Mesons. See hadrons.
Metals. In astrophysics, all elements heavier than helium. MeV. One million (106) electron 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.
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br /> 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 invest
igation 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 space. See 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.
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