projectile to move away from a planet and never return to its point of launch. It equals 2^½ × circular velocity. (See circular velocity.)
eukaryote Cell with a nucleus, that is, with DNA contained by an interior membrane; a multicelled organism. Eukaryotes first appeared about 1.4 billion years ago.
Europa One of Jupiter's four large moons, notable for its smooth, bright, icy surface.
event horizon Imaginary surface at the distance from a black hole where the escape velocity is the speed of light. Matter and energy cannot escape from inside the event horizon.
evidence for planets near other stars Although planets the size of Jupiter or smaller near other stars are beyond our current detection capability, new techniques of imaging and astrometry will allow their detection within a few years, if they exist. Already some objects larger than Jupiter but smaller than stars have been found.
evidence for present-day star formation The fact that the solar system is much younger than the galaxy; existence of young open clusters; existence of short-lived massive stars.
evidence of spiral structure (in Milky Way) Spiral distribution of hydrogen gas mapped by 21-cm radio line; spiral distribution of open clusters; spiral patterns observed in other galaxies.
evolution of galaxies Changes in form and stellar populations of galaxies as a result of consuming gas and dust during star formation and production of heavy elements during star evolution.
evolutionary correction A correction that must be made to the data associated with many cosmological tests, because distant objects are younger than nearby objects.
evolutionary theory A theory in which changes occur by relatively slow processes or processes commonly growing out of the initial conditions, rather than by sudden or unusual processes.
evolutionary track The sequence of points on the H-R diagram occupied by a star as it evolves.
excitation The process of causing an atom or molecule to go into an excited state, that is, having some electrons in elevated energy levels; the state of being excited.
excited atoms and molecules Atoms or molecules in which electrons are not all in the lowest possible energy levels.
excited state The state of an atom or molecule when not all electrons are in the lowest possible energy levels.
exobiology Study of life beyond the Earth.
expanding universe A term popularized by Eddington to describe the mutual recession of galaxies.
explorative interval The hypothetical interval of time during which a species actively engages in the exploration of other planets.
explosive nucleosynthesis In the explosive expanding shell of a supernova, the formation of nuclei of heavy elements up to plutonium by means of rapid neutron capture.
extragalactic standard of rest An assumed stationary frame of reference defined by using the nearby galaxies as reference objects.

fault A fracture along which displacement has occurred on the solid surface of a planet or other celestial body.
fields Entities dispersed in space but having a measurable value or magnitude that can be measured at any point in space. Examples are gravitational, electric, and magnetic fields.
fireball An unusually bright meteor, which may yield meteorites.
first quarter The phase of the Moon when it is one-fourth of the way around its orbit from new moon; the first quarter moon is seen in the evening sky with a straight terminator and half the disk illuminated.
flare (1) On the Sun, a sudden, short-lived, localized outburst of energy, often ejecting gas at speeds exceeding 1000 km/s; (2) an outburst from certain types of variable stars, sometimes called flare stars.
flat geometry A geometry in which parallel lines never meet, as described by Euclidean rules.
flat universe A universe with exactly the critical mass density and zero space curvature. It will continue to expand ever more slowly.
focal length The distance from a lens or mirror to the point where it focuses the image of a very distant object, such as the Moon.
focus One of the two interior points around which planets or stars move in an elliptical orbit.
forbidden line Spectral line arising from a metastable state in atoms.
force In physics, a specific phenomenon producing acceleration of mass. Forces can be generated in many ways, such as by gravity, pressure, and radiation.
formation of galaxies Processes that led to subdivision of the universe's gas after the big bang and its collapse into individual galaxies.
free-fall Motion under the influence of gravity only, without any other force or acceleration, such as rocket firing.
free-fall contraction Contraction of a cloud or system of particles by gravity only, unresisted by any other force.
frequency Number of electromagnetic oscillations per second corresponding to electromagnetic radiation of any given wavelength.
full moon The phase of the Moon when it is closest to 180° from the Sun and therefore fully illuminated. galactic bulge The spheroidal distribution of stars toward the center of the Milky Way that are intermediate in age between the disk stars and halo stars.
galactic cannibalism The absorption of one galaxy by another during a collision, forming a new, larger galaxy.
galactic disk The thin formation of gas and dust on circular orbits where most of the young stars in the Milky Way are found.
galactic equator The plane of the Milky Way galaxy projected on the sky.
galactic halo A spherical swarm of globular clusters above and below the galactic disk, centered on a point in the direction of the constellation Sagittarius.
galactic latitude Angular distance around the galactic equator from the galaxy's center.
galactic longitude Angular distance from the galactic equator.
galactic morphology The appearance of a galaxy in terms of its different components: disk, bulge, halo, bar, nucleus.
galactic nucleus The center of a galaxy.
galaxy Any of the largest groupings of stars, usually of mass 10⁸ to 10¹³ M.
Galilean satellites The four large satellites of Jupiter, discovered by Galileo.
Galileo Galilei (1564-1642): Italian scientist who first applied the telescope to observe other planets, discovering lunar craters, Jupiter's moons, and other celestial phenomena.
Galileo program A NASA mission to put an unmanned probe in orbit around Jupiter in the 1990s.
gamma-ray burster The theoretical interpretation is unclear, but these sources display erratic bursts of high energy gamma-rays and no preference for the disk of the galaxy.
Ganymede Largest of the four Galilean satellites of Jupiter, with a fractured and cratered ice surface.
general theory of relativity Einstein's theory that deals with accelerations caused by gravity and other forces. It begins to differ substantially from Newton's theory only when gravitational fields begin to get very strong. It predicts that the structure of space-time is affected by the mass-energy of the universe.
geological time scale The sequence of events in the history of the Earth.
giant elliptical galaxy Massive galaxies with diffuse elliptical form, somewhat resembling globular clusters but much larger. They are often found in the middle of rich clusters and may be formed by the cannibalism of smaller galaxies.
giant planet (1) Jupiter, Saturn, Uranus, or Neptune; (2) any planet much more massive than the Earth.
giant star Highly luminous star larger than the Sun. O and B main-sequence stars are sometimes called blue giants; evolved stars of extremely large radius are called red giants.
gibbous A phase between half-illuminated and fully illuminated, with a convex terminator (pronounced with hard g, as in ``give'').
globular cluster X radiation X-ray radiation from globular clusters, discovered unexpectedly in the 1970s; it indicates energetic environments somewhere within them.
globular star cluster A dense spheroidal cluster of stars, usually old, with mass of 10<sup>4</sup> to 10<sup>6</sup> M.
grain Small (usually microscopic) solid particle in space.
grand unified theories (GUTs) Speculative theories that seek to unify electromagnetism and the strong and weak nuclear forces, probably at a temperature around 10<sup>28</sup> K.
granite A rock type of modest density and high silica content, formed in association with differentiation processes and, therefore, found primarily on the Earth.
granitic rock A silica-rich, light-colored rock type common in the Earth's continents. Granites, being low in density, tend to float to the surfaces of planets that have had extensive melting in the outer layers.
granules Convection cells 1000 to 2000 km across, rising from the subphotospheric layers of the Sun. Each granule rises at a speed of 2 to 3 km/s and lasts for a few minutes.
gravitational contraction Slow contraction of a cloud, star, or planet due to gravity, causing heat and radiation.
gravitational lensing The creation of a distorted image of a distant quasar or galaxy when its light is focused by the gravity of a galaxy between it and us.
gravitational mass The mass of an object that reflects the way it moves in a gravitational field.
gravitational microlensing The amplification of the light of a background star due to the presence of an object between us and the star.
gravitational redshift Radiation leaving an intense gravitational field suffers a loss in energy, corresponding to a shift to longer or redder wavelengths.
gravitational waves One of the predictions of general relativity, gravitational waves result when compact objects change their state. These ``ripples'' in the structure of space travel at the speed of light, and have been indirectly detected in binary pulsar systems.
gravity The force by which all masses attract all other masses.
great circle Any circle on the surface of a sphere (especially the Earth or sky) generated by a plane passing through the center of the sphere; the shortest distance between two points on a sphere.
Great Red Spot A large, reddish, oval, semipermanent cloud formation on Jupiter.
greenhouse effect Heating of an atmosphere by absorption of outgoing infrared radiation.
Gregorian calendar Essentially the modern calendar system, introduced around a.d. 1600 under Pope Gregory XIII, and containing the modern system of reckoning leap years.
ground state The lowest energy state of an atom, in which all electrons are in the lowest possible energy levels.
Gum Nebula A large, relatively nearby nebula in the Southern Hemisphere sky, detected in hydrogen alpha light and formed by a supernova explosion estimated to have occurred around 9000 b.c.

HI region Interstellar region in which hydrogen is predominantly neutral.
HII region Interstellar region in which hydrogen is predominantly ionized.
half-life In any phenomenon, the time during which the main variable changes by half its original value; often used loosely to indicate the characteristic time scale of a phenomenon. In radioactive decay, the time for half the atoms in a system to disintegrate.
Halley's comet The most famous comet, which visits the inner solar system every 76 years, most recently in 1986 when close-up photos were made by space probes.
Hayashi track A sharply descending evolutionary track in the H-R diagram covering the early period of stellar evolution from the high-luminosity phase to the main sequence.
Heisenberg uncertainty principle A fundamental limitation to the precision of physical measurements. It states that we cannot know with arbitrary accuracy both the position and momentum of a system, or the energy of the system at every instant of time.
heliacal rising: A star's first visible rising during the yearly cycle.
heliacal setting A star's last visible setting during the yearly cycle.
helium flash Runaway helium ``burning'' inside a star as it evolves off the main sequence and into the giant phase of evolution. It occurs when degenerate gas at the star's center reaches a temperature of about 10⁸ K.
Helmholtz contraction Slow contraction of a cloud or system of particles by the force of gravity, which is retarded by outward gas pressure and the limited rate at which radiation can escape.
high-luminosity phase A star's short-lived stage of maximum brightness during pre-main-sequence evolution.
high-velocity star A star with a high velocity relative to the Sun; generally associated with the galactic halo.
homogeneous Uniform in composition throughout the volume considered.
horizon The boundary of the observable universe, defined by the distance that light can travel in the age of the universe.
host galaxy The galaxy surrounding an active quasar core.
hot dark matter Weakly interacting particles that were traveling at relativistic speeds when the first structures formed. Candidates include the neutrino.
hour angle The number of hours since a star (or other body) last crossed the local meridian.
H-R diagram A technique for representing stellar data by plotting spectral type (or color or temperature) against luminosity (or absolute magnitude), named after its early proponents, Hertzsprung and Russell.
Hubble flow The component of a galaxy's motion that is caused by the smooth expansion of the universe. See also peculiar velocity of a galaxy.
Hubble law The observed linear relationship between the distance of a galaxy and its velocity of recession. In modern cosmology, this is interpreted as evidence of an expanding universe.
Hubble parameter The ratio of the recession velocity of a galaxy to its distance. In modern cosmology, this is important because it determines the size and age of the expanding universe, but it currently has a large uncertainty of at least 30%.
Hubble Space Telescope A large orbiting telescope with 2.4-m (94-in.) mirror. Design flaws made it less efficient than intended, but it is currently producing results beyond those possible from the ground.
Hubble time The inverse of the Hubble parameter, this gives an upper bound to the age of the universe. Deceleration due to matter means that the age of the universe will be less than the Hubble time.
hydrogen alpha line The designation of hydrogen's red spectral line at 656.3 nm, more properly called hydrogen Balmer alpha.
hydrogen Balmer series The series of all hydrogen spectral lines from 364.6 to 656.3 nm, caused by electron transitions between the second and higher energy levels.
hydrostatic equilibrium The balance that exists at every point in a stable star between the inward force of gravity and the outward pressure due to energy released from nuclear reactions.
hyperbola The orbital curve followed by any free-falling body moving faster than escape velocity.
Hyperion An irregularly shaped outer moon of Saturn, noted for its chaotic rotation.
hypothesis A proposed explanation of an observed phenomenon or a proposal that a certain observable phenomenon occurs.

Iapetus An outer moon of Saturn noteworthy because one hemisphere has a bright, icy surface, and the other, a black carbonaceous surface.
ice ages Intervals of geologic history during which much larger fractions of Earth's surface were covered by glaciers than is true today.
IC number The catalog number of a cluster, nebula, or galaxy in the Index Catalog.
igneous rock Rock crystallized from molten material.
impact crater A roughly circular depression of any size (known examples range from microscopic size to diameters greater than 1000 km) caused by a meteorite impact.
impacts of interplanetary debris Collisions of interplanetary rocky or icy bodies with Earth (or with other worlds). Small impacts are much more common than large impacts.
impact-trigger theory The leading theory of the moon's origin, in which material was blasted off Earth's mantle and then reaccumulated to form the Moon.
incidence of multiplicity Among stars, the fraction of systems that contain more than one star. Probably 50 to 70% of systems have companion stars, many of these having more than one companion.
inertial mass The mass of an object that reflects its resistance to a change in its motion.
inferior planet Mercury or Venus.
inflationary universe A modification of the standard big bang model; it says that the universe went through a brief early period of unusually rapid expansion just after the big bang itself. Inflation helps to explain the smoothness of the microwave background radiation and the fact that the geometry of the universe is close to flat.
infrared Radiation of wavelength too long to see, usually about 1 to 100 µm.
infrared star A star detected primarily by infrared light.
intense early bombardment The very intensive bombardment of planetary bodies by meteorites, from 4.6 to about 4 billion years ago, following planet formation.
interferometry A system for obtaining high-resolution astronomical observations by linking several physically separated telescopes electronically, in effect creating a single, much larger telescope.
interstellar atom Atom of gas in interstellar space.
interstellar grain Microscopic solid grain in interstellar space; interstellar dust.
interstellar molecule Molecule of gas in interstellar space.
interstellar obscuration Absorption of starlight by interstellar dust, causing distant objects to appear fainter.
interstellar reddening Loss of blue starlight due to interstellar dust, causing distant objects to appear redder and fainter.
interstellar snowball Hypothetical interstellar particle larger than an interstellar grain.
inverse square law The relation describing any entity, such as radiation or gravity, that varies as 1/r², where r is the distance of the entity from the source.
Io The innermost Galilean moon of Jupiter, famous for its active volcanism, which is unique among moons.
ion Charged atom or molecule.
ionization The process of knocking one or more electrons off a neutral atom or molecule.
ionized gas A gas in which many of the atoms have lost at least one electron, thus becoming charged particles, or ions.
iron meteorite Meteorite composed of a nearly pure nickel-iron alloy.
irregular galaxy A galaxy of amorphous shape. Most have relatively low mass (10⁸-10¹⁰ M).
irregular variable A star that fluctuates in brightness irregularly.
isochrone fitting The method of making theoretical models of stellar evolution and fitting them to observed H-R diagrams, as a way of measuring the ages of populations of stars.
isotope A form of an element with a specified number of neutrons in the nucleus. Each element may have many possible isotopic forms, but only a few are stable.
isotropic Appearing uniform no matter what the direction of view.

joule The unit of energy in the SI metric system of units.
Julian date The date based on a running tabulation of days, starting January 1, 4713 b.c.
Jupiter's atmospheric composition Mostly hydrogen, with additional helium, hydrogen-based compounds, and other gases.
Jupiter's infrared thermal radiation Infrared radiation from Jupiter due to slow contraction of its interior and exceeding the incoming solar radiation.
Jupiter's interior Beneath Jupiter's atmosphere, a high-pressure region of liquid hydrogen, liquid metallic hydrogen, and a small central core of rocky material.
Jupiter's temperature Temperatures around -200° F are measured at and above the cloud tops in Jupiter's atmosphere, but the air temperature increases below the clouds to values around room temperature and even warmer.
Kelvin scale The absolute temperature scale, with 0 K = absolute zero. A kelvin is the same size (some temperature difference) as a centigrade degree.
Johannes Kepler (1571-1630): Astronomer who first deduced the shapes and relations of planets' elliptical orbits.
Kepler's laws The three laws of planetary motion that describe how the planets move, show that the Sun is the central body, and allow accurate prediction of planetary positions.
kiloparsec (kpc) 1000 parsecs.
Kirchhoff's laws Laws describing conditions that produce emission, absorption, and continuous spectra.
Kuiper belt The reservoir of comet nuclei in the Pluto region, from about 30 to 100 AU.

L The luminosity of the Sun, 4×10²⁶ watts.
LSR (See local standard of rest.)
Lagrangian points In an orbiting system with one large and one small body, an array of five points where a still smaller body would retain a fixed position with respect to the other two.
Lagrangian surface An imaginary surface with a figure-8 cross section surrounding two coorbiting bodies in circular orbits and constraining motions of particles within the system.
large-scale structure The distribution of matter in the universe on the largest scales, describing the space distribution of clusters and superclusters of galaxies.
lava Molten rock on the surface of a planet.
life A process in which complex, carbon-based materials organized in cells take in additional material from their environment, replicate molecules, reproduce, and do other weird things like writing books.
life in the solar system Hypothetical biological activity on other planetary bodies, such as on Mars, under Europa's ice, or in Jupiter's atmosphere. Now regarded as unlikely.
life on Mars Although long sought and believed possible by many scientists, biological processes on Mars were apparently ruled out in 1976 when Viking landers found no organic material there.
life outside the solar system Hypothetical biological activity beyond the outskirts of our solar system, as on hypothetical planets around other stars. Such life is regarded as plausible, even if sparsely scattered, by most astronomers.
light curve A plot of brightness of a star (or other object) versus time.
light-gathering power The ability of a telescope or binoculars to gather light. It is proportional to the area of the objective, that is, the square of the aperture.
light-year The distance light travels in one year, 9.46×10¹² km.
limb The apparent edge of a celestial object.
line of nodes A line formed by the intersection of an orbit and some other reference plane, such as the plane of the solar system.
linear measure Measurement involving linear distances, as opposed to angles or angular distances.
lithosphere The solid rocky layer in a partially molten planet.
Local Group The cluster of galaxies to which the Milky Way and 26 nearby galaxies belong.
local standard of rest (LSR) A frame of reference moving with the average velocity of the nearby stars (out to about 50 pc from the Sun).
Local Supercluster A large flattened structure of about 10¹⁵ M centered on the Virgo cluster, of which the Milky Way is a member.
long-period comet A comet moving on a nearly parabolic orbit and thus having an orbital period of hundreds of thousands of years.
look-back time The idea in cosmology that light takes a significant time to reach us from distant objects, so we inevitably see them as they were in the past.
luminosity The total energy radiated by a source per second. The luminosity of the Sun (L) is 4×20²⁶ watts.
luminosity function General term for the distribution of brightness of astronomical objects, as a function of their density in space. Can be derived for stars, galaxies, quasars, or any astronomical source.
lunar eclipse Dimming of the Moon as it passes into the Earth's shadow.

M The mass of the Sun, 2×10³⁰ kg.
M giant A giant star of spectral class M.
Magellanic clouds The two galaxies nearest the Milky Way, irregular in form and visible to the naked eye in the Southern Hemisphere.
Magellanic stream Gas filaments connecting the magellanic clouds to the Milky Way.
magma Underground molten rock.
magma ocean Primordial layer of molten lava on the initial surface of the Moon and (by inference) planets.
magnetic braking The slowing of rotation of a star or planet by interaction of its magnetic field with surrounding ionized material.
magnetic field Region of space in which a compass (or other detector) would respond to magnetism of some body.
magnetic support The support provided when an interstellar gas cloud is laced with a magnetic field. The magnetic field resists the gravitational collapse of the cloud and so affects the rate and efficiency of star formation.
magnification Apparent angular size of a telescope image divided by the angular size of the object seen by the naked eye.
main-sequence star One of the group of stars defined on the H-R diagram that have a relatively stable interior configuration and are consuming hydrogen in nuclear reactions; a star on the main sequence.
mantle A region of intermediate density surrounding the core of planets.
mare (pl. maria): A dark-colored region on a planet or satellite; a region of basaltic lava flow on the Moon.
Mars-crossing asteroids Asteroids whose orbits cross that of Mars.
Mars' rotation period The day on Mars--24^h 37^m, only slightly longer than the Earth's.
Martian air pressure The pressure exerted by the very thin Martian air. On Mars' surface, the air pressure is only about 0.7% of that at the Earth's surface.
Martian air temperature Soil temperatures approach or exceed freezing in the day; the air is colder. Night temperatures around -123°F are recorded.
Martian atmosphere The thin gases around Mars, composed almost entirely of carbon dioxide (CO₂).
Martian meteorites A handful of meteorites about 1.3 billion years old, believed to have been blasted off Mars about 0.2 billion years ago.
maser (microwave amplification by simulated emission of radiation): (1) A device that amplifies microwave radio waves through special electronic transitions in atoms; (2) an interstellar cloud that acts in this way.
maser emission Particularly intense emission from molecules in dense molecular clouds, which occurs when a large number of molecules are excited to energy levels above the ground state.
mass (1) Material; (2) the amount of material.
mass-energy equivalence Einstein's statement of the relationship between mass and energy, given by the famous equation E = mc².
mass extinctions Extinctions of many species 65 million years ago, probably triggered by the impact of an asteroid.
mass/luminosity ratio The mass per unit of light or total radiation emitted from an object such as a galaxy.
mass-luminosity relations The relation between the mass of a main-sequence star and its total radiation rate; the more massive, the greater the luminosity.
mass of the galaxy About 4 ×10⁴¹ kg, or 2×10¹¹ M, as estimated from the circular velocity equation.
matter-dominated universe The universe more than 10,000 years after the big bang, in which the mass-energy of particles exceeds the energy density of photons and gravitational structures can begin to form.
Maunder minimum The interval from 1645 to 1715, when solar activity was minimal.
mean density Mass of an object divided by its volume.
mean solar time Time shown by conventional clocks, determined by the Sun's mean rate averaged over the year.
megaparsec One million parsecs.
meridian (1) A north-south line on a planet, moon, or star; (2) a great circle through the celestial pole and the zenith.
Messier number The catalog number of a nebula, star cluster, or galaxy in Messier's Catalog.
metallic hydrogen A high-pressure form of hydrogen with free electrons.
metastable state In an atom, a configuration of electrons that is relatively long-lived, but is rarely found on the Earth because it is disrupted by collisions with other atoms; it may be found in interstellar atoms, creating forbidden spectral lines.
meteor A rapidly moving luminous object visible for a few seconds in the night sky (a ``shooting star'').
meteorite An interplanetary rock or metal object that strikes the ground.
meteorite impact crater Circular depression in planetary surfaces, caused by explosions as meteorites crash into the surfaces at high speeds.
meteoroid A particle in space, generally smaller than a few meters across.
meteor shower A concentrated group of meteors, seen when the Earth's orbit intersects debris from a comet.
meter 39.4 in.
microwave background fluctuations Tiny spatial variations in the temperature of the microwave background radiation; they form the seeds for eventual galaxy formation.
Microwave Observing Project (MOP) The most ambitious SETI experiment yet attempted. This NASA-sponsored project uses radio techniques to monitor 1000 solar-type stars for artificial, intelligent signals and to scan the entire sky.
Milky Way galaxy The spiral galaxy in which we live.
Miller-Urey experiments A series of experiments in which amino acids and other organic molecules were created in laboratory conditions that simulate the conditions of the early Earth.
millisecond pulsar A small class of pulsars that rotate at speeds of up to 700 times per second, thought to be old neutron stars ``spun up'' by the infall of material from a binary companion.
Mimas The innermost large moon of Saturn, icy and heavily cratered.
minerals Chemical compounds, usually in the form of crystals, that constitute rocks.
minute of arc An angle equaling 1/60 of a degree.
Miranda The innermost of five large moons of Uranus, noted for puzzling fractured and grooved terrain.
mks system A metric system of units expressing length in meters, mass in kilograms, and time in seconds. (See also cgs system.)
molecular cloud An interstellar cloud of gas and dust, with greater than average density, dust content, and high concentration of molecules.
molecules in space More than 80 varieties of molecules have been discovered in space, some containing as many as 13 atoms. The sites for molecules are interstellar gas clouds; some have been trapped in comets and meteorites.
Moon (1) The Earth's natural satellite; (2) any satellite.
morphology-density relation The observation that the relative number of spiral and elliptical galaxies depends on the density of the region. High-density regions or clusters have a larger fraction of ellipticals than low-density regions.
multicelled organisms Life forms that represented a significant increase in complexity and originated on Earth about 600 million years ago.
multiple scattering Redirection of electromagnetic radiation (such as light waves) by repeated interaction with atoms, molecules, or dust grains in space or in an atmosphere.
multiple star system A system of three or more stars orbiting around each other.
mutation The fundamental mechanism for generating change in genetic material. Mutations alter the molecular structure of the DNA molecule in ways that can be helpful, harmful, or neutral to the organism.
mutual recession of galaxies The phenomenon that all distant galaxies are moving away from us, and the farther away they are, the faster they are receding.

natural selection The theory that states that those individuals best adapted to the ever-changing environment produce a greater number of offspring.
nebula A cloud of dense gas and/or dust in interstellar space or surrounding a star.
negative hydrogen ions Hydrogen atoms that have temporarily captured an extra electron, responsible for opacity in the photosphere of the Sun and many stars.
negatively curved geometry A geometry in which parallel lines diverge; sometimes called a hyperbolic geometry.
Neptune The outermost gas giant planet in the outer solar system.
neutrino A subatomic particle created in certain nuclear reactions inside stars. It can pass through most matter. Its mass is uncertain, being either zero or a tiny fraction of an electron's mass.
neutrino burst During the explosive death of a massive star, 0.1-0.2 solar masses of material and a vast amount of energy is carried away in the form of neutrinos. The neutrino burst was seen for the first time in Supernova 1987A.
neutron One of the two major particles constituting the atomic nucleus; it has zero charge and mass 1.6749×10^-27 kg.
neutron star A star with a core composed mostly of neutrons, with density 10¹⁶ to 10¹⁸ kg/m³. Many or most neutron stars are pulsars.
new moon The phase of the moon when it is nearest the Earth-Sun line, hence invisible from Earth because of the Sun's glare.
Isaac Newton (1642-1727): English physicist who discovered the spectrum and laws of gravitation and motion; he also developed calculus and made other discoveries. Possibly the greatest physicist in history.
Newtonian cosmology A model of the universe with infinite volume, no expansion, and Euclidean geometry.
Newton's laws of motion Three rules describing motion and forces. Briefly, (1) a body remains in its state of motion unless a force acts on it; (2) force equals mass times acceleration; (3) for every action there is an equal and opposite reaction.
NGC number The catalog number of a nebula, cluster, or galaxy in the New General Catalog.
node One of two points where an orbit crosses a reference plane.
noncosmological redshift A hypothetical redshift of distant galaxies not caused by the Doppler effect.
nonthermal radiation Radiation not due to the heat of the source; for example, synchrotron radiation.
North Star (1) Polaris; (2) any bright star that happens to be within a few degrees of the north celestial pole during a given era.
nova A type of suddenly brightening star (from the Latin for ``new'') resulting from explosive brightening when gas is dumped from one member of a binary star pair onto the other.
nuclear reaction Reaction involving the nuclei of atoms in which a nucleus changes mass.
nucleocosmochronology A method of measuring the age of the Milky Way by using the decay of radioactive isotopes within it.
nucleus (1) The matter at the center of an atom, composed of protons and neutrons; (2) the bright central core (or solid body) of a comet; (3) the bright central core of a galaxy.

O association An association of O-type stars.
objective The major light-gathering element of a telescope; the mirror in a reflector and the lens in a refractor.
oblate spheroid The shape assumed by a sphere deformed by rotation.
obliquity The angle by which a planet's rotation axis is tipped to its orbit.
Occam's razor The principle that the simplest hypothesis, with the fewest assumptions, is most likely to be correct. Named after its use to cut away false hypotheses.
ocean tide The Moon's tidal stretching of the Earth, as observed in the ocean surface (as opposed to body tide).
Olbers' paradox The problem of why the sky is dark at night if the universe is filled with stars.
Oort cloud The swarm of comets surrounding the solar system.
opacity The extent to which gaseous (or other) material absorbs light.
open star cluster A grouping of relatively young Population I stars (usually 10²-10³ M), sometimes called a galactic cluster.
open universe A universe with negative space curvature and less than the critical mass density. If the universe is open, it will expand forever.
opposition The period when a superior planet lies in an opposite direction from the Sun as seen from the Earth. At opposition, a planet appears in the midnight sky, well placed for observation.
optical double star A pair of stars that have small angular separation but are not coorbiting.
orbital precession A slow, cyclical change in the orientation of the plane of an orbit.
organic chemistry Chemistry involving organic molecules.
organic molecule Molecule based on the carbon atom, usually large and complex, but not necessarily part of living organisms.
origin of the heavy elements The process by which light elements fused to form nuclei of heavy elements, primarily inside massive stars.
Orion Nebula Several hundred solar masses of gas and dust composing the core of a star-forming region about 460 parsecs away. One of the most prominent nebulae in our sky, it forms the central ``star'' in Orion's sword.
Orion star-forming region The larger region in which stars are forming around the Orion Nebula.
ozone layer An atmospheric layer rich in ozone (O<sub>3</sub>), created by the interaction between oxygen molecules (O<sub>2</sub>) and solar radiation. On Earth, its altitude is about 20-60 km. parabola (1) The curved trajectory followed by a particle moving at escape velocity; (2) the curve of a Newtonian telescope's primary mirror.
parallax An angular shift in apparent position due to an observer's motion; more specifically, a small angular shift in a star's apparent position due to the Earth's motion around the Sun. Stellar parallax, used to measure stellar distance, is defined as the angle subtended by the radius of the Earth's orbit as seen from the star.
parent body A body from which a meteorite formed and later broke off as a fragment.
parent isotope A radioactive isotope that disintegrates and forms a daughter isotope.
parsec A distance of 206 265 AU, 3.26 ly, or 3.09×10<sup>13</sup> km; defined as the distance corresponding to a parallax of 1 second of arc.
partial solar eclipse An eclipse in which the light source is not totally obscured from an observer.
particlelike properties of light Characteristics of light, such as concentration of energy and momentum in discrete microscopic packets (photons) that mimic the properties of particles.
Pauli exclusion principle A principle of subatomic physics specifying that no two electrons in a very small volume have exactly the same properties of energy, motion, and so on.
peculiar galaxy Galaxy that does not fit into the standard Hubble sequence; its morphology can be caused by a tidal interaction with another galaxy.
peculiar velocity (of a galaxy) The component of a galaxy's motion that is not caused by the smooth expansion of the universe. Since galaxies interact gravitationally, they usually have a measurable peculiar velocity. See also Hubble flow.
peculiar velocity (of a star) A star's velocity with respect to the local standard of rest.
penumbra (1) The outer, brighter part of a shadow, from which the light source is not totally obscured; (2) the outer, lighter part of a sunspot.
perigee The point in an orbit around the Earth that is closest to the Earth.
permafrost Semipermanent underground ice.
phase The apparent shape of an illuminated body, varying with the ``phase angle'' from observer to body to illumination source.
Phoebe The outermost moon of Saturn, a small dark moon believed to be captured.
photometry The measurement of the amount of light, either total or in different specified colors, coming from an object.
photon The quantum unit of light, having some properties of a wave. For each wavelength of radiation, the photon has a different energy.
photosphere The light-emitting surface layer of the Sun.
photosynthesis Process that converts sunlight into stored chemical energy, essential for the proliferation of advanced forms of life.
physical binary stars Two stars orbiting around a common center of mass.
Planck's constant The fundamental constant that relates the energy of an electromagnetic wave to its frequency.
Planck's law A formula that describes the energy associated with each wavelength (color) in the spectrum. It shows that photons of blue light are more energetic than photons of red light.
planet A solid (or partially liquid) body orbiting around a star but too small to generate energy by nuclear reactions.
``Planet X'' A term sometimes used for a hypothetical tenth planet in the solar system, beyond Pluto.
planetary nebula A type of circumstellar gas cloud that has spheroidal shape and often appears as a faint disk in telescopes; it has nothing to do with planets except for the rough resemblance to a planet's shape when seen telescopically.
planetesimal One of the small bodies from which planets formed, usually ranging from micrometers to kilometers in diameter.
planetology The study of the planets' origins, evolution, and conditions.
plasma A high-temperature gas consisting entirely of ions, instead of neutral atoms or molecules. Because of the high temperature, the atoms strike each other hard enough to keep at least the outer electrons knocked off.
plate Moving unit of the Earth's lithosphere, typically of continental scale.
plate tectonics Motions of a planet's lithosphere, causing fracturing of the surface into plates. Primary example occurs on the Earth.
Pluto Cataloged as the outermost and smallest planet in the solar system, but possibly one of many small worldlets on the fringe of the solar system.
Polaris The North Star.
Population I Stars with a few percent heavy elements (heavier than helium), found in the disks of spiral galaxies and in irregular galaxies.
Population II Stars composed of nearly pure hydrogen and helium, found in the halo and center of spiral galaxies, in elliptical galaxies, and to a limited extent in irregulars.
positively curved geometry A geometry where parallel lines converge, sometimes called a spherical geometry.
powers of 10 The number of times 10s must be multiplied together to give a specific number; the exponent of 10. (Example: 10² = 100; the power, or exponent, is 2; see also Appendix 1.)
precession The wobble in the position of a planet's rotation axis caused by external forces. Also, the change in a coordinate system (tied to any planet) caused by such a wobble.
pre-main-sequence star Evolutionary state of stars prior to arrival on the main sequence, especially just before the main sequence is reached.
principle of relativity The principle that observers can measure only relative motions, since there is no absolute frame of reference in the universe by which to specify absolute motions.
prograde and retrograde satellite orbits Satellite orbits in which motion is in the same or opposite direction, respectively, as the planet's rotation.
prograde rotation Spinning on an axis from west to east, or counterclockwise as seen from the North Pole (as in the case of the Earth).
prokaryote Cell that contains a single long strand of DNA but no nucleus. Prokaryotes were the first and simplest life forms on the Earth, dating back 3.8 billion years.
prominence A radiating gas cloud extending from the solar surface into the thinner corona.
prominent star One of the brightest stars in our sky, but not necessarily one of the nearest.
proper motion The angular rate of motion of a star or other object across the sky. (Most stars have proper motions less than a few seconds of arc per year.)
protein Any of several types of complex organic molecules made from amino acids inside plants and animals, which are essential in living organisms.
proteinoid Cell-like, nonliving spheroid of protein molecules created in the laboratory by heating amino acids and adding water; a possible step in the evolution of life.
protogalaxy Enormous gas cloud that will collapse by gravity to form a galaxy.
proton One of the two basic particles in an atomic nucleus, with positive charge and mass 1.6726 = 10^-27 kg.
proton-proton chain A series of thermonuclear reactions that convert hydrogen nuclei to helium nuclei, converting a tiny amount of mass into energy.
protoplanet A planet shortly before its final formation. Sometimes hypothesized to have a massive atmosphere and greater mass than in its present state.
protostar A gravitationally stable cloud of stellar mass contracting in an early pre-main-sequence evolutionary state.
pseudoscience Research that has the trappings of science but does not follow the scientific method, usually lacking review and repetition of observations by independent researchers.
Ptolemaic model The ancient Earth-centered model of the solar system, with the Sun, Moon, and other planets moving in epicycles.
pulsar A rapidly rotating neutron star with a strong magnetic field, observed to emit pulses of radiation.

quantum Fundamental unit of energy that governs all particle interactions and dictates the discreteness of the physical world.
quasar An active galaxy seen at such a large distance that the bright nucleus overwhelms the surrounding galaxy, and the image appears stellar. Most quasars have very large redshifts.
quasar absorption lines When the light from a distant quasar intercepts an intervening galaxy, an absorption line is created in the quasar spectrum at the red shift of the galaxy. Such lines are called quasar absorption lines.
quasar luminosity function The density of quasars in space (number per Mpc³) as a function of luminosity. The most luminous quasars are the rarest.

r-process reactions Rapid reactions, probably occurring inside supernovae, in which heavy elements are formed as atomic nuclei capture neutrons. (See also s-process reactions.)
radial velocity The velocity component along the line of sight toward or away from an observer. Recession is positive; approach is negative.
radiation (1) Any electromagnetic waves or atomic particles that transmit energy across space; (2) one of three modes of heat (energy) transmission through stars or planets from warm regions to cool regions.
radiation-dominated universe The universe less than 10,000 years after the big bang, in which the energy per unit volume in the form of photons is greater than the energy per unit volume represented by particles of mass.
radiation pressure An outward pressure on small particles exerted by electromagnetic radiation in a direction away from the light source.
radioactive atom Any atom whose nucleus spontaneously disintegrates.
radio communication Communications sent by electromagnetic waves at radio wavelengths. This is the favored form of communication for SETI experiments, because of the high speed of transmission and the low amount of energy required to send a message.
radio galaxy A galaxy that emits unusually large amounts of radio radiation.
radioisotopic dating: Dating of rock or other material by measuring amounts of parent and daughter isotopes.
radio jets One-sided or two-sided jets of glowing material that are seen in some active galaxies.
radio telescope A device that gathers and concentrates radio waves.
random error Error that can be reduced when separate observations are combined. For example, four times the data will reduce the error by a factor of two.
ray A bright streak of material ejected from a crater on the Moon or other planet.
Rayleigh scattering Scattering of light by particles smaller than the light's wavelength. This process favors scattering of blue light.
recombination The process in the early universe, completed a million years after the big bang, by which protons and electrons combined to form hydrogen atoms.
red giant A post-main-sequence star whose surface layers have expanded to many solar radii and have relatively low temperatures.
redshift A Doppler shift of spectral features toward longer wavelengths, indicating recession of the source.
redshift of galaxies The shift toward longer wavelengths in light of distant galaxies, due to their recession from the solar system. It increases with galaxies' distances.
reflected radiation Radiation that has arrived from outside a body and bounced off, as opposed to thermal radiation.
reflector A type of telescope using a mirror as the light collector.
refractor A type of telescope using a lens as the light collector.
refractory element An element least likely to be driven out of a material by heating. These elements are usually concentrated in the last components to melt when a material such as rock is heated.
regolith A powdery soil layer on the Moon and some other bodies caused by meteorite bombardment.
regression of nodes A shifting of the R.A. = Dec. coordinate system, relative to the stars, as a result of the 26,000-y wobble of the Earth's rotation axis.
relativistic Moving at speeds near that of light.
relativity (See principle of relativity.)
replication The basic process of reproduction for life, and the means by which genetic information is propagated. It involves the ability of some carbon-based molecules to split into two halves that can assemble copies of themselves.
representative stars A sample of stars randomly drawn from the total population of stars in space.
resolution The smallest angle that can be discerned with an optical system; for example, the eye can resolve about 2 minutes of arc.
resonance A close, or simple-number, relationship between periodicities in two phenomena. For example, if one body has half the orbital period of another, they are said to be in orbital resonance.
retrograde motion Revolution or rotation from east to west contrary to the usual motion in the solar system.
retrograde rotation Spinning on an axis from east to west, or clockwise as seen from the North Pole (opposite to the spin direction of Earth).
Rhea (See Tethys, Dione, and Rhea.)
rift A major split in a planet's lithosphere due to active or incipient plate tectonic stresses.
right ascension Longitude lines projected onto the celestial sphere. (Abbreviation: R.A.)
rille A type of lunar valley.
RNA (ribonucleic acid) The molecule that assembles proteins from DNA instructions.
Roche's limit The distance from a large body within which tidal forces would disrupt a satellite.
rock Solid aggregation of minerals.
rotation curve Orbital velocity as a function of distance from the center of a galaxy. The flat rotation curves observed for many spiral galaxies provide good evidence for dark matter.
rotational line broadening Broadening of spectral lines due to rotation of the source.
rotational support An outward force provided by a rotating gas cloud to resist the inward force of gravity.
RR Lyrae star A type of variable star similar to the Cepheids that has been found associated with Population II and not Population I.
runaway star A star rapidly moving away from a region of recent star formation.
Russell-Vogt theorem The theorem stating that the equilibrium structure of a star is determined by its mass and chemical composition.

s-process reactions Slow reactions in giant stars in which heavy elements are built up as atomic nuclei capture neutrons. (See also r-process reactions.)
Saha equation An equation derived in 1920, by the Indian physicist Saha, that tells the percent of atoms in each different excited state, given the conditions in a gas. This in turn controls what spectral lines are emitted or absorbed by that gas.
saros cycle An interval of 6585 d (about 18 y) separating cycles of similar eclipses, used by ancient people to predict eclipses.
satellite Any small body orbiting a larger body.
Saturn The sixth planet out from the Sun, famous for its prominent rings.
Saturn's atmosphere The thick, cloudy gases around Saturn, composed mostly of hydrogen.
Saturn's ring system A system of innumerable icy particles orbiting Saturn.
Saturn's satellite system A family of at least 18 moons orbiting Saturn, ranging from 20 km diameter up to a size slightly exceeding that of the planet Mercury.
scale error In distance measurement, an error that affects all measurements of larger distances.
Schwarzschild radius The radius corresponding to the event horizon of a black hole, proportional to the mass of the black hole.
science Study of nature using the scientific method. (See scientific method.)
scientific method The method of learning about nature from making observations, formulating hypotheses, and constructing observational or experimental tests to see if the hypotheses are accurate.
search for extraterrestrial intelligence (SETI) A scientific field devoted to calculating the probability of intelligent life in the universe and designing the optimum strategies for making contact.
seasonal changes on Mars Changes in shape and darkness of the dusky patches on Mars from summer to winter and year to year. Once thought to indicate Martian vegetation, the changes are now known to result from blowing dust deposits.
secondary atmosphere A planet's atmosphere after modification by outgassing and other processes. (See also early dense atmosphere.)
second of arc An angle equaling 1/3600 of a degree.
sedimentary rock Rock formed from sediments.
seeing The quality of stillness or lack of shimmer in a telescopic image, associated with atmospheric conditions. If the atmosphere is very still and the image is sharp, the seeing is said to be good.
seismic waves Waves passing through the interior or surface layers of a planet due to a seismic disturbance, such as an earthquake or large meteorite impact.
seismology Study of vibrational waves passing through planets, revealing internal structure.
selection effect Any effect that systematically biases observations or statistics away from a correct understanding.
Seyfert galaxy A type of galaxy with a bright, bluish nucleus, possibly marking a transition between ordinary galaxies and quasars.
shepherd satellites Satellites that move near planetary rings and act to confine the ring particles onto certain orbits.
short period comet A comet with a revolution period less than about 100 y.
SI metric system The internationally standardized scientific system of units, in which length is given as meters, mass as kilograms, and time as seconds.
sidereal Referring to stars.
sidereal period A period of rotation or revolution where the movement is measured relative to the stars.
sidereal time Time measured by the apparent motion of the stars (instead of the Sun), used by astronomers to point telescopes toward celestial targets; it is the right ascension that is on the meridian at any given location.
significant figures The number of digits known for certain in a quantity.
small-angle equation The equation giving the relation between the distance D of an object, its diameter d, and its angular size a - alpha (expressed in seconds of arc):

tangential velocity The velocity component perpendicular to the line of sight.
Tarantula Nebula (30 Doradus) A huge HII emission nebula in the Large Magellanic Cloud.
T association An association of T Tauri stars.
tectonics Disruption of planetary or satellite surfaces by large-scale mass movements, such as faulting.
telescope An instrument for collecting electromagnetic radiation and producing magnified images of distant objects.
temperature A measure of the average energy of a molecule of a material.
terminator The dawn or dusk line separating night from day on a planet or satellite.
terrestrial planet (1) Mercury, Venus, Earth, or Mars; (2) a planet composed primarily of rocky material.
Tethys, Dione, and Rhea Intermediate-sized icy moons of Saturn.
theory A body of hypotheses, often with mathematical backing and having passed some observational tests; often implying more validity than the term hypothesis.
theory of cosmological redshifts The theory that galaxies' redshifts are all due to recessional motion, increase with distance, and thus give an indicator of distance.
theory of galaxy formation The (speculative) theory that describes how galaxies form from the gravitational collapse of enormous clouds of gas in the early universe.
theory of natural selection Darwin's theory that evolution of species is driven, at least in large part, by competition among species and ``survival of the fittest.''
theory of noncosmological redshifts The theory that at least some galaxies' redshifts are not Doppler shifts due to recession, but are due to some other cause.
theory of star formation The theory that describes how stars form by the gravitational collapse of interstellar clouds of dust and gas.
thermal escape Escape of the fastest-moving gas atoms or molecules from the top of a planet's atmosphere by means of their thermal motion.
thermal infrared radiation Radiation from bodies at room temperature or planetary temperatures, primarily at infrared wavelengths. See also thermal radiation.
thermal motion Movement of atoms and molecules associated with the temperature of the material; they grow faster as the temperature increases.
thermal radiation Electromagnetic radiation emitted by a body and associated with an object's temperature; it grows greater and bluer in color as the temperature increases.
third quarter The phase of the Moon when it is three-fourths of the way around its orbit from new moon; the third quarter moon is seen in the dawn sky with a straight terminator and half the disk illuminated.
thrust The force generated by a high-speed discharge, as from a rocket or airplane.
tidal interaction Close encounter between galaxies that leads to observable effects on the morphology.
tidal recession Recession of the Moon (or other satellite) from the Earth (or other planet) caused by tidal forces.
tide A bulge raised in a body by the gravitational force of a nearby body.
Titan Saturn's largest moon, famous for its thick, smoggy-orange nitrogen atmosphere.
total solar eclipse (1) An eclipse in which the light source is totally obscured from a specified observer; (2) an eclipse in which a body is entirely immersed in another's shadow. (See also eclipse.)
transit (1) Passage of a planet across the Sun's disk; (2) any passage of a body with a small angular size across the face of a body with a large angular size.
triple-alpha process A nuclear reaction in which helium is transformed into carbon in red giant stars.
Triton Neptune's largest moon.
Trojan asteroids Asteroids caught near the Lagrangian points in Jupiter's orbit, 60° ahead of and 60° behind the planet.
tsunami A large ocean wave generated by earthquake of volcanic activity (the correct name for a tidal wave).
T Tauri star A type of variable star, often shedding mass, believed to be still forming and contracting onto the main sequence.
21-cm emission line The important radio radiation at 21-cm wavelength from interstellar neutral atomic hydrogen.
21-cm radio waves Produced by neutral hydrogen, these waves are especially useful for galactic mapping because they allow us to detect HI clouds, which are concentrated in the spiral arms.

ultrabasic rock A rock of high density, low silica content, and high iron content, often derived from the upper mantle of a planet or satellite.
ultraviolet radiation Radiation of wavelength too short to see, but longer than that of X rays.
umbra (1) The dark inner part of a shadow, in which the light source is totally obscured; (2) the dark inner part of a sunspot.
unification The idea that the four forces of nature are just different manifestations of one basic superforce; the properties of the superforce can be realized only at phenomenally high temperatures or energies.
uniformity of nature The fundamental assumption in astronomy that the laws of physics that are derived in terrestrial laboratories also apply throughout the universe.
universe Everything that exists. Astronomers distinguish between the observable universe, the region from which light has had time to reach us in the age of the universe, and the physical universe, which may be much larger.
Uranus The seventh planet outward from the Sun.
Uranus' rotation axis Notable for its almost right-angle tilt (obliquity) of 97° to Uranus' orbital plane.
Uranus' satellites A system of five large moons discovered from Earth, and another ten discovered by Voyager 2 in 1986.
Urey reaction Reaction by which the Earth's carbon dioxide was concentrated in carbonate rocks after dissolving in seawater.

Van Allen belts Doughnut-shaped zones around the Earth (or another planet with a strong magnetic field) that traps energetic ions from the Sun.
variable star A star that varies in brightness.
velocity dispersion The range of velocities in a self-contained dynamical system, like a globular cluster or a galaxy.
Venera 7 First spacecraft to land successfully on another planet; it transmitted data from the surface of Venus in 1970.
Viking 1 The first successful probe to land on Mars (July 20, 1976). It made the first surface photos and measures of soil composition.
Viking 2 The second successful probe to land on mars (September 3, 1976).
virtual pairs Particle/antiparticle pairs that appear and disappear for an instant, as allowed by Heisenberg's uncertainty principle.
visible light Electromagnetic radiation at wavelengths that can be perceived by the eye.
visual apparent magnitude An apparent magnitude estimate based only on visual radiation from an object (excluding infrared, ultraviolet, X rays, and so on).
visual binary A binary in which both components can be seen.
Voids Low-density regions in the large-scale distribution of galaxies.
volatile element Element easily driven out of a material by heating.
volcanic crater A circular depression caused by volcanic processes such as explosion or collapse.
volcanism Eruption of molten materials at the surface of a planet or satellite.
volcanoes Sites where molten materials erupt from inside a planet or satellite.

wavelength (1) The length of the wavelike characteristic of electromagnetic radiation; (2) in any wave, the distance from one maximum to the next.
wavelike properties of light Characteristics of light, such as frequency and diffraction, that mimic properties of waves.
wave-particle duality The concept that particles can show wavelike properties and that radiation can show particlelike properties.
weak nuclear force The force of nature that converts neutrons into protons and is responsible for radioactive decay.
white dwarf star A planet-sized star of roughly solar mass and very high density (10⁸ to 10¹¹ kg/m³) produced as a terminal state after nuclear fuels have been consumed.
white light A mixture of light of all colors in proportions as found in the solar spectrum.
Wien's law A formula giving the wavelength W at which the maximum amount of radiation comes from a body of temperature T. The formula is W = 0.00290/T.
Wolf-Rayet star A type of very hot star ejecting mass.

X ray Electromagnetic radiation of wavelength about 0.01 to 10 nm.
X-ray burster A binary system composed of a neutron star and a main-sequence star, in which the thermonuclear detonation of a layer of helium on the neutron star leads to an intense burst of X rays.
X-ray source Celestial object emitting X rays; many are probably binary systems where mass is transferred.

zenith The point directly overhead.
zero-age main sequence The main sequence defined by a population of stars all of which have just evolved onto the main sequence. (Further evolution modifies the main sequence shape on the H-R diagram slightly.)
zodiac A band around the sky about 18° wide, centered on the ecliptic, in which the planets move.
zodiacal light A glow, barely visible to the eye, caused by dust particles spread along the ecliptic plane.
zone Light cloud band on a giant planet.
zone of avoidance A band around the sky, centered on the Milky Way, in which galaxies are obscured by the Milky Way's dust.