Coming of Age in the Milky Way

by Timothy Ferris

  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 o
nly 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).