z	Notation for redshift. Objects of higher z are further away. A redshift of z near 0 corresponds to objects nearby; a redshift of z = 1 corresponds to objects at a distance of about 10 billion light years. [LB90]
Z-number	See atomic number. [H76]
z pinch	A diffuse toroidal pinch in which the magnetic field runs around the plasma column. [H76]
Zanstra's theory	A theory of emission lines in planetary nebulae which supposes that the emission lines in hydrogen (and helium) arise from a process of ionization (by the ultraviolet radiation of the central star) and recombination, and that the forbidden lines arise from the collisional excitations of the metastable state. [H76]
Zeeman effect	(a) The splitting of atomic spectral lines into two or more components in a transverse magnetic field. [DC99] (b) The splitting of spectral lines by a magnetic field. [A84] (c) Line broadening due to the influence of magnetic fields. A multiplet of lines is produced, with distinct polarization characteristics. The Zeeman effect is measured by measuring the difference between right-hand and left-hand polarization across a spectral line. [H76]
Zel'dovich Spectrum	A particular prescription for how much clumping of matter should occur on each length scale. Specifically, the Zel'dovich spectrum proposes that the strength of inhomogeneity, of clumping, should be the same for each length scale at the moment when that length scale is equal to the size of the horizon. (See horizon.) [LB90]
Zener Diode	A semiconductor diode with high doping levels on each side of the junction. If the junction is reverse-biased, breakdown occurs at a well-defined potential, giving a sharp increase in current. The effect is called Zener breakdown; it occurs because electrons are excited directly from the valence band into the conduction band. Zener diodes are used as voltage regulators. See also diode. [DC99]
Zenith	(a) The point in the sky directly overhead. [McL97] (b) The point on the celestial sphere directly above the observer's head - i.e., opposite to the direction of gravity (cf. nadir). [H76]
Zenith Distance	Angular distance on the celestial sphere measured along the great circle from the zenith to the celestial object. Zenith distance is 90° minus altitude. [S92]
Zeolite	An absorbent material (in the form of small pellets) used in low-temperature cryostats to trap gases released gradually after active pumping has ceased. See outgassing. [McL97]
Zepto-	Symbol: z A prefix denoting 10-21. For example, 1 zeptometer (zm) = 10-21 meter (m). [DC99]
Zerilli's Equation	A Schrödinger-type equation for even-parity perturbations on the Schwarzschild metric. [H76]
Zero-Age Main Sequence	The position on the H-R diagram for stars which have attained hydrostatic equilibrium and have started hydrogen burning in their cores, but which have not yet had time to produce an observable change in their chemical composition (ZAMS). [H76]
Zeropoint	The magnitude corresponding to one data number per second from the array detector for a star of zero color term (like Vega) corrected for absorption in the Earth's atmosphere. [McL97]
Zero-Point Energy	(a) The energy of the lowest state of a quantum system. Amount of vibrational energy allowed by quantum mechanics to be associated with atomic particles at 0 K, whereas classical mechanics requires this to be zero. Also, the energy of an electron in its ground state. [H76] (b) In quantum mechanics nothing of interest can have zero energy and quantum fluctuations lead to a kind of jittering motion even at lowest energies. This minimum energy due to these quantum fluctuations is termed zero-point energy. [D89]
Zero-Point Pressure	The pressure contributed by degenerate electrons, which do not come to rest even at absolute zero. [H76]
Zeroth Law of Thermodynamics	Law of thermal equilibrium. [H76]
Zetta-	Symbol: Z A prefix denoting 1021. For example, 1 zettameter (Zm) = 1021 meter (m). [DC]
Zinc	A bluish-white transition metal, applied as a coating (galvanizing) to protect steel from corrosion. Symbol: Zn; m.p. 419.58°C; b.p. 907°C; r.d. 7.133 (20°C); p.n. 30; r.a.m. 65.39. [DC99]
Zino	The supersymmetric partner of the Z boson.
Zirconium	(a) Element with atomic number 40. It arises almost entirely from the s-process, in red giant stars. [C95] (b) A hard lustrous silvery transition element that occurs in a gemstone, zircon (ZrSiO4). It is used in some strong alloy steels. Symbol: Zr; m.p. 1850°C; b.p. 4380°C; r.d. 6.506 (20°C); p.n. 40; r.a.m. 91.224. [DC99]
Zodiac	(a) A band about 8° wide on the celestial sphere, centered on the ecliptic. [H76] (b) Twelve constellations originally only representing a calendar of the Sun's apparent progress in the heavens during one Earth-year. The principal planets are to be found along much the same path (the ecliptic) and so, probably early in human history, caused each constellation to become a focus for divination according to the pseudo-science of astrology. Since then, however, the millennia that have passed have taken the Sun out of phase with the original calendar. The 12 constellations are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. [A84]
Zodiacal Light	A faint glow that extends away from the Sun in the ecliptic plane of the sky, visible to the naked eye in the western sky shortly after sunset or in the eastern sky shortly before sunrise. Its spectrum indicates it to be sunlight scattered by interplanetary dust. (Pioneer 10 has determined that its brightness varies inversely as the square of the distance out to 2.25 AU and then decreases more rapidly.) The zodiacal light contributes about a third of the total light in the sky on a moonless night. [H76]
Zone of Avoidance	An irregular zone near the plane of the Milky Way where the absorption due to interstellar dust is so great that no external galaxies can be seen through it. [H76]
Z particle	(a) A particle that is identical to the photon in all respects except mass. It conveys part of the weak force between hadrons and leptons, and its existence was a distinctive prediction of the Glashow-Salam-Weinberg theory of the amalgamated weak-electromagnetic force. [D89] (b) Massive bosons thought to have been abundant in the early universe, when the unified electroweak force was manifest. [F88]
Zwicky Compact Galaxy	A type of very high-surface-brightness galaxy first brought to general attention by Fritz Zwicky. [BFM02]