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Pronunciation Guide
ā pay

ē be

ī pie

ō so

ŭ cut

ă hat

ĕ pet

ĭ pit

ŏ pot


ä father


î pier

ô paw





ōō food


Numbers in parentheses refer to the page where the term is first discussed in the text.
absolute visual magnitude (Mv) (106) Intrinsic brightness of a star. The apparent visual magnitude the star would have if it were 10 pc away.

absolute zero (79) The theoretical lowest possible temperature at which a material contains no extractable heat energy. Zero on the Kelvin temperature scale.

absorption line (86) A dark line in a spectrum produced by the absence of photons absorbed by atoms or molecules.

absorption spectrum (dark-line spectrum) (86) A spectrum that contains absorption lines.

accretion (253) The sticking together of solid particles to produce a larger particle.

accretion disk (156) The whirling disk of gas that forms around a compact object such as a white dwarf, neutron star, or black hole as matter is drawn in.

active galactic nuclei (AGN) (210) The centers of active galaxies that are emitting large amounts of excess energy. See also active galaxy.

active galaxy (210) A galaxy whose center emits large amounts of excess energy, often in the form of radio emission. Active galaxies are suspected of having massive black holes in their centers into which matter is flowing.

active region (93) A magnetic region on the solar surface that includes sunspots, prominences, flares, and the like.

adaptive optics (63) A computer-controlled optical system in an astronomical telescope used to partially correct for seeing.

albedo (273) The ratio of the light reflected from an object divided by the light that hits the object. Albedo equals 0 for perfectly black and 1 for perfectly white.

amino acid (ŭ·mē'nō) (316) Carbon-chain molecule that is the building block of protein.

angstrom (Å) (ăng'strŭm) (58) A unit of distance commonly used to measure the wavelength of light. 1 Å = 10-10 m.

angular diameter (17) The angle formed by lines extending from the observer to opposite sides of an object.

angular distance (17) The angle formed by lines extending from the observer to two locations.

angular momentum (155) A measure of the tendency of a rotating body to continue rotating. Mathematically, the product of mass, velocity, and radius.

annular eclipse (27) A solar eclipse in which the solar photosphere appears around the edge of the moon in a bright ring, or annulus. The corona, chromosphere, and prominences cannot be seen.

anorthosite (ăn·ôr'thŭ·sīt) (274) Rock of aluminum and calcium silicates found in the lunar highlands.

antimatter (226) Matter composed of antiparticles, which upon colliding with a matching particle of normal matter annihilate and convert the mass of both particles into energy. The antiproton is the antiparticle of the proton, and the positron is the antiparticle of the electron.

aphelion (ŭ·fē'le·ŭn) (21) The orbital point of greatest distance from the sun.

apparent visual magnitude (mv)(14) The brightness of a star as seen by human eyes on Earth.

arc minute (17) 1⁄60th of a degree.

arc second (17) 1⁄60th of a minute of arc.

array detector (67) Device for collecting and recording electromagnetic radiation using multiple individual detectors arrayed on the surface of a chip, for example a CCD electronic camera.

asterism (12) A named grouping of stars that is not one of the recognized constellations. Examples are the Big Dipper and the Pleiades.

asteroid (244) Small, rocky world. Most asteroids lie between Mars and Jupiter in the asteroid belt.

Astronomical Unit (AU) (5) Average distance from Earth to the sun; 1.53108 km, or 933106 mi.

atmospheric window (60) Wavelength region in which our atmosphere is transparent—at visual, infrared, and radio wavelengths.

atom (79) The smallest unit of a chemical element, consisting of a nucleus containing protons and neutrons plus a surrounding cloud of electrons.

aurora (ô·rôr'ŭ) (97) The glowing light display that results when a planet’s magnetic field guides charged particles toward the north and south magnetic poles, where they strike the upper atmosphere and excite atoms to emit photons.

autumnal equinox (20) The point on the celestial sphere where the sun crosses the celestial equator going southward. Also, the time when the sun reaches this point and autumn begins in the Northern Hemisphere—about September 22.

Babcock model (94) A model of the sun’s magnetic cycle in which the differential rotation of the sun winds up and tangles the solar magnetic field in a 22-year cycle. This is thought to be responsible for the 11-year sunspot cycle.

Balmer series (87) A series of spectral lines produced by hydrogen in the near-ultraviolet and visible parts of the spectrum. The three longest-wavelength Balmer lines are visible to the human eye.

barred spiral galaxy (200) A spiral galaxy with an elongated nucleus resembling a bar from which the arms originate.

basalt (270) Dark igneous rock, characteristic of solidified lava.

belt–zone circulation (293) The atmospheric circulation typical of Jovian planets in which dark belts and bright zones encircle the planet parallel to its equator.

big bang (224) The high-density, high-temperature state from which the expanding universe of galaxies began.

big rip (234) The fate of the universe if dark energy increases with time and galaxies, stars, and even atoms are eventually ripped apart by the accelerating expansion of the universe.

binary stars (113) Pairs of stars that orbit around their common center of mass.

binding energy (82) The energy needed to pull an electron away from its atom.

biological evolution (315) The process of mutation, variation, and natural selection by which life adjusts itself to its changing environment.

blackbody radiation (79) Radiation emitted by a hypothetical perfect radiator. The spectrum is continuous, and the wavelength of maximum emission depends on the body’s temperature.

black dwarf (133) The end state of a white dwarf that has cooled to low temperature.

black hole (167) A mass that has collapsed to such a small volume that its gravity prevents the escape of all radiation. Also, the volume of space from which radiation may not escape.

blazars See BL Lac objects.

BL Lac objects (215) Objects that resemble quasars, thought to be highly luminous cores of distant active galaxies.

blueshift (84) A Doppler shift toward shorter wavelengths caused by a velocity of approach.

Bok globule (140) Small, dark cloud only about 1 ly in diameter that contains 10 to 1000 solar masses of gas and dust, believed to be related to star formation.

breccia (brĕch'ē·ŭ) (274) Rock composed of fragments of earlier rocks bonded together.

calibration (177) The establishment of the relationship between a parameter that is easily determined and a parameter that is more difficult to determine. For example, the periods of Cepheid variables have been calibrated to reveal absolute magnitudes, which can then be used to find distance. Thus astronomers say Cepheids have been calibrated as distance indicators.

Cambrian explosion (kăm'brē·ŭn) (321) A geological period 0.6 to 0.5 billion years ago during which life on Earth became diverse and complex. Cambrian rocks contain the oldest easily identifiable fossils.

carbonaceous chondrite (kär·bŭ·nā'·shŭs kŏn'-drīt) (248) Stony meteorite that contains both chondrules and volatiles. These chondrites may be the least-altered remains of the solar nebula still present in the solar system.

carbon–nitrogen–oxygen (CNO) cycle (131) A series of nuclear reactions that use carbon as a catalyst to combine four hydrogen atoms to make one helium atom plus energy, effective in stars more massive than the sun.

Cassegrain focus (kăs'ŭ·grān) (65) The optical design in which the secondary mirror reflects light back down the tube through a hole in the center of the objective mirror.

catastrophic theory (257) An explanation of a phenomenon involving special, sudden, perhaps violent events.

CCD See charge-coupled device.

celestial equator (16) The imaginary line around the sky directly above Earth’s equator.

celestial sphere (14) An imaginary sphere of very large radius surrounding Earth to which the planets, stars, sun, and moon seem to be attached.

center of mass (51) The balance point of a body or system of masses. The point about which a body or system of masses rotates in the absence of external forces.

Cepheid variable star (sĕ·fē'ĭd) (177) Variable star with a period of 60 days whose period of variation is related to luminosity.

Chandrasekhar limit (shăn'drä·sā'·kär) (154) The maximum mass of a white dwarf, about 1.4 solar masses. A white dwarf of greater mass cannot support itself and will collapse.

charge-coupled device (CCD) (67) An electronic device consisting of a large array of light-sensitive elements used to record very faint images.

chemical evolution (320) The chemical process that led to the growth of complex molecules on primitive Earth. This did not involve the reproduction of molecules.

chromatic aberration (krō·măt'ĭk) (61) A distortion found in refracting telescopes because lenses focus different colors at slightly different distances. Images are consequently surrounded by color fringes.

chromosome (317) A body within a living cell that contains genetic information responsible for the determination and transmission of hereditary traits.

chromosphere (krō'mŭ·sfîr) (88) Bright gases just above the photosphere of the sun.

circular velocity (50) The velocity an object needs to stay in orbit around another object.

circumpolar constellation (17) A constellation so close to one of the celestial poles that it never sets or never rises as seen from a particular latitude.

closed orbit (51) Either a circular orbit or an elliptical orbit that returns to the same starting point over and over..

closed universe (230) A model universe in which the average density is great enough to stop the expansion and make the universe contract.

CNO cycle See carbon–nitrogen–oxygen cycle.

cold dark matter (231) Mass in the universe, as yet undetected except for its gravitational influence, which is made up of slow-moving particles.

comet (244) One of the small, icy bodies that orbit the sun and produce tails of gas and dust when they approach the sun.

compact object (154) A stellar remnant that generates no nuclear energy and is much smaller and denser than a star.

comparative planetology (264) The study of planets in relation to one another.

condensation (253) The growth of a particle by addition of material from surrounding gas, atom by atom.

condensation sequence (252) The sequence in which different materials condense from the solar nebula as we move outward from the sun.

conservation of energy (126) One of the basic laws of stellar structure: The amount of energy flowing out of the top of a shell must equal the amount coming in at the bottom plus whatever energy is generated within the shell.

conservation of mass (126) One of the basic laws of stellar structure: The total mass of the star must equal the sum of the masses of the shells, and the mass must be distributed smoothly through the star.

constellation (12) One of the stellar patterns identified by name, usually of mythological gods, people, animals, or objects. Also, the region of the sky containing that star pattern.

continuous spectrum (86) A spectrum in which there are no absorption or emission lines.

convection (81) Circulation in a fluid driven by heat. Hot material rises and cool material sinks.

convective zone (94) The region inside a star where energy is carried outward as rising hot gas and sinking cool gas.

corona (88) On the sun, the faint outer atmosphere composed of low-density, high-temperature gas.

coronae (279) On Venus, large, round geological faults in the crust caused by the intrusion of magma below the crust.

coronagraph (89) A telescope designed to photograph the inner corona of the sun.

coronal hole (97) An area of the solar surface that is dark at X-ray wavelengths, thought to be associated with divergent magnetic fields and the source of the solar wind.

coronal mass ejection (CME) (97) Matter ejected from the sun’s corona in powerful surges guided by magnetic fields.

cosmic microwave background radiation (225) Radiation from the hot clouds of the big bang explosion. The large red shift makes it appear to come from a body whose temperature is only 2.7 K.

cosmological constant (233) A constant in Einstein’s equations of space and time that represents a force of repulsion.

cosmological principle (229) The assumption that any observer in any galaxy sees the same general features of the universe.

cosmology (220) The study of the nature, origin, and evolution of the universe.

Coulomb barrier (kōō·lôm) (130) The electrostatic force of repulsion between bodies of like charge, commonly applied to atomic nuclei.

Coulomb force (82) The electrostatic force of repulsion or attraction between charged bodies.

critical density (230) The average density of the universe needed to make its curvature flat.

dark age (228) The period of time after the glow of the big bang faded into the infrared and before the birth of the first stars, during which the universe expanded in darkness.

dark energy (234) The energy believed to fill empty spaces and drive the acceleration of the expanding universe.

dark halo (182) The low-density extension of the halo of our galaxy, believed to be composed of dark matter.

dark-line spectrum See absorption spectrum.

dark matter (182) Nonluminous matter that is detected only by its gravitational influence.

dark nebula (137) A cloud of gas and dust seen silhouetted against a brighter nebula.

debris disk (259) A disk of dust found by infrared observations around some stars. The dust is debris from collisions among asteroids, comets, and Kuiper belt objects.

degenerate matter (151) Extremely high-density matter in which pressure no longer depends on temperature due to quantum mechanical effects.

density (78) Mass per volume.

density wave theory (184) Theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of the galaxy.

deoxyribonucleic acid (316) The long carbon-chain molecule that records information to govern the biological activity of the organism. DNA carries the genetic data passed to offspring.

deuterium (130) An isotope of hydrogen in which the nucleus contains a proton and a neutron.

differential rotation (94) The rotation of a body in which different parts of the body have different periods of rotation. This is true of the sun, the Jovian planets, and the disk of the galaxy.

differentiation (254) The separation of planetary material according to density.

diffraction fringe (62) Blurred fringe surrounding any image, caused by the wave properties of light. Because of this, no image detail smaller than the fringe can be seen.

digitized (67) Information that is converted to numerical data that can be read directly into a computer memory for later analysis.

disk component (179) All material confined to the plane of the galaxy.

distance ladder (202) The calibration used to build a distance scale reaching from the size of Earth to the most distant visible galaxies.

DNA See deoxyribonucleic acid.

Doppler effect (84) The change in the wavelength of radiation due to relative radial motion of source and observer.

double-lobed radio source (212) A galaxy that emits radio energy from two regions (lobes) located on opposite sides of the galaxy.

Drake equation (328) The equation that estimates the total number of communicative civilizations in our galaxy.

dwarf planet (309) A body that orbits the sun, is not a satellite of a planet, is massive enough to pull itself into a spherical shape but not massive enough to clear out other bodies in and near its orbit. For example, Pluto, Eris, and Ceres.

dynamo effect (94) The process by which a rotating, convecting body of conducting matter, such as Earth’s core, can generate a magnetic field.

east point (16) One of the four cardinal directions, the point on the horizon directly east.

eccentricity (e)(42) A number between 1 and 0 that describes the shape of an ellipse, the distance from one focus to the center of the ellipse divided by the semimajor axis.

eclipsing binary system (115) A binary star system in which the stars eclipse each other.

ecliptic (19) The apparent path of the sun around the sky.

ejecta (274) Pulverized rock scattered by meteorite impacts on a planetary surface.

electromagnetic radiation (58) Changing electric and magnetic fields that travel through space and transfer energy from one place to another; examples are light or radio waves.

electron (79) Low-mass atomic particle carrying a negative charge.

ellipse (41) A closed curve around two points, called the foci, such that the total distance from one focus to the curve and back to the other focus remains constant.

elliptical galaxy (200) A galaxy that is round or elliptical in outline and contains little gas and dust, no disk or spiral arms, and few hot, bright stars.

emission line (85) A bright line in a spectrum caused by the emission of photons from atoms.

emission nebula (136) A cloud of glowing gas excited by ultraviolet radiation from hot stars.

empirical (43) Description of a phenomenon without explaining why it occurs.

energy level (82) One of a number of states an electron may occupy in an atom, depending on its binding energy.

energy transport (126) Flow of energy from hot regions to cooler regions by one of three methods: conduction, convection, or radiation.

enzyme (316) Special protein that controls processes in an organism.

epicycle (ĕp´ŭ·s ī ·kŭl) (37) The small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (the deferent) around Earth.

escape velocity (51) The initial velocity an object needs to escape from the surface of a celestial body.

evening star (19) Any planet visible in the sky just after sunset.

event horizon (167) The boundary of the region of a black hole from which no radiation may escape. No event that occurs within the event horizon is visible to a distant observer.

evolutionary theory (257) An explanation of a phenomenon involving slow, steady processes of the sort seen happening in the present day.

excited atom (82) An atom in which an electron has moved from a lower to a higher energy level.

extrasolar planet (259) A planet orbiting a star other than the sun.

eyepiece (60) A short-focal-length lens used to enlarge the image in a telescope. The lens nearest the eye.

false-color image (67) A representation of graphical data with added or enhanced color to reveal detail.

field of view (4) The area visible in an image. Usually given as the diameter of the region.

filament (89) A solar prominence, seen from above, silhouetted against the bright photosphere.

filtergram (89) A photograph (usually of the sun) taken in the light of a specific region of the spectrum—for example, an Hα filtergram.

first principle (34) A principle that seems obviously true to everyone and supposedly needs no further examination.

flare (97) A violent eruption on the sun’s surface.

flatness problem (232) In cosmology, the peculiar circumstance that the early universe must have contained almost exactly the right amount of matter to make space-time flat.

flat universe (230) A model of the universe in which space-time is not curved.

flux (14, 105) A measure of the flow of energy out of a surface. Usually applied to light.

focal length (60) The focal length of a lens is the distance from the lens to the point where it focuses parallel rays of light.

folded mountain range (270) A long range of mountains formed by the compression of a planet’s crust.

forward scattering (294) The optical property of finely divided particles to preferentially direct light in the original direction of the light’s travel.

galactic cannibalism (208) The theory that large galaxies absorb smaller galaxies.

galaxy (7) A large system of stars, star clusters, gas, dust, and nebulae orbiting a common center of mass.

Galilean satellites (găl·ŭ·lē'ŭn) (247) The four largest satellites of Jupiter, named after their discoverer, Galileo.

gamma ray (59) The shortest-wavelength electromagnetic waves.

gamma-ray burst (171) A sudden, powerful burst of gamma rays.

gene (317) A unit of DNA—or sometimes RNA—information responsible for controlling an inherited physiological trait.

general theory of relativity (164) A theory that describes gravity as due to curvature of space-time.

geocentric universe (34) A model universe with Earth at the center, such as the Ptolemaic universe.

giant (110, 148) Large, cool, highly luminous star in the upper right of the H–R diagram, typically 10 to 100 times the diameter of the sun.

global warming (272) The gradual increase in the surface temperature of Earth caused by human modifications to Earth’s atmosphere.

globular cluster (152) A star cluster containing 100,000 to 1 million stars in a sphere about 75 ly in diameter, generally old, metal-poor, and found in the spherical component of the galaxy.

granulation (81) The fine structure of bright grains covering the sun’s surface.

grating (68) A piece of material in which numerous microscopic parallel lines are scribed. Light encountering a grating is dispersed to form a spectrum.

gravitational collapse (254) The process by which a forming body such as a planet gravitationally captures gas from its surroundings.

gravitational redshift (168) The lengthening of the wavelength of a photon due to its escape from a gravitational field.

greenhouse effect (268) The process by which a carbon dioxide atmosphere traps heat and raises the temperature of a planetary surface.

ground state (83) The lowest permitted electron energy level in an atom.

habitable zone (325) A region around a star within which planets have temperatures that permit the existence of liquid water.

half-life (250) The time required for half of the atoms in a radioactive sample to decay.

halo (179) The spherical region of a spiral galaxy, containing a thin scattering of stars, star clusters, and small amounts of gas.

heat (79) Energy stored in a material as agitation among its particles.

heat of formation (258) In planetology, the heat released by infalling matter during the formation of a planetary body.

heavy bombardment (255) The intense cratering during the first 0.5 billion years in the history of the solar system.

heliocentric universe (35) A model of the universe with the sun at the center, such as the Copernican universe.

helioseismology (91) The study of the interior of the sun by the analysis of its modes of vibration.

Herbig–Haro object (140) A small nebula that varies irregularly in brightness, believed to be associated with star formation.

Hertzsprung–Russell (H–R) diagram (hĕrt' sprŭng·rŭs·ŭl) (110) A plot of the intrinsic brightness versus the surface temperature of stars. It separates the effects of temperature and surface area on stellar luminosity and is commonly plotted as absolute magnitude versus spectral type but also as luminosity versus surface temperature or color.

homogeneity (229) The assumption that, on the large scale, matter is uniformly spread through the universe.

horizon (16) The circular boundary between the sky and Earth.

horizon problem (232) In cosmology, the circumstance that the primordial background radiation seems much more isotropic than can be explained by the standard big bang theory.

horizontal branch (149) In the H–R diagram, stars fusing helium in a shell and evolving back toward the red giant region.

H–R diagram See Hertzsprung–Russell diagram.

HII region (136) A region of ionized hydrogen around a hot star.

Hubble constant (H) (203) A measure of the rate of expansion of the universe, the average value of velocity of recession divided by distance, presently believed to be about 70 km/s/Mpc.

Hubble law (203) The linear relation between the distances to galaxies and their velocity of recession.

Hubble time (224) The age of the universe, equivalent to 1 divided by the Hubble constant. The Hubble time is the age of the universe if it has expanded since the big bang at a constant rate.

hydrostatic equilibrium (126) The balance between the weight of the material pressing downward on a layer in a star and the pressure in that layer.

hypernova (171) Produced when a very massive star collapses into a black hole. Thought to be a possible source of gamma-ray bursts.

hypothesis (42) A conjecture, subject to further tests, that accounts for a set of facts.

ice line (252) Boundary beyond which water vapor could freeze to form ice.

inflationary big bang (232) A version of the big bang theory, derived from grand unified theories, that includes a rapid expansion when the universe was very young.

infrared (IR) (59) The portion of the electromagnetic spectrum with wavelengths longer than red light, ranging from 700 nm to about 1 mm, between visible light and radio waves.

instability strip (177) The region of the H–R diagram in which stars are unstable to pulsation. A star passing through this strip becomes a variable star.

interferometer (62) Separated telescopes combined to produce a virtual telescope with the resolution of a much larger-diameter telescope.

interstellar dust (134) Microscopic solid grains in the interstellar medium.

interstellar medium (134) The gas and dust distributed between the stars.

interstellar reddening (135) The process in which dust scatters blue light out of starlight and makes the stars look redder.

intrinsic brightness (105) A measure of the amount of energy a star produces.

inverse square relation (48) A rule that the strength of an effect (such as gravity) decreases in proportion as the distance squared increases.

ion (82) An atom that has lost or gained one or more electrons.

ionization (82) The process in which atoms lose or gain electrons.

irregular galaxy (201) A galaxy with a chaotic appearance, large clouds of gas and dust, and both population I and II stars, but without spiral arms.

isotopes (82) Atoms that have the same number of protons but a different number of neutrons.

isotropy (ī·sŏt'rŭ·pē) (229) The assumption that, in its general properties, the universe looks the same in every direction.

Jovian planet (246) Jupiter-like planet with a large diameter and low density.

Kelvin temperature scale (79) A temperature scale using Celsius degrees and based on zero at absolute zero.

kiloparsec (kpc) (179) A unit of distance equal to 1000 pc or 3260 ly.

Kirchhoff’s laws (86) A set of laws that describe the absorption and emission of light by matter.

Kuiper belt (248) The collection of icy planetesimals believed to orbit in a region from just beyond Neptune out to 100 AU or more.

large-impact hypothesis (274) The theory that the moon formed from debris ejected during a collision between Earth and a large planetesimal.

Large Magellanic Cloud (201) An irregular galaxy, visible in the southern sky, that is a satellite of our Milky Way Galaxy.

large-scale structure (234) The distribution of clusters and superclusters of galaxies in filaments and walls enclosing voids.

late heavy bombardment (275) The sudden temporary increase in the cratering rate in our solar system that occurred about 4 billion years ago.

light curve (115) A graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries.

light-gathering power (62) The ability of a telescope to collect light, proportional to the area of the telescope’s objective lens or mirror.

lighthouse model (163) The explanation of a pulsar as a spinning neutron star sweeping beams of electromagnetic radiation around the sky.

light pollution (64) The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the observation of faint objects.

light-year (ly) (6) The unit of distance light travels in 1 year.

liquid metallic hydrogen (293) A form of liquid hydrogen that is a good electrical conductor, found in the interiors of Jupiter and Saturn.

Local Group (205) The small cluster of a few dozen galaxies that contains our Milky Way Galaxy.

look-back time (202) The amount by which you look into the past when you look at a distant galaxy, a time equal to the distance to the galaxy in light-years.

luminosity (L)(106) The total amount of energy a star radiates in 1 second.

luminosity class (112) A category of stars of similar luminosity, determined by the widths of lines in their spectra.

lunar eclipse (27) The darkening of the moon when it moves through Earth’s shadow.

Lyman series (lī'mĕn) (87) Spectral lines in the ultraviolet spectrum of hydrogen produced by transitions whose lowest energy level is the ground state.

magma ocean (275) The exterior of the newborn moon, a shell of molten rock hundreds of kilometers deep.

magnetic carpet (89) The network of small magnetic loops that covers the solar surface.

magnetosphere (măg·nē'tō·sfîr) (293) The volume of space around a planet within which the motion of charged particles is dominated by the planetary magnetic field rather than the solar wind.

magnifying power (63) The ability of a telescope to make an image larger.

magnitude scale (13) The astronomical brightness scale. The larger the number, the fainter the star.

main sequence (110) The region of the H–R diagram running from upper left to lower right, which includes roughly 90 percent of all stars.

mantle (266) The layer of dense rock and metal oxides that lies between the molten core and Earth’s surface or a similar layer in another planet.

mare (mä'rā) (plural: maria) (273) One of the lunar lowlands filled by successive flows of dark lava, from the Latin for “sea.”

mass (47) A measure of the amount of matter making up an object.

mass–luminosity relation (117) The more massive a star is, the more luminous it is.

Maunder butterfly diagram (môn'dŭr) (92) A graph showing the latitude of sunspots versus time, first plotted by W. W. Maunder in 1904.

Maunder minimum (môn'dŭr) (93) A period between 1645 and 1715 of less numerous sunspots and other solar activity.

megaparsec (Mpc) (202) A unit of distance equal to 1,000,000 pc.

metals (187) In astronomical usage, all atoms heavier than helium.

meteor (248) A small bit of matter heated by friction to incandescent vapor as it falls into Earth’s atmosphere.

meteorite (248) A meteor that survives its passage through the atmosphere and strikes the ground.

meteoroid (248) A meteor in space before it enters Earth’s atmosphere.

meteor shower (249) A multitude of meteors that appear to come from the same region of the sky, believed to be caused by comet debris.

micrometeorite (276) Meteorite of microscopic size.

midocean rise (270) One of the undersea mountain ranges that push up from the seafloor in the center of the oceans.

Milky Way (7) The hazy band of light that circles our sky, produced by the glow of our galaxy.

Milky Way Galaxy (7) The spiral galaxy containing our sun, visible in the night sky as the Milky Way.

Miller experiment (318) An experiment that reproduced the conditions under which life began on Earth and manufactured amino acids and other organic compounds.

millisecond pulsar (164) A pulsar with a pulse period of only a few milliseconds.

model See scientific model.

molecular cloud (138) A dense interstellar gas cloud in which atoms are able to link together to form molecules such as H2 and CO.

molecule (79) Two or more atoms bonded together.

monolithic collapse model (188) An early hypothesis that says that the galaxy formed from a single large cloud of turbulent gas over 13 billion years ago.

morning star (19) Any planet visible in the sky just before sunrise.

multicellular (321) An organism composed of more than one cell.

multiringed basin (275) Large impact feature (crater) containing two or more concentric rims formed by fracturing of the planetary crust.

mutant (315) Offspring born with altered DNA.

nadir (16) The point on the celestial sphere directly below the observer; the opposite of the zenith.

nanometer (nm) (58) A unit of distance equaling one-billionth of a meter (10–9 m).

natural law (42) A theory that is almost universally accepted as true.

natural selection (315) The process by which the best traits are passed on, allowing the most able to survive.

neap tide (49) Ocean tide of low amplitude occurring at first- and third-quarter moon.

nebula (134) A glowing cloud of gas or a cloud of dust reflecting the light of nearby stars.

NEO (Near Earth Object) (256) A small solar system body (asteroid or comet) with an orbit near enough to Earth that it poses some threat of eventual collision.

neutrino (131) A neutral, massless atomic particle that travels at or nearly at the speed of light.

neutron (79) An atomic particle with no charge and about the same mass as a proton.

neutron star (161) A small, highly dense star, with radius about 10 km, composed almost entirely of tightly packed neutrons.

Newtonian focus (65) The optical design in which a diagonal mirror reflects light out the side of the telescope tube for easier access.

nonbaryonic matter (231) Proposed dark matter made up of particles other than protons and neutrons (baryons).

north celestial pole (16) The point on the celestial sphere directly above Earth’s North Pole.

north point (16) One of the four cardinal directions, the point on the horizon directly north.

nova (146) From the Latin, meaning “new,” a sudden brightening of a star making it appear as a new star in the sky, believed to be associated with eruptions on white dwarfs in binary systems.

nuclear bulge (179) The spherical cloud of stars that lies at the center of spiral galaxies.

nuclear fission (129) Reactions that break the nuclei of atoms into fragments.

nuclear fusion (130) Reactions that join the nuclei of atoms to form more massive nuclei.

nucleus (of an atom) (79) The central core of an atom containing protons and neutrons that carries a net positive charge.

oblateness (293) The flattening of a spherical body, usually caused by rotation.

observable universe (222) The part of the universe that you can see from your location in space and in time.

occultation (306) The passage of a larger body in front of a smaller body.

Olbers’s paradox (ôl'bŭrs) (222) The conflict between observation and theory about why the night sky should or should not be dark.

Oort cloud (ōrt) (258) The hypothetical source of comets, a swarm of icy bodies believed to lie in a spherical shell extending to 100,000 AU from the sun.

opacity (127) The resistance of a gas to the passage of radiation.

open cluster (152) A cluster of 100 to 1000 stars with an open, transparent appearance, usually relatively young and located in the disk of the galaxy. The stars are not tightly grouped.

open orbit (51) An orbit that carries an object away, never to return to its starting point.

open universe (230) A model of the universe in which the average density is less than the critical density needed to halt the expansion.

optical telescope (61) Telescopes that gather visible light.

outflow channel (284) Geological feature on Mars produced by the rapid motion of floodwaters.

outgassing (254) The release of gases from a planet’s interior.

ovoid (306) The oval features found on Miranda, a satellite of Uranus.

paradigm (39) A commonly accepted set of scientific ideas and assumptions.

parallax (36) The apparent change in position of an object due to a change in the location of the observer. Astronomical parallax is measured in seconds of arc.

parsec (pc) (pär'sĕk) (105) The distance to a hypothetical star whose parallax is 1 second of arc.1 pc = 206,265 AU = 3.26 ly.

Paschen series (pä'shŭn) (87) Spectral lines in the infrared spectrum of hydrogen produced by transitions whose lowest energy level is the third.

penumbra (pĭ·nŭm'brŭ) (26) The portion of a shadow that is only partially shaded.

perihelion (pĕr·ŭ·hē'lē·ŭn) (21) The orbital point of closest approach to the sun.

period–luminosity relation (177) The relation between period of pulsation and intrinsic brightness among Cepheid variable stars.

permafrost (282) Permanently frozen soil.

permitted orbit (82) One of the energy levels that an electron may occupy in an atom.

photographic plate (67) The first image-recording device used with telescopes; it measures the brightness of objects, but with only moderate precision.

photometer (67) Sensitive astronomical instrument that measures the brightness of individual objects very precisely.

photon (59) A quantum of electromagnetic energy that carries an amount of energy that depends inversely on its wavelength.

photosphere (81) The bright visible surface of the sun.

planet (5) A small, nonluminous body formed by accretion in a disk around a protostar.

planetary nebula (151) An expanding shell of gas ejected from a star during the latter stages of its evolution.

planetesimal (plăn·ŭ·tĕs'ŭ·mŭl) (253) One of the small bodies that formed from the solar nebula and eventually grew `into protoplanets.

plate tectonics (270) The constant destruction and renewal of Earth’s surface by the motion of sections of crust.

plutino (309) One of the icy Kuiper belt objects that, like Pluto, is caught in a 3:2 orbital resonance with Neptune.

poor galaxy cluster (205) An irregularly shaped cluster that contains fewer than 1000 galaxies, many of which are spiral, and no giant ellipticals.

population I (187) Stars rich in atoms heavier than helium, nearly always relatively young stars found in the disk of the galaxy.

population II (187) Stars poor in atoms heavier than helium, nearly always relatively old stars found in the halo, globular clusters, or the nuclear bulge.

precession (14) The slow change in the direction of Earth’s axis of rotation. One cycle takes nearly 26,000 years.

primary atmosphere (268) A planet's first atmosphere.

primary lens (60) In a refracting telescope, the largest lens.

primary mirror (60) In a reflecting telescope, the largest mirror.

prime focus (65) The point at which the objective mirror forms an image in a reflecting telescope.

primordial soup (319) The rich solution of organic molecules in Earth’s first oceans.

prominence (96) Eruption on the solar surface. Visible during total solar eclipses.

proper motion (177) The rate at which a star moves across the sky, measured in seconds of arc per year.

protein (316) Complex molecule composed of amino acid units.

proton (79) A positively charged atomic particle contained in the nucleus of an atom. The nucleus of a hydrogen atom.

proton–proton chain (130) A series of three nuclear reactions that builds a helium atom by adding together protons. The main energy source in the sun.

protoplanet (253) Massive object, destined to become a planet, resulting from the coalescence of planetesimals in the solar nebula.

protostar (140) A collapsing cloud of gas and dust destined to become a star.

pulsar (162) A source of short, precisely timed radio bursts, believed to be spinning neutron stars.

P wave (267) A pressure wave. a type of seismic wave produced in Earth by the compression of the material.

quantum leap (83) Jumps of electrons from one orbit or energy state to another.

quantum mechanics (82) The study of the behavior of atoms and atomic particles.

quasar (quasi-stellar object, or QSO) (kwā'zär) (212) Small, powerful source of energy believed to be the active core of a very distant galaxy.

quintessence (234) The postulated energy that fills empty space and drives the acceleration of the universe.

radial velocity (Vr) (85) That component of an object’s velocity directed away from or toward Earth.

radio galaxy (210) A galaxy that is a strong source of radio signals.

radio telescope (61) Telescopes that gather radio radiation.

rays (380) Ejecta from meteorite impacts forming white streamers radiating from some lunar craters.

recombination (228) The stage, within 300,000 years of the big bang, when the gas became transparent to radiation.

reconnection (97) On the sun, the merging of magnetic fields to release energy in the form of flares.

red dwarf (112) A faint, cool, low-mass, main-sequence star.

redshift (84) A Doppler shift toward longer wavelengths caused by a velocity of recession.

reflection nebula (136) A nebula produced by starlight reflecting off dust particles in the interstellar medium.

refracting telescope (60) A telescope that forms images by bending (refracting) light with a lens.

reionization (228) The stage in the early history of the universe when ultraviolet photons from the first stars ionized the gas filling space.

resolving power (62) The ability of a telescope to reveal fine detail. Depends on the diameter of the telescope objective.

retrograde motion (36) The apparent backward (westward) motion of planets as seen against the background of stars.

revolution (18) Orbital motion about a point located outside the orbiting body. See also rotation.

ribonucleic acid (RNA) (317) Long carbon-chain molecules that use the information stored in DNA to manufacture complex molecules necessary to the organism.

rich galaxy cluster (205) A cluster containing over 1000 galaxies, mostly elliptical, scattered over a volume about 3 Mpc in diameter.

rift valley (270) A long, straight, deep valley produced by the separation of crustal plates.

ring galaxy (209) A galaxy that resembles a ring around a bright nucleus, believed to be the result of a head-on collision of two galaxies.

RNA See ribonucleic acid.

Roche limit (rōsh) (294) The minimum distance between a planet and a satellite that holds itself together by its own gravity. If a satellite’s orbit brings it within its planet’s Roche limit, tidal forces will pull the satellite apart.

Roche lobe (155) The volume of space a star controls gravitationally within a binary system.

rotation (18) Motion around an axis passing through the rotating body. See also revolution.

rotation curve (182) A graph of orbital velocity versus radius in the disk of a galaxy.

rotation curve method (203) A method of determining a galaxy’s mass by observing the orbital velocity and radius of stars in the galaxy.

runaway greenhouse (281) A situation in which the greenhouse effect grows more severe.

Sagittarius A* (190) The powerful radio source located at the core of the Milky Way Galaxy.

Saros cycle (sĕ'rōs) (29) An 18-year, 111⁄3-day period after which the pattern of lunar and solar eclipses repeats.

Schmidt-Cassegrain focus (65) The optical design that uses a thin corrector plate at the entrance to the telescope tube. A popular design for small telescopes.

Schwarzschild radius (RS)(schwôrts' shēld) (167) The radius of the event horizon around a black hole.

scientific model (14) A tentative description of a phenomenon for use as an aid to understanding.

scientific notation (5) The system of recording very large or very small numbers by using powers of 10.

secondary atmosphere (268) The gases outgassed from a planet’s interior; rich in carbon dioxide.

secondary mirror (65) In a reflecting telescope, a mirror that directs the light from the primary mirror to a focal position.

seeing (62) Atmospheric conditions on a given night. When the atmosphere is unsteady, producing blurred images, the seeing is said to be poor.

self-sustaining star formation (186) The process by which the birth of stars compresses the surrounding gas clouds and triggers the formation of more stars, proposed to explain spiral arms.

semi-major axis (a)(42) Half of the longest diameter of an ellipse.

SETI (326) The Search for Extra-Terrestrial Intelligence.

Seyfert galaxy (sē'fûrt) (210) An otherwise normal spiral galaxy with an unusually bright, small core that fluctuates in brightness, believed to indicate that the core is erupting.

shepherd satellite (299) A satellite that, by its gravitational field, confines particles to a planetary ring.

shield volcano (283) Wide, low-profile volcanic cone produced by highly liquid lava.

shock wave (139) A sudden change in pressure that travels as an intense sound wave.

sidereal tracking (sī·dîr'ē·ŭl) (64) The continuous movement of a telescope to keep it pointed at a star.

singularity (167) The object of zero radius into which the matter in a black hole is believed to fall.

Small Magellanic Cloud (201) An irregular galaxy, visible in the southern sky, that is a satellite of our Milky Way Galaxy.

solar eclipse (23) The event that occurs when the moon passes directly between Earth and the sun, blocking your view of the sun.

solar nebula hypothesis (243) The hypothesis that the planets formed from the same cloud of gas and dust that formed the sun.

solar system (5) The sun and its planets, asteroids, comets, and so on.

solar wind (89) Rapidly moving atoms and ions that escape from the solar corona and blow outward through the solar system.

south celestial pole (16) The point on the celestial sphere directly above Earth’s South Pole.

south point (16) One of the four cardinal directions, the point on the horizon directly south.

spectral class (108) A star’s position in the temperature classification system O, B, A, F, G, K, M, based on the appearance of the star’s spectrum.

spectral line (76) A line in a spectrum at a specific wavelength produced by the absorption or emission of light by certain atoms.

spectral sequence (108) The arrangement of spectral classes (O, B, A, F, G, K, M) ranging from hot to cool.

spectrograph (68) A device that separates light by wavelengths to produce a spectrum.

spectroscopic binary system (115) A star system in which the stars are too close together to be visible separately. We see a single point of light, and only by taking a spectrum can we determine that there are two stars.

spectroscopic parallax (112) The method of determining a star’s distance by comparing its apparent magnitude with its absolute magnitude as estimated from its spectrum.

spectrum (68) A range of electromagnetic radiation spread into its component wavelengths (colors), for example a rainbow; also, representation of a spectrum as a graph showing intensity of radiation as a function of wavelength or frequency.

spherical component (179) The part of the galaxy including all matter in a spherical distribution around the center (the halo and nuclear bulge).

spicule (spĭk'yōōl) (89) A small, flamelike projection in the chromosphere of the sun.

spiral arm (7, 179) Long spiral pattern of bright stars, star clusters, gas, and dust. Spiral arms extend from the center to the edge of the disk of spiral galaxies.

spiral galaxy (200) A galaxy with an obvious disk component containing gas; dust; hot, bright stars; and spiral arms.

spiral tracer (183) Object used to map the spiral arms—for example, O and B associations, open clusters, clouds of ionized hydrogen, and some types of variable stars.

spring tide (49) Ocean tide of high amplitude that occurs at full and new moon.

standard candle (202) Object of known brightness that astronomers use to find distance—for example, Cepheid variable stars and supernovae.

star (5) A globe of gas held together by its own gravity and supported by the internal pressure of its hot gases, which generate energy by nuclear fusion.

star cluster (139) A group of stars that formed together and orbit a common center of mass.

starburst galaxy (207) A galaxy undergoing a rapid burst of star formation.

Stefan-Boltzmann law (80) A law stating that hotter objects emit more energy than cooler objects of the same size, in proportion to the fourth power of temperature.

stellar association (139) A group of stars that are not gravitationally bound to one another.

stellar model (128) A table of numbers representing the conditions in various layers within a star.

stellar parallax (p) (104) A measure of stellar distance. See also parallax.

stromatolite (318) A layered fossil formation caused by ancient mats of algae or bacteria, which built up mineral deposits season after season.

subduction zone (270) A region of a planetary crust where a tectonic plate slides downward.

summer solstice (20) The point on the celestial sphere where the sun is at its most northerly point. Also, the time when the sun passes this point, about June 22, and summer begins in the Northern Hemisphere.

sunspot (81) Relatively dark spot on the sun that contains intense magnetic fields.

supercluster (8, 234) A cluster of galaxy clusters.

supergiant (112, 148) Exceptionally luminous star whose diameter is 10 to 1000 times that of the sun.

supernova (146) The explosion of a star in which it increases its brightness by a factor of about a million.

supernova remnant (160) The expanding shell of gas marking the site of a supernova explosion.

S wave (267) A shear wave. a type of seismic wave produced in Earth by the lateral motion of the material.

synchrotron radiation (sĭn'krŭ·trŏn) (159) Radiation emitted when high-speed electrons move through a magnetic field.

temperature (79) A measure of the agitation among the atoms and molecules of a material, the intensity of heat.

terrestrial planet (246) An Earthlike planet—small, dense, rocky.

theory (42) A system of assumptions and principles applicable to a wide range of phenomena that has been repeatedly verified.

tidal heating (296) The heating of a planet or satellite because of friction caused by tides.

tidal tail (208) A long streamer of stars, gas, and dust torn from a galaxy during its close interaction with another passing galaxy.

time dilation (168) The slowing of moving clocks or clocks in strong gravitational fields.

transition (87) The movement of an electron from one atomic energy level to another.

transit of Venus (78) Rare occasions when Venus can be seen as a tiny dot directly between Earth and the sun.

T Tauri star (tôrē) (140) A young star surrounded by gas and dust, believed to be contracting toward the main sequence.

turnoff point (152) The point in an H–R diagram at which a cluster’s stars turn off of the main sequence and move toward the red-giant region, revealing the approximate age of the cluster.

type I supernova (158) A supernova explosion caused by the collapse of a white dwarf.

type II supernova (158) A supernova explosion caused by the collapse of a massive star.

ultraviolet (UV) (59) The portion of the electromagnetic spectrum with wavelengths shorter than violet light, between visible light and X rays.

umbra (ŭm'brŭ) (23) The region of a shadow that is totally shaded.

uncompressed density (252) The density a planet would have if its gravity did not compress it.

unified model (214) An attempt to explain the different types of active galactic nuclei using a single model viewed from different directions.

uniform circular motion (36) The classical belief that the perfect heavens could move only by the combination of uniform motion along circular orbits.

valley network (284) A system of dry drainage channels on Mars that resembles the beds of rivers and tributary streams on Earth.

vernal equinox (20) The place on the celestial sphere where the sun crosses the celestial equator moving northward. Also, the time of year when the sun crosses this point, about March 21, and spring begins in the Northern Hemisphere.

visual binary system (114) A binary star system in which the two stars are separately visible in the telescope.

volatile (244) Easily vaporized.

water hole (326) The interval of the radio spectrum between the 21-cm hydrogen radiation and the 18-cm OH radiation; these are likely wavelengths to use in the search for extraterrestrial life.

wavelength (58) The distance between successive peaks or troughs of a wave, usually represented by l.

wavelength of maximum intensity (λmax) (80) The wavelength at which a perfect radiator emits the maximum amount of energy. Depends only on the object’s temperature.

weight (47) The force that gravity exerts on an object.

west point (16) One of the four cardinal directions, the point on the horizon directly west.

white dwarf (112) Dying star at the lower left of the H–R diagram that has collapsed to the size of Earth and is slowly cooling off.

Wien's law (80) A law stating that the hotter an object is, the shorter will be the wavelength of its most intense output.

winter solstice (20) The point on the celestial sphere where the sun is farthest south. Also, the time of year when the sun passes this point, about December 22, and winter begins in the Northern Hemisphere.

X ray (59) Electromagnetic waves with wavelengths shorter than ultraviolet light.

Zeeman effect (93) The splitting of spectral lines into multiple components when the atoms are in a magnetic field.

zenith (16) The point on the sky directly above the observer.

zero-age main sequence (ZAMS) (133) The location in the H–R diagram where stars first reach stability as hydrogen-burning stars.

zodiac (19) A band centered on the ecliptic and encircling the sky.