Coming of Age in the Milky Way - Timothy Ferris (2003)

GLOSSARY

The breaking of a wave cannot explain the whole sea.

—Vladimir Nabokov

ABT. Abbreviation employed in this book to mean “after the beginning of time,” which is here defined as the beginning of the expansion of the universe.

Absolute luminosity. See luminosity.

Absolute magnitude. See magnitude.

Absolute space. Newtonian space, hypothesized to define a cosmic reference frame independent of its content of matter or energy. The existence of absolute space, enshrined in aether theory, was denied in relativity.

Aberration of starlight. Displacement in the apparent location of stars in the sky, introduced by the motion of the earth.

Absorption lines. Dark lines in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object closer to the observer. Like emission lines, absorption lines betray the chemical composition and velocity of the material that produces them.

Acceleration. An increase in velocity over time.

Accelerator. A machine for speeding subatomic particles to high velocity, then colliding them with a stationary target or with another beam of particles moving in the opposite direction. (In the latter instance, the machine may be called a collider.) At velocities approaching that of light, the mass of the particles increases dramatically, adding greatly to the energy released on impact. The resulting explosion promotes the production of exotic particles, which are analyzed according to their behavior as they fly away through a particle detector.

Aether. (1) In Aristotelian physics, the fifth element, of which the stars and planets are made. (2) In classical physics, an invisible medium that was thought to suffuse all space.

Alchemy. Art of bringing parts of the universe to the perfect state toward which they were thought to aspire—e.g., gold for metals, immortality for human beings.

Andromeda galaxy. Major spiral galaxy, 2.2 million light-years from Earth, Gravitationally bound to the Milky Way galaxy, with which it shares membership in the Local Group, it is currently approaching us, rather than receding as is the case for most galaxies.

Angular momentum. The product of mass and angular velocity for an object in rotation; similar to linear momentum. In quantum mechanics, angular momentum is quantized, i.e., is measured in indivisible units equivalent to Planck’s constant divided by 2π.

Anisotropy. The characteristic of being dependent upon direction. (Light coming with equal intensity from all directions is isotropic; a spotlight’s beam is anisotropic.) The cosmic background radiation is generally isotropic—i.e., its intensity is the same in all parts of the sky—but small anisotropics have been detected which are thought to reflect the earth’s proper motion relative to the framework of the universe as a whole.

Anthropic principle. The doctrine that the value of certain fundamental constants of nature can be explained by demonstrating that, were they otherwise, the universe could not support life and therefore would contain nobody capable of worrying about why they are as they are. Were the strong nuclear force slightly different in strength, for instance, the stars could not shine and life as we know it would be impossible.

Antimatter. Matter made of particles with identical mass and spin as those of ordinary matter, but with opposite charge. Antimatter has been produced experimentally, but little of it is found in nature. Why this should be so is one of the questions that must be answered by any adequate theory of the early universe.

Apparent magnitude. See magnitude.

Aristotelian physics. Physics as promulgated by Aristotle; includes the hypothesis that our world is comprised of four elements, and that the universe beyond the moon is made of a fifth element and so is fundamentally different from the mundane realm.

Asteroids. Low-mass, solid objects that orbit the sun and shine by reflected light. Most belong to the “asteroid belt,” a zone located between the orbits of Mars and Jupiter. Though they number in the millions, their total mass is but a tiny fraction of the earth’s. Also called minor planets.

Astrolabe. Sighting instrument employed since antiquity to determine the elevation above the horizon of celestial objects. Eventually replaced by the sextant.

Astrology. The belief that human affairs and people’s personalities and characters are influenced by (or encoded in) the positions of the planets.

Astronomical unit. The mean distance from the earth to the sun, equal to 92.81 million miles or 499.012 light-seconds.

Astronomy. The science that studies the natural world beyond the earth.

Astrophysics. The science that studies the physics and chemistry of extraterrestrial objects. The alliance of physics and astronomy, which began with the advent of spectroscopy, made it possible to investigate what celestial objects are and not just where they are.

Asymmetry. A violation of symmetry.

Asymptotic freedom. Orwellian liberty enjoyed by quarks, which move freely when close together but are reined in by an increasingly powerful strong nuclear force whenever they begin to drift apart.

Atoms. The fundamental units of a chemical element. An atom consists of a nucleus, which may contain protons and neutrons, and electrons, which occupy shells that surround the nucleus and are centered on it.

Avoidance. The fact that galaxies appear to “avoid” the Milky Way, and are most numerous in other parts of the sky. When galaxies were known as spiral nebulae and their nature was not yet understood, avoidance was thought by some researchers to indicate a connection between them and the Milky Way. Now the effect is understood to be due to dark clouds of dust and gas in our galaxy, which obscure our view of the universe beyond in those quarters of the sky.

Background radiation. See cosmic background radiation.

Baryon number. The total number of baryons in the universe, minus the total number of antibaryons. An index, therefore, of the cosmic matter-antimatter asymmetry.

Baryons. Massive elementary particles with half-integral spin that experience the strong nuclear force. Protons and neutrons are baryons. See hadrons.

GeV. One billion (10’) electron volts. See GeV.

Big bang theory. Model of cosmic history in which the universe begins in a state of high density and temperature, both of which decrease as the universe expands. Less a theory than a school of theories that attempt to trace how the universe evolved.

Billion. This book employs the American billion, equal to one thousand million or 109.

Binary star. A double star system, in which the two stars are bound together by their mutual gravitational force.s

Biology. The scientific study of life and living matter.

Black-body curve. Plot of energy level against wavelength for heat or other radiation emitted by an object capable of absorbing all the energy that strikes it. The curve has a pronounced hump that moves toward shorter wavelengths as the temperature increases. The cosmic background radiation, thought to consist of photons emitted during the big bang, conforms to a black-body curve.

Black holes. Objects with a gravitational field so intense that their escape velocity exceeds that of light. No macroscopic object inside the black hole, therefore, can escape it. In terms of general relativity, space surrounding a black hole is said to reach infinite curvature, making it a singularity.

Bosons. Elementary particles with integer spin that do not obey the Pauli exclusion principle. They include the photons and the W and Z particles, carriers of the electromagnetic and the electroweak forces respectively.

Boundary condition. Restriction on the limits of applicability of an equation. Examples include the definition of a “closed system” in thermodynamics, and the theater within which one collapses the wave function in quantum mechanics. Every equation in physics may in principle be reduced to two fundamentals—the initial conditions and the boundary conditions.

Broken symmetry. In cosmology and particle physics, a state in which traces of an earlier symmetry may be discerned. A broken symmetry condition differs from chaos in that its parts, can in theory be united in a symmetrical whole, like the pieces of a jigsaw puzzle.

Caltech. The California Institute of Technology, in Pasadena.

Carbon reaction. An important nuclear fusion process that occurs in stars. Carbon-12 both initiates it and, following interactions with nuclei of nitrogen, hydrogen, oxygen, and other elements, reappears at its conclusion.

Catastrophism. Nineteenth-century hypothesis that depicted the many changes evinced by the geological record as having resulted from cataclysms occurring during a relatively brief period of history. Compare uniformitarianism.

Causation, causality. The doctrine that every new situation must have resulted from a previous state. Causation underlay the original atomic hypothesis of the Greeks, and was popular in classical physics. It is eroded in quantum mechanics and has, in any case, never been proved essential to the scientific world view. See chance, determinism.

Centauras A. Giant elliptical galaxy, located between the Local Group and the center of the Virgo Supercluster.

Cepheid variable. A pulsating variable star whose periodicity—i.e., the time it takes to vary in brightness—is directly related to its absolute magnitude. This correlation between brightness and period makes Cepheids useful in measuring intergalactic distances. CERN. The Center for European Nuclear Research, located outside Geneva, Switzerland.

Chance. Characteristic of a regime in which predictions cannot be made exactly, but only in terms of probabilities. In classical physics, chance was thought to pertain only where ignorance limited our understanding of an underlying mechanism of strict causation. But in the Copenhagen interpretation of quantum mechanics, chance is portrayed as inherent to all observations of nature.

Charm. The fourth flavor of quarks. Predicted by theory, charmed quarks were discovered in 1974.

Chromatic aberration. Introduction of spurious colors by a lens. This defect flawed the performance of refracting telescopes for centuries, until attenuated by the introduction of corrective elements into a compound lens.

Chronometer. A highly accurate timepiece.

Circle. An ellipse possessing but one focus.

Classical physics. Physics prior to the introduction of the quantum principle. Classical physics incorporates Newtonian mechanics, views energy as a continuum, and is strictly causal.

Closed universe. Cosmological model in which the universe eventually stops expanding and begins to collapse, presumably to end in a fireball like that of the big bang. Compare open universe.

Cloud chamber. A glass-walled enclosure containing a vapor in which particles can be detected by photographing the tracks of water droplets they leave behind when they pass through the chamber.

Collider. See accelerator.

Color. Property of quarks that expresses their behavior under the strong force. Analogous to the concept of charge in electromagnetism, except that, whereas there are two electrical charges (plus and minus), the strong force involves three color charges—red, green, and blue. The term is whimsical, and has nothing to do with color in the conventional sense, any more than quark “flavor,” which determines the weak force behavior of quarks, has anything to do with taste.

Comets. Minor members of the solar system, thought to be lumps of dirt and ice left over from the formation of the solar system. Millions of comets are believed to reside in the Oort Cloud, a spherical region with a radius of some thirty to one hundred thousand astronomical units centered on the sun. Comets falling in from the Oort Cloud are heated by the sun and grow glowing tails, which can make them conspicuous in the skies of Earth.

Confinement. The inability of quarks to escape the bonds that hold them in pairs and triplets at the energy levels found in the universe today. See gluon lattice, asymptotic freedom.

Conservation laws. Laws that identify a quantity, such as energy, that remains unchanged throughout a transformation. All conservation laws are thought to involve symmetries.

Copernicanism. Broadly, the hypothesis that the earth and the other planets orbit the sun.

Cosmic background radiation. Microwave radio emission coming from all directions and corresponding to a black-body curve; its properties coincide with those predicted by the big bang theory as having been generated by photons released from the big bang when the universe was less than one million years old. The big bang theory suggests the existence of neutrino and gravitational background radiations as well, though the means to detect such do not yet exist.

Cosmic matter density. The average number of fermions per unit volume of space throughout the universe. Since matter is depicted in general relativity as bending space, the value of the cosmic matter density, if known, could reveal the overall curvature of cosmic space. See critical density, omega.

Cosmic rays. Subatomic particles, primarily protons, that speed through space and strike the earth. The fact that they are massive, combined with their high velocities, means that they pack considerable energy—from 108 to more than 1022electron volts.

Cosmogony. The study of the origin of the universe.

Cosmology. (1) The science concerned with discerning the structure and composition of the universe as a whole. Combines astronomy, astrophysics, particle physics, and a variety of mathematical approaches including geometry and topology. (2) A particular cosmological theory.

Cosmological constant. A term sometimes employed in cosmology to express a force of “cosmic repulsion,” such as the energy released by the false vacuum thought to power exponential expansion of the universe in the inflationary universe models. Whether any such thing as cosmic repulsion exists or ever played a role in cosmic history remains an open question.

Coulomb barrier. Electromagnetic zone of resistance surrounding protons (or other electrically charged particles) that tends to repel other protons (or other particles of like charge).

Creationism. Belief that the universe was created by God in the relatively recent past, as implied by literal interpretations of biblical chronology, and that the species of terrestrial life did not arise through Darwinian evolution but, rather, all came into existence at once.

Critical density. The cosmic density of matter required to “close” the universe and so eventually to halt cosmic expansion. Its value amounts to about ten hydrogen atoms per cubic meter of space. The observed density is so close to the critical value that the question of whether the universe is open or closed has not yet been resolved by observation. See open universe, closed universe.

Dark matter. Matter whose existence is inferred on the basis of dynamical studies—e.g., the orbits of stars in galaxies—but which does not show up as bright objects such as stars and nebulae. Its composition is unknown: It might consist of subatomic particles, or of dim dwarf stars or black holes, or a combination of various sorts of objects.

Darwinism. Theory that species arise through the natural selection of random mutations that better fit changing conditions in a generally uniformitarian Earth.

Dead reckoning. Navigation by recording one’s heading, velocity, and elapsed time, with little or no reference to the stars.

Deceleration parameter. Quantity designating the rate at which the expansion of the universe is slowing down, owing to the braking effect of the galaxies’ gravitational tug on one another. It is a function of the cosmic matter density.

Declination. Location on the sky in a north-south direction. Lines of declination are the celestial equivalent of latitude on Earth. Compare right ascension.

Decoupling. Separation of classes of particles from regular interaction with one another, as in the decoupling of photons from particles of matter that produced the cosmic background radiation.

Deduction. Process of reasoning in which a conclusion is derived from a given premise or premises, without a need for additional information. Compare induction.

Degree. (1) A measure of temperature: Unless otherwise specified, all temperatures in this book are in degrees Kelvin. (To convert to Celsius, subtract 273.) (2) An angle subtended in the sky: From the zenith to the horizon is 90 degrees; the distance between the pointer stars of the Big Dipper is 5 degrees.

Detector. Device for recording the presence of subatomic particles. A typical modern detector consists of an array of electronic sensors connected to a computer, capable of recording the paths of the particles as they fly out from the collision site in a particle accelerator.

Determinism. The doctrine that all events are the predictable effects of prior causes. See causation.

Deuterium. An isotope of hydrogen, the nucleus of which comprises one neutron plus one proton.

Dimension. A geometrical axis.

Dirac equation. Mathematical description of the electron, derived by Paul Dirac, that incorporates both quantum mechanics and special relativity.

DNA. Deoxyribonucleic acid, the macromolecule that carries the genetic information requisite to life on Earth.

Doppler shift. Change in the apparent wavelength of radiation (e.g., light or sound) emitted by a moving body. A star moving away from the observer will appear to be radiating light at a lower frequency than if at rest; consequently, lines in the star’s spectrum will be shifted toward the red (lower frequency) end of the spectrum. The existence of a direct relationship between the redshift of light from galaxies and their distances is the fundamental evidence for the expansion of the universe. Double star. See binary star.

Dwarf stars. Main-sequence stars with masses equal to or less than that of the sun. More generally, any star on or below the main sequence in the Hertzsprung-Russell diagram.

Dynamics. Study, in physics, of the motion and equilibrium of systems under the influence of force.

Dynamo. An electric generator that employs a spinning magnetic field to produce electricity.

Eccentrics. In Ptolemaic cosmology, displacement of the center of a rotating celestial sphere from the center of the universe.

Eclipse. Obscuration of one astronomical object (such as the sun) by another such object (such as the moon).

Electrodynamics. Study of the behavior of electromagnetic force in motion.

Electromagnetic force (or interaction). Fundamental force of nature that acts on all electrically charged particles. Classical electromagnetics is based on Maxwell’s and Faraday’s equations, quantum electromagnetics on the theory of quantum electrodynamics (QED).

Electrons. Light elementary particles with a negative electrical charge. Electrons are found in shells surrounding the nuclei of atoms; their interactions with the electrons of neighboring atoms create the chemical bonds that link atoms together as molecules.

Electron shells. Zones in which the electrons in atoms reside. Their radius is determined by the quantum principle, their population by the exclusion principle.

Electronuclear force. Single fundamental force thought to have functioned in the very early universe and to have combined the attributes thereafter parceled out to the electromagnetic and the strong and weak nuclear forces. See grand unified theory.

Electron volt. Measure of energy, equal to 1.6 × 10−12 erg.

Electroweak theory. Theory demonstrating links between the electromagnetic and the weak nuclear forces. Indicates that in the high energies that characterized the very early universe, electromagnetism and the weak force functioned as a single, electroweak force. Also known as the Weinberg-Salam theory. Ellipse. A plane curve in which the sum of the distances of each point along its periphery from two points—its “foci”—are equal. Emission lines. Bright lines produced in a spectrum by a luminous source, such as a star or a bright nebula. Compare absorption lines. Empiricism. An emphasis on sense data as a source of knowledge, in opposition to the rationalist belief that reasoning is superior to experience. Energy. (1) The capacity to do work. (2) Manifestation of a particular variety of force.

Epicycles. In Ptolemaic cosmology, a circular orbit around a point that itself orbits another point. Escape velocity. The speed at which an object can leave another object behind, without being recalled by its gravitational force. The escape velocity of Earth —which must, for instance, be attained by a spacecraft if it is to reach another planet—is 25,000 miles per hour. Euclidean geometry. See geometry.

Evolution. (1) In biology, the theory that complex and multifarious living things developed from generally simpler and less various organisms. (2) In astronomy, the theory that more complex and varied atoms develop from simpler ones, as through the synthesis of heavy atomic nuclei in stars.

Exclusion principle. The rule that no two fermions can occupy the same quantum state.

Expansion of universe. Constant increase, with time, in the distance separating distant galaxies from one another. Expansion does not take place within individual galaxies or clusters of galaxies, which are bound together gravitationally, but evidences itself on the supercluster level.

Fermilab. The Fermi National Accelerator Laboratory, in Batavia, Illinois.

Fermions. Particles with half-integral spin. Fermions obey the exclusion principle, which says that no two fermions can exist in an atom in the same quantum state; in practice this restricts the number of electrons, which are fermions, permitted in each electron shell.

Fermi’s question. The question of why, if spacefaring extraterrestrial civilizations exist, their representatives haven’t visited Earth.

Feynman diagram. Schematic representation of an interaction between particles.

Field. Domain or environment in which the real or potential action of a force can be described mathematically at each point in space.

Fission, nuclear. Interaction in which nucleons previously united in an atomic nuclei are disjoined, releasing energy. Fission powers “atomic” bombs. Compare fusion.

Flatness problem. The riddle of why the universe is neither dramatically open nor closed, but appears to be almost perfectly balanced between these states. Flavor. Designation of quark types—up, down, strange, charmed, top, and bottom. Flavor determines how the weak nuclear force influences quarks. Force. Agency responsible for a change in a system. In Newtonian mechanics, gravitational force bends the moon away from the straight trajectory it would otherwise pursue. Fossils. Geological remains of what was once a living thing. Fraunhofer lines. Dark lines in a spectrum.

Fusion, nuclear. Interaction in which nucleons are forged together, creating new atomic nuclei and releasing energy. Fusion powers “hydrogen” bombs.

Galactic disk. The plate-shaped component of a spiral galaxy, in which the spiral arms are found.

Galactic halo. A spherical aggregation of stars, globular star clusters, and thin gas clouds, centered on the nucleus of the galaxy and extending beyond the known extremities of the galactic disk.

Galaxy. A large aggregation of stars, bound together gravitationally. There are three major classifications of galaxies—spiral, elliptical, and irregular—and several subclassifications. The sun belongs to a spiral galaxy, the Milky Way galaxy.

Gamma rays. Extremely short-wavelength electromagnetic energy. Gauge theory. Account of forces that views them as arising from broken symmetries.

Geocentric cosmology. School of ancient theories that depicted the earth as standing, immobile, at the center of the universe.

Geology. Scientific study of the dynamics and history of the earth, as evidenced in its rocks, chemicals, and fossils.

Geometry. The mathematics of lines drawn through space. In euclidean geometry, space is postulated to be “flat,” i.e., to be the three-dimensional analog of a plane. In noneuclidean geometry, space is “curved,” i.e., is the three-dimensional analog of a sphere or a hyperbola.

GeV. One billion (10’) electron volts. Sometimes written as one BeV.

Giant stars. High-luminosity stars that lie above the main sequence on the Hertz-sprung-Russell diagram.

Globular clusters. See star clusters.

Glueballs. Theoretical particles made exclusively of gluons. Tentative evidence of the existence of glueballs had been found in accelerator experiments by the mid-1980s.

Gluon lattice. Force field generated by the strong nuclear force that holds quarks together. See gluons.

Gluons. Quanta that carry the strong nuclear force. Like photons, vector bosons, and gravitons—the carriers respectively of electromagnetism, the weak force, and gravitation—gluons are massless bosons. Consequently, for simplicity’s sake, some physicists lump together all the force-carrying quanta under the term “gluons.”

Grand unified theories. Class of theories that purport to reveal identities linking the strong and electroweak forces. The differences between these forces in nature today is attributed to the breaking of symmetrical relationships among force-carrying particles as the very early universe expanded and cooled.

Gravitational force (or interaction). Fundamental force of nature, generated by all particles that possess mass. Interpreted by means of Newtonian mechanics or by the general theory of relativity.

Gravitinos. Hypothetical force-carrying particles predicted by supersymmetry theories. The gravitino’s spin would be Vi. Its mass is unknown.

Gravitons. The quanta thought to convey gravitational force; analogous to the photons, gluons, and intermediate vector bosons of electromagnetism and the strong and weak nuclear forces. Predicted by quantum theory of gravity, gravitons have not yet been detected.

Gravity. (1) In Aristotelian physics, an innate tendency of the elements earth and water to fall. (2) In Newtonian physics, the universal, mutual, attraction of all massive objects for one another; its force is directly proportional to the mass of each object, and decreases by the square of the distance separating the objects involved. (3) In Einstein’s general relativity, gravity is viewed as a consequence of the curvature of space induced by the presence of a massive object. In quantum mechanics the gravitational field is said to be conveyed by quanta called gravitons.

Great year. Ancient concept of a celestial and historical cycle, its duration roughly a thousand or ten thousand years, at the end of which there is universal destruction and a new great year begins.

GUT. Acronym for grand unified theory.

Hadrons. Elementary particles that are influenced by the strong nuclear force. There are two sorts of hadrons—mesons, which have integral spin, and baryons, which have spin ½ or 3/2.

Half-life. The time it takes for half of a given quantity of radioactive material to decay. Halo, galactic. See galactic halo.

Heliocentric cosmology. School of models in which the sun was portrayed as standing at the center of the universe.

Hermetic. Of or relating to Hermes Trismegistus, a mythical philosopher beloved of the Neoplatonists and usually identified with ancient Egypt.

Hertz. A unit of frequency equal to one cycle (or wave) per second.

Hertzsprung-Russell diagram. Plot that reveals a relationship between the colors and absolute magnitudes of stars.

Higgs field. Mechanism operating in symmetry-breaking events; in electroweak theory, the Higgs field is said to have imparted mass to the W and Z particles.

High-energy physics. See particle physics.

Horizon problem. A quandary in standard big bang theory, which indicates that few of the particles of the early universe would have had time to be in causal contact with one another at the outset of cosmic expansion. It appears to have been resolved in the inflationary universe theory.

Hubble constant. The rate at which the universe expands, equal to approximately fifty kilometers of velocity per megaparsec of distance.

Hubble diagram. Plot of galaxy redshifts against their distances. This was the first evidence of the expansion of the universe.

Hubble law. That distant galaxies are found to be receding from one another at velocities directly correlated to their distances apart.

Hyperbolic space. See geometry.

Hyperdimensional. Involving more than the customary four dimensions (three of space plus one of time) of relativistic space-time.

Hypothesis. A scientific proposition that purports to explain a given set of phenomena; less comprehensive and less well established than a theory.

Indeterminacy principle. Quantum precept indicating that the position and trajectory of a particle cannot both be known with perfect exactitude. Indeterminacy thus indicates the existence of a basic quantum of knowledge of the particle world. And, since information about one quantity can be extracted at the expense of another, it demonstrates that the answers we obtain about natural events result to some extent from the questions we choose to ask about them.

Induction. System of reasoning in which the conclusion, though implied by the premises and consistent with them, does not necessarily follow from them.

Inertia. Quality of mass, such that any massive particle tends to remain at rest relative to a given reference frame, and to remain in constant motion once in motion, unless acted upon by a force.

Inflationary universe. Theory that the expansion of the very early universe proceeded much more rapidly than it does today—at an exponential rather than a linear rate.

Infrared light. Electromagnetic radiation of a slightly longer wavelength than that of visible light.

Infrared slavery. Inability of quarks to escape the bonds of the strong force that confines them to the company of other quarks.

Initial condition. (1) In physics, the state of a system at the time at which a given interaction begins—e.g., the approach of two electrons that are about to undergo an electromagnetic interaction. (2) In cosmology, a quantity inserted as a given in cosmogonic equations describing the early universe.

Intelligence. Defined in SETI as the ability and willingness to transmit electromagnetic signals across interstellar space.

Interaction. Event involving an exchange between two or more particles. Since the fundamental forces are portrayed by quantum theory as involving the exchange of force-carrying particles (the bosons), the forces are more correctly described as interactions.

Interferometer. A device for observing the interference of waves of light or similar emanations caused by a shift in the phase or wavelength of some of the waves.

Intermediate vector bosons. See W, Z particles.

Inverse-square law. In Newtonian mechanics, the rule that the measured intensity of light diminishes by the square of the distance of its source—so that, e.g., if stars A and B are of equal absolute magnitude but star B is twice as distant, it will appear to be one quarter as bright as star A.

Invisible astronomy. The study of celestial objects by observing their radiation at wavelengths other than those of visible light.

Island universe hypothesis. Assertion that the sun belongs to a galaxy and that the spiral nebulae are other galaxies of stars, which in turn are separated from one another by vast voids of space. Compare nebular hypothesis.

Isotopes. Atoms having the same number of protons in their nuclei but different numbers of neutrons, with the result that their mass differs though they may have the same number of electrons.

Isotropy. Quality of being the same in all directions. Compare anisotropy.

Jet Propulsion Laboratory. NASA installation near Pasadena, operated by Caltech and specializing in unmanned space exploration.

Jovian. Giant planets that have a gaseous surface; the sun’s known Jovian planets are Jupiter, Saturn, Uranus, and Neptune.

Kaluza-Klein theory. Five-dimensional relativity theory that played a role in the development of unified theory.

Latitude. On Earth, distance north or south of the equator along a line connecting the poles.

Lattice. See gluon.

Law. A theory of such wide and invariable application that its violation is thought to be impossible.

Leptons. Elementary particles that have no measurable size and are not influenced by the strong nuclear force. Electrons, muons, and neutrinos are leptons. Light. Electromagnetic radiation with wavelengths of or close to those detectable by the eye.

Light-year. The distance light travels in one year, equal to 5.8 × 1012 (about six trillion) miles.

Local Group. The association of galaxies to which the Milky Way galaxy belongs. Longitude. On Earth, distance east or west of Greenwich, England, measured along lines drawn parallel to the equator. Lookback time. Phenomenon that, owing to the finite velocity of light, the more distant an object being observed, the older is the information received from it. A galaxy one billion light-years away, for instance, is seen as it looked one billion years ago.

Lorentz contraction. Diminution in the observed length of an object along the axis of its motion, as perceived by an external observer who does not share its velocity.

Luminosity. The intrinsic brightness of a star. Usually defined in terms of absolute magnitude.

M. Designation of objects in the Messier catalog of nebulae, star clusters, and galaxies, published in the eighteenth century.

Mach’s principle. Precept that the inertia of objects results not from their relationship to Newtonian absolute space, but to the rest of the mass and energy distributed throughout the universe. Though unproved and perhaps unprovable, Mach’s principle inspired Einstein, who sought with partial success to incorporate it into the general theory of relativity.

Magellanic Clouds. Two galaxies that lie close to the Milky Way galaxy. They are visible in the southern skies of Earth.

Magnetic monopole. A massive particle with but one magnetic pole, the production of which is indicated in some theories of the early universe.

Magnitude. The brightness of a star or planet, expressed on a scale in which lower numbers mean greater brightness. Apparent magnitude indicates the brightness of objects as we see them from Earth, regardless of their distance. Absolute magnitude is defined as the apparent magnitude a star would have if viewed from a distance of ten parsecs, or 32.6 light-years. Each step in magnitude equals a difference of 2.5 times in brightness: The brightest stars in the sky are apparent magnitude 1; the dimmest, 6. The magnitudes of extremely bright objects are expressed in negative values—e.g., the apparent magnitude of the sun is about •26.

Main sequence. The curving path in the Hertzsprung-Russell diagram along which most stars lie.

Many body problem. The difficulty of calculating the interactions—e.g., the Newtonian gravitational interactions—of three or more objects.

Mass. Measure of the amount of matter in an object. Inertial mass indicates the object’s resistance to changes in its state of motion. Gravitational mass indicates its response to the gravitational force. In the general theory of relativity, gravitational and inertial mass are revealed to be aspects of the same quantity.

Materialism. Belief that material objects and their interactions constitute the complete reality of all phenomena, including such seemingly insubstantial phenomena as thoughts and dreams. Compare spiritualism.

Matter waves. Characteristic by virtue of which matter, like energy, displays the qualities of waves as well as of particles. See wave-particle duality.

Mechanics. The study, in physics, of the influence of forces.

Megaparsec. One million (106parsecs.

Mesons. See hadrons.

Metals. In astrophysics, all elements heavier than helium. MeV. One million (106electron volts.

Micrometry. The measurement of the apparent sizes and separations of astronomical objects by use of knife blades or crosshairs in the eyepiece of a telescope. If the distance of an object is known, its size can be determined through micrometry.

Microwave background. See cosmic background radiation. Microwaves. Radio radiation with wavelengths of about 10•4 to 1 meter, equal to 109 to 1013hertz.

Mile. The mile employed in this book is the statute mile, equal to 5,280 feet. Milky Way. A softly glowing band of light that bisects the skies of Earth, produced by light from stars and nebulae in the galactic disk. Milky Way galaxy. The spiral galaxy in which the sun resides. Million. A thousand thousand (106). Minor planets. See asteroids. Missing matter. Alternate term for dark matter. MIT. Massachusetts Institute of Technology, in Cambridge, Mass. Molecules. The smallest units of a chemical compound. A molecule is composed of two or more atoms, linked by interactions of their electrons. Monopole. See magnetic monopole.

Muon. Short-lived elementary particle with negative electrical charge. Muons are leptons. They resemble electrons, but are 207 times more massive.

Natural philosophy. A term widely employed in the seventeenth century to mean what today is encompassed in the word science.

Natural selection. Tendency of individuals better suited to their environment to survive and perpetuate their species, leading to changes in the genetic makeup of the species and, eventually, to the origin of new species. See evolution.

Nautical mile. Equals 1.15 statute miles.

Nebulae. Indistinct, nonterrestrial objects visible in the night sky. “Bright” nebulae glow with light emitted by the gas of which they are composed (“emission” nebulae) or by reflected starlight (“reflection” nebulae) or both. “Dark” nebulae consist of clouds of gas and dust that are not so illuminated. “Planetary” nebulae are shells of gas ejected by stars. Spiral nebulae are galaxies.

Nebular hypothesis. Hypothesis, maintained in the nineteenth and early twentieth century, that the spiral nebulae are not galaxies but are instead whirlpools of gas from which new systems of stars and planets are condensing. Compare island universe theory.

Neutrinos. Electrically neutral, massless particles that respond to the weak nuclear force but not the strong nuclear and electromagnetic forces.

Neutrons. Electrically neutral, massive particles found in the nuclei of atoms. Each neutron is composed of one up quark and two down quarks; its mass is 939.6 MeV, slightly more than that of the proton. Stable within the nucleus, the neutron if isolated decays, with a half-life of fifteen minutes.

Neutron stars. Stars with gravitational fields so intense that most of their matter has been compressed into neutrons. They are formed when massive stars run out of nuclear fuel and collapse. Many rotate rapidly and generate radio pulses; when detected by radiotelescopes, they are known as pulsars.

NGC. Designates entries in the New General/Catalog of nonstellar objects.

Noneuclidean geometry. See geometry.

Nova. A star that brightens suddenly and to an unprecedented degree, creating the impression that a new star has appeared where none was before. Hence the name, from nova for “new.” See supernova. Nuclei. (1) The central part of atoms, composed of protons and neutrons (which are made of quarks) and containing nearly all of each atom’s mass. (2) The central region of a galaxy. Nucleons. Protons and neutrons, the constituents of atomic nuclei. Nucleosynthesis, nucleogenesis. The fusion of nucleons to create the nuclei of new atoms. Nucleosynthesis takes place in stars, and, at an accelerated rate, in supernovae.

Observational cosmology. The application of observational data to the study of the universe as a whole. Open clusters. See star clusters.

Omega. Index of the matter density of the universe, defined as the ratio between its actual density and the “critical density” required to “close” the universe and eventually halt its expansion. If omega equals less than 1, the universe is “open” and will expand forever. See cosmic matter density.

Oort Cloud. Home of most solar system comets.

Open universe. Cosmological model in which the universe continues to expand forever; its space-time geometry is hyperbolic, or “open.” Optics. The science of light.

Oscillating universe. Cosmological model in which the universe is “closed” and its expansion is destined to stop, to be succeeded by collapse and “then” (if ordinary temporal terms may be said to apply) by a rebound into a new expansion phase.

Panstellar. Of or pertaining to more than one star.

Paradox. A self-contradictory proposition. Paradoxes are most useful when they seem most likely to be true, for it is then that they best serve to expose flaws in the data or reasoning that led to their appearance.

Parallax. The apparent displacement in the position of a star or planet occasioned by its being viewed from two different locations—e.g., by observing it from two widely separated stations on Earth, or at intervals of six months, when the earth is at either extreme of its orbit around the sun. The resulting angle can be used, by triangulation, to determine the distance of the star or planet.

Parsec. Astronomical unit of distance, equal to 3.26 light-years. Particle accelerator. See accelerator.

Particle physics. The branch of science that deals with the smallest known structures of matter and energy. As their experimental investigation usually involves the application of considerable energy, particle physics overlaps with high-energy physics.

Particles. Fundamental units of matter and energy. All may be classed as fermions, which have half-integral spin and obey the exclusion principle, and bosons, which have integral spin and do not obey the exclusion principle. The term particle is metaphoric, in that all subatomic particles also evince aspects of wave-like behavior.

Pauli exclusion principle. See exclusion principle.

Period-luminosity function. The relationship between the absolute magnitude and period of variability of Cepheid variable stars.

Phase transition. An abrupt change in the equilibrium state of a system, as evoked by the cooling of the early universe as it expanded.

Photon decoupling. The release of photons from constant collisions with massive particles as the universe expanded and its matter density diminished. See decoupling.

Photons. The quanta of the electromagnetic force. The name comes from the fact that light is a form of electromagnetism. Photons have zero rest mass and can therefore travel infinitely far.

Physics. The scientific study of the interactions of matter and energy.

Planck epoch, Planck time. The first instant following the beginning of the expansion of the universe, when the cosmic matter density was still so high that gravitational force acted as strongly as the other fundamental forces on the subatomic scale.

Planck’s constant. The fundamental quantity of action in quantum mechanics.

Planet. An astronomical object more massive than an asteroid but less so than a star. Planets shine by reflected light; stars generate light of their own.

Plasma. A state in which matter consists of electrons and other subatomic particles without any structure of an order higher than that of atomic nuclei.

Platonic solids. The five regular polyhedrons—the tetrahedron, octahedron, hexahedron, icosahedron, and dodecahedron—esteemed by Plato as embodying aesthetic and rational ideals.

Plurality of worlds. Hypothesis that the universe contains inhabited planets other than Earth.

Pole star. The star—Polaris—that lies near the direction in the sky toward which the north pole of the earth points. Positron. The antimatter twin of the electron.

Post hoc fallacy. The erroneous assumption that, because B follows A, B therefore was caused by A. More strictly, the fallacy of calculating, in retrospect, the odds of B’s having occurred by adding up a long sequence of such putative causes.

Precession. The slow (once per twenty-six thousand years) gyration of the earth’s axis.

Proper motion. Individual drifting of stars through space. Protogalaxy. A galaxy in the process of formation. None are observed nearby, indicating that all or most galaxies formed long ago.

Proton. A massive particle with positive electrical charge found in the nuclei of atoms. Composed of two up quarks and one down quark. The proton’s mass is 938.3 MeV, slightly less than that of the neutron. Proton decay. Spontaneous disintegration of the proton, predicted by grand unified theory but never observed experimentally. Proton-proton reaction. An important nuclear fusion reaction that occurs in stars. It begins with the fusion of two hydrogen nuclei, each of which consists of a single proton. Pulsars. See neutron stars. QCD. See quantum chromodynamics. QED. See quantum electrodynamics.

Quadrant. An instrument, based on a quarter of a circle, employed to measure the altitude above the horizon of astronomical bodies. Eventually replaced by the sextant.

Quanta. Fundamental units of energy.

Quantum chromodynamics. The quantum theory of the strong nuclear force, which it envisions as being conveyed by quanta called gluons. The name derives from the assignment of a quantum number called color to designate how quarks function in response to the strong force.

Quantum electrodynamics. The quantum theory of the electromagnetic force, which it envisions as being carried by quanta called photons.

Quantum genesis. Hypothesis that the origin of the universe may be understood in terms of a quantum chance.

Quantum leap. The disappearance of a subatomic particle—e.g., an electron—at one location and its simultaneous reappearance at another. The counterintuitive weirdness of the concept results in part from the limitations of the particle metaphor in describing a phenomenon that is also in many respects a wave.

Quantum mechanics. See quantum physics.

Quantum physics. Physics based upon the quantum principle, that energy is emitted not as a continuum but in discrete units.

Quantum space. Vacuum with the potential to produce virtual particles.

Quantum tunneling. A quantum leap through a barrier.

Quarks. Fundamental particles from which all hadrons are made. According to the theory of quantum chromodynamics, protons, neutrons, and their higher-energy cousins are composed of trios of quarks, while the mesons are each made of one quark and one antiquark. Held together by the strong nuclear force, quarks are not found in isolation in nature today; see asymptotic freedom.

Quasars. Pointlike sources of light whose redshifts indicate that they lie at distances of billions of light-years. Thought to be the nuclei of young galaxies.

Radio. Long-wavelength electromagnetic radiation.

Radioactivity. Emission of particles by unstable elements as they decay.

Radio astronomy. Study of the universe at the radio wavelengths of electromagnetic energy.

Radiocarbon dating. Determination of the age of a substance containing radioactive carbon by means of its radioactive half-life. Radiometric dating. Determination of the age of objects—e.g., earth and moon rocks—by means of the half-life of the unstable elements they contain.

Radiotelescopes. Sensitive radio antennae employed to detect the radio energy emitted by nebulae, galaxies, pulsars, etc.

Recombination. The capture of an electron by a proton. Numerous recombinations are thought to have occurred when the universe was a little less than one million years old, resulting in the formation of electron shells around helium and hydrogen nuclei to create complete atoms.

Red giants. Large stars with an atmosphere that is relatively cool, and therefore looks redder in color than does that of a main sequence star.

Redshift. Displacement of the spectral lines in light coming from the stars of distant galaxies, thought to be produced by the velocity of the galaxies outward in the expanding universe. See Hubble law.

Redshift-distance relation. The correlation between redshift in the spectra of galaxies and their distances. See Hubble law.

Relativistic. Approaching the velocity of light. Particles moving at these speeds demonstrate effects predicted by the special theory of relativity—increased mass, slowing of time, etc.—that must be taken into account by combining relativity with quantum theory if accurate predictions are to be made.

Relativity, general theory of. Einstein’s theory of gravitational force.

Relativity, special theory of. Einstein’s theory of the electrodynamics of moving systems.

Renaissance. Generally, the period of cultural awakening in the West beginning at about 1350 and ending with the death of Giordano Bruno in 1600 or of Shakespeare in 1616.

Renormalization. The removal of nonsensical infinities from quantum mechanics equations by a mathematical procedure in which other infinities are introduced in order to cancel them.

Retrograde. Apparent motion of a planet in a direction opposite to its normal progress across the sky, produced by the orbital motion of the earth.

Right ascension. Location in the sky along an east-west direction; the celestial equivalent of longitude. Compare declination.

Royal Society. English organization founded in the seventeenth century and dedicated to the advancement of science.

Satellite. An object in orbit around another, more massive object.

Scholastics. Adherents to the philosophy and cosmology of Aristotle. Their dominance in the universities, which had been founded largely to study Aristotle, constituted an obstacle to acceptance of the Copernican system advocated by Kepler and Galileo.

Science. Systematic study of nature, based upon the presumption that the universe is based upon rationally intelligible principles and that its behavior can therefore be predicted by subjecting observational data to logical analysis.

SETI. The Search for Extraterrestrial Intelligence, by using radiotelescopes to listen for signals transmitted by intelligent alien beings.

Sextant. Instrument employed to measure the elevation of astronomical objects above the horizon. Based upon an arc equal to a sixth of a circle, sextants are more compact and easier to use than are the quadrants that preceded them.

Shadow matter. Theoretical classes of particles, their existence intimated by supersymmetry theory, that participate in few if any of the four known fundamental forces. Planets, stars, and galaxies made of shadow matter could conceivably exist in the same space and time we occupy without our sensing their presence.

Singularity. A point of infinite curvature of space where the equations of general relativity break down. A black hole represents a singularity; so, perhaps, did the universe at the first moment of time.

Solar system. The sun, its planets, and the asteroids and comets that, like the planets, orbit the sun.

Space. Traditionally, the three-dimensional theater within which events transpire, explicable by means of euclidean geometry. In relativity, space is depicted in terms of noneuclidean geometries as well. In quantum physics, space may be constructed out of any of a variety of abstractions, such as a “charge space” employed in dealing with electrically charged particles or the “color space” in which quarks can for convenience be plotted. See geometry.

Spacetime. Arena in which events are depicted in the theory of relativity. The orbit of a planet, for instance, can be described as a “world line” in a four-dimensional spacetime continuum.

Spectrograph. A device, usually based on a finely etched grate that performs the function of a prism, for breaking up light into its constituent parts and making a photographic or electronic record of the resulting spectrum. When lacking a means for recording the spectrum, the device is called a spectroscope.

Spectroscopic binary. A double star in which the individual stars cannot be resolved, but can be detected through their effects on the spectrum of the system—e.g., the relative motions of the stars may be detected from Doppler shifts in the spectral lines of starlight.

Spectroscopy. Scientific investigation of an object by studying its spectrum.

Spectrum. A record of the distribution of matter or energy (e.g., light) by wavelength. Spectra can be studied to learn the chemical composition and motion of stars and galaxies.

Spherical spaceSee geometry.

Spin. The intrinsic angular momentum of an elementary particle, as by the particle’s spinning on its axis. Spin is quantized in units of Planck’s constant of action, h, so that, e.g., “spin 1,” means spin = 1h. Particles with integral spin (0, 1) are called bosons; those with half spin are fermions.

Spiral nebulae. See nebulae.

Spiritualism. Belief that material interactions alone cannot account for all phenomena, and that some—e.g., thought—are due to the fundamentally insensible actions of intangibles.

Standard model. The theories of the four forces, which, taken together, can predict the outcome of every known fundamental interaction.

Star. A celestial object that generates energy by means of nuclear fusion at its core. To do this it must have more than about 0.08 the sun’s mass. If, for instance, the planet Jupiter were some fifty to one hundred times more massive than it is, fusion reactions would transpire in its core and it would be a star. See planet.

Star clusters. Gravitationally bound aggregations of stars, smaller and less massive than galaxies. “Globular” clusters are the largest category; they are old, and may harbor hundreds of thousands to millions of stars, and are found both within and well away from the galactic disk. “Open” clusters are smaller, have a wide range of ages, and reside within the disk. Statute mile. See mile.

Steady state. Theory that the expanding universe was never in a state of appreciably higher density—i.e., that there was no “big bang”—and that matter is constantly being created out of empty space in order to maintain the cosmic matter density.

Stellar evolution. The building of complex atomic nuclei from simpler nuclei in stars, with the result that succeeding generations of stars and planets contain a greater variety of chemical elements than did their predecessors. See evolution.

Stochastic cooling. The gathering (i.e., focusing) of clouds of subatomic particles in an accelerator by monitoring their scattering vectors and altering the magnetic environment in an accelerator storage ring to keep them close together. First employed in storing particles of antimatter, which are expensive to manufacture and ought not to be wasted.

Storage ring. A ring in which particles are kept in a circular motion, suspended in a magnetic field, until they can be injected into the larger ring of an accelerator.

String theory. Theory that subatomic particles actually have extension along one axis, and that their properties are determined by the arrangement and vibration of the strings.

Strong nuclear force (or interaction). Fundamental force of nature that binds quarks together, and holds nucleons (which are comprised of quarks) together as the nuclei of atoms. Portrayed in quantum chromodynamics as conveyed by quanta called gluons.

Subatomic. Of a scale smaller than that of an atom. Subatomic particles. See particles.

Sum over histories. Probabilistic interpretation of a system’s past, in which quantum indeterminacy is taken into account and the history is reconstructed in terms of each possible path and its relative likelihood.

Sun. The star orbited by the earth.

Supercluster. A cluster of clusters of galaxies. Superclusters are typically about one hundred million (108) light-years in diameter and contain tens of thousands of galaxies.

Superconducting super collider. A proposed accelerator of great size and high energy.

Supergiants. The largest and brightest class of stars. Supernovae. The explosions of giant stars. Superstring theory. Alternate name for string theory.

Supersymmetry. Class of theories that seek to identify symmetrical relationships linking fermions and bosons—i.e., particles of half-integral spin, like electrons, protons, and neutrinos, with those of integral spin, like photons and gluons. If attainable, a fully realized supersymmetry theory would provide a unified account of all four fundamental forces, and might well shed light on the very early evolution of the universe as well.

Superunified theory. Hypothetical theory that presumably would show how all four fundamental forces of nature functioned as a single force in the extremely early universe. The best current candidates for such a potential achievement are thought to be supersymmetry and string theory. Symmetry. State of a system such that it has a significant quantity that remains invariant after a transformation. More generally, an apt or pleasing proportion based upon such a state. Symmetry breaking. The loss of symmetry in a transformation. See broken symmetry.

Symmetry group. A mathematical group with a common property that unites its members and evinces a symmetry.

Telescope. A device for gathering and amplifying light or other energy. Refracting telescopes gather light by means of a lens, reflecting telescopes by means of a mirror. Radiotelescopes gather radio energy, typically by using a metallic dish antenna. Telescopes have also been built that can gather X rays, gamma rays, and other forms of energy.

TeV. Equal to one teraelectron volt, or 1,000 GeV.

Tevatron. A particle accelerator capable of attaining an energy of 1 TeV.

Theory. A rationally coherent account of a wider range of phenomena than is customarily accounted for by a hypothesis.

Thermodynamics. The study of the behavior of heat (and, by implication, other forms of energy) in changing systems.

Thought experiment. An experiment that cannot be or is not carried out in practice, but can—given sufficient imagination and rigor—be reasoned through by thought and intuition alone.

Time. A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space.

Transit. The passage of a smaller, nearer astronomical object across the face of a larger object in the background, as in a transit of Venus across the sun.

Triangulation. Measurement of the distance of a planet or nearby star by sighting its apparent position against background stars from two or more separate locations. See parallax.

Trillion. A thousand billion (1012).

Ultraviolet light. Electromagnetic radiation of a wavelength slightly shorter than that of visible light.

Unified theory. In particle physics, any theory exposing relationships between seemingly disparate classes of particles. More generally, a theory that gathers a wide range of fundamentally different phenomena under a single precept, as in Maxwell’s discovery that light and magnetism are aspects of a single, electromagnetic force.

Uniformitarianism. The hypothesis that the extensive changes in the earth, as evinced in the geological record, have resulted, not from massive catastrophes, but from the slow operation of wind, weather, volcanism, and the like over many millions of years. Compare catastrophism.

Vacuum genesis. Hypothesis that the universe began as nothingness, from which matter and energy arose by a process analogous to the appearance of virtual particles from a vacuum.

Variable star. A star that changes in brightness periodically.

Virgo Cluster. A nearby cluster of galaxies.

Virgo Supercluster. An aggregation of galaxies—roughly ten thousand of them—to which the Virgo Cluster and our own galaxy belong. Virtual particles. Short-lived particles that arise from a vacuum. Their existence is permitted by the indeterminacy principle. Voyager. Pair of unmanned American spacecraft launched in 1977 on missions to Jupiter, Saturn, and beyond. Wave function. A quantum mechanical expression that describes all the relevant properties of a particle. Wave-particle duality. Quantum realization that particles of matter and energy also exhibit many of the characteristics of waves. Waves. Propagation of energy by means of coherent vibration. Weak nuclear force (or interaction). Fundamental force of nature that governs the process of radioactivity. It is currently accounted for by the electroweak theory. Weinberg-Salam theory. See electroweak theory.

World line. In relativity, the path traced out in four-dimensional space-time by a given object or particle. W particles. Massive bosons thought to have been abundant in the early universe, when the unified electroweak force was manifest. X rays. Short-wavelength electromagnetic energy. The X ray portion of the electromagnetic spectrum lies between the realms of gamma rays and that of ultraviolet light. Yang-Mills theory. See gauge theory.

Zoo hypothesis. Hypothesis that life on Earth has been detected by intelligent extraterrestrials who scruple not to visit us because they do not wish to interfere with our development.

Z particles. Massive bosons thought to have been abundant in the early universe, when the unified electroweak force was manifest.