Astronomical Glossary

U(1)
the symmetry group associated with electromagnetic gauge invariance. [D89]
U line
A sodium line at 3302 Å. [H76]
UBV Photometric System
(a) Measurement of the astronomical color index of a star, utilizing the ultraviolet, blue and yellow visual images over two pre-set wavelengths obtained by photoelectric filtering. Other standardized filter wavebands are also used. [A84]
(b) A system of stellar magnitudes devised by Johnson and Morgan at Yerkes which consists of measuring an object's apparent magnitude through three color filters: the ultraviolet (U) at 3600 Å; the blue (B) at 4200 Å; and the "visual" (V) in the green-yellow spectral region at 5400 Å. It is defined so that, for A0 stars, B - V = U - B = 0; it is negative for hotter stars and positive for cooler stars. The Stebbins-Whitford-Kron six-color system (U, V, B, G, R, I) is defined so that B + G + R = 0. [H76]
UCD
Ultra-Compact Dwarf galaxy. [BFM03]
UBVRI
Designations for parts of the optical waveband, isolated by means of special glass filters which eliminate the unwanted regions, and used for standard astronomical intensity measurements. [McL97]
UHF
Ultra-High Frequency A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m). [DC99]
Uhuru
A satellite devoted entirely to the study of cosmic X-ray sources. It was launched off the coast of Kenya on 1970 December 12. [H76]
ULIRG
Ultra-Luminous InfraRed Galaxy

Ultra-High Frequency
UHF A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m). [DC99]
Ultramicroscopic
Length scales shorter than the Planck length (and also time scales shorter than the Planck time). [G99]
Ultra-Relativistic Having velocities very nearly equal to the velocity of light (E >> mc²). [H76]
Ultrashort-Period Cepheids see Delta Scuti Stars. [H76]
Ultraviolet
(a) Part of the electromagnetic spectrum immediately above visible light (but below Gamma-rays and X-rays); it therefore comprises a range of radiation of shorter wavelength and higher frequency than those of visible light. (UV) [A84]
(b)A form of electromagnetic radiation, shorter in wavelength than visible light. Ultraviolet wavelengths range between about 1 nm and 400 nm. Ordinary glasses are not transparent to these waves; quartz is a much more effective material for making lenses and prisms for use with ultraviolet.
Like light, ultraviolet radiation is produced by electronic transitions between the outer energy levels of atoms. The distinction between the two types of radiation is in fact physiological rather than physical. However, having a higher frequency, ultraviolet photons carry more energy than those of light. [DC99]
Ultraviolet Astronomy
Astronomy carried out in the waveband 300nm to about 10nm. At these wavelengths, the atmosphere is opaque to radiation and hence these astronomies have to be conducted from above the Earth's atmosphere. The wavelength band 300 to 120nm can be successfully explored using telescopes which form a natural extension of optical techniques. At shorter wavelengths, new approaches have to be taken because the mirror materials become nonreflecting. In addition at short ultraviolet wavelengths, lambda < 91.2 nm, the inrerstellar medium is likely to become opaque because of Lyman-continuum absorption. [D89]
Ultraviolet-Bright Stars
Stars that are brighter than the horizontal-branch stars and bluer than the giant-branch stars. [H76]
Ultraviolet Catastrophe
In the late nineteenth century it was realized that the short-wavelength region of black-body radiation could not be explained by the theories of physics of the time (classical physics). The problem - sometimes called the ultraviolet catastrophe - was resolved by the concept of quantization of energy. (see Planck's Radiation Law) [DC99]
Ultraviolet Excess
Property of a star that emits more ultraviolet radiation than one would have expected, based on its visual color. In general, the greater the ultraviolet excess, the lower the star's metallicity, because metals in a star's atmosphere absorb ultraviolet radiation. [C95]
Ultraviolet Excess Screening
Technique devised by Sir Martin Ryle and Allan Sandage to measure the spectral red shift of suspected quasars. It was this process that resulted in the discovery of quasi-stellar objects. [A84]
Ultraviolet Ga Stars
A small group of Bp stars, which in the ultraviolet spectrum exhibit a strong 1414 line of GaII. [JJ95]
Ultraviolet Light
Electromagnetic radiation of a wavelength slightly shorter than that of visible light. [F88]
Ultraviolet Radiation
Electromagnetic radiation "beyond the violet" with wavelengths in the approximate range 100-4000 Å. [H76]
Ultraviolet Stars
Very hot prewhite-dwarf stars; usually the hot central stars of planetary nebulae which are contracting toward the white-dwarf state. [H76]
Umbra
The portion of a shadow cone in which none of the light from an extended light source (ignoring refraction) can be observed. [S92]
Umbriel
A satellite of Uranus about 400 km in diameter (period 4.1 days). Discovered by Lassell in 1851. [H76]
Umklapp Scattering
The contribution to scattering caused when the exchange of momentum crosses the boundary of a Brillouin zone. [H76]
Uncertainty Principle
(a) The principle that the fundamental uncertainty in a variable times that in its canonical conjugate is of the order of Planck's constant: Delta x Delta p = h. Thus the uncertainty in the measurement of the position of an electron varies inversely as the uncertainty in the measurement of its momentum. A corollary is that it is impossible to measure an atomic or nuclear process without at the same time disturbing or altering the process. [H76]
(b) Principle of quantum mechanics, discovered by Heisenberg, that there are features of the universe, like the position and velocity of a particle, that cannot be known with complete precision. Such uncertain aspects of the microscopic world become ever more severe as the distance and time scales on which they are considered become ever smaller. Particles and fields undulate and jump between all possible values consistent with the quantum uncertainty. This implies that the microscopic realm is a roiling frenzy, awash in a violent sea of quantum fluctuations. [G99]
(c) Also called the Heisenberg Uncertainty Principle, a fundamental result of quantum mechanics stating that the position and speed of a particle cannot be simultaneously known with complete certainty. If one is known with high certainty, the other becomes very uncertain. The product of uncertainty in position and uncertainty in momentum (mass multiplied by speed) is equal to a constant, Planck's constant. Since both initial position and initial speed are required to forecast the future position of a particle, the uncertainty principle prevents completely accurate predictions of the future from the past, even if all the laws of physics are known. [LB90]
(d) The uncertainty principle is a consequence of quantum theory. It implies that a pair of observables cannot both be measured simultaneously to arbitrary accuracy. It can often be used to understand quantum theory results in a simple way. [K2000]
Unification
Scientists have sought for centuries to unify the descript ions of apparently different phenomena by showing that they were due to the same underlying natural laws and that complex levels of matter were made of simpler levels. This unification process is a subject of very active research about the forces of nature today. The possible unification of the strong, electromagnetic, and weak forces is called a grand unification. There is a continuing effort to unify these forces with gravity. String theories seem to do that successfully. [K2000]
Unified Field Theory
A single theory to account for the electromagnetic, gravitational, strong, and weak interactions by one set of equations. So far, attempts to find such a theory have been unsuccessful, although there has been some progress in unifying the weak and electromagnetic interactions. [DC99]
Unified Theories
(a) 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. [F88]
(b) Any theory that describes all four forces and all of matter within a single, all-encompassing framework. Also called the Unified Field Theory. [G99]
(c) Attempts to unite the theories of the strong, electromagnetic, and weak forces of nature. Ultimately it is hoped that gravity will also be incorporated in this scheme. [D89]
Uniform Vibration
The overall motion of a string in which it moves without changes in shape. [G99]
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. [F88]
Unitarity
The principle of conservation of probability. An example would be that if a particle can decay by several modes, the sum of the fractions taking each particular mode should add up to one. [H76]
Unitary Transformation A transformation (U) whose reciprocal is equal to its Hermitian conjugate. [H76]
Universal Attraction
see Law of Universal Attraction. [A84]
Universal Time (UT)
(a) The local mean time of the prime meridian. It is the same as Greenwich mean time, counted from 0 hours beginning at Greenwich mean midnight. UT0 is uncorrected; UT1 is corrected for the Chandler wobble; UT2 is corrected both for the Chandler wobble and for seasonal changes in Earth's rotation rate. [H76]
(b) A measure of time that conforms, within a close approximation, to the mean diurnal motion of the Sun and serves as the basis of all civil timekeeping. UT is formally defined by a mathematical formula as a function of sidereal time. Thus UT is determined from observations of the diurnal motions of the stars. The timescale determined directly from such observations is designated UT0; it is slightly dependent on the place of observation. When UT0 is corrected for the shift in longitude (see Longitude; Terrestrial) of the observing station caused by polar motion, the timescale UT1 is obtained. Whenever the designation UT is used in this volume, UT1 is implied. [S92]
Universality
The phenomenon whereby many microscopically quite different physical systems exhibit critical point behavior in with quantitatively identical features such as critical indices. [D89]
Universality Class
This is a way of classifying the behavior of systems near the critical points of continuous phase transitions. Systems in the same universality class have the same behavior in the critical region, when an appropriate matching is made between the physical variables, and have the same critical exponents. [D89]
UTC
Universal Time Coordinated [LLM96]
Universe
The total celestial cosmos. According to Gott et al. the universe seems to be on a large scale isotropic, homogeneous, matter-dominated, and with negligible pressure. The total proper mass content of about 10²³ M (Sandage derives 10⁵⁶ g from his determination of the deceleration parameter q₀) and radius of about 2 × 10²⁸ cm are the order of magnitude that most cosmologists would accept if the universe is bounded. Total mass contributed by luminous matter, about 3 × 10⁵³ g (see Mass Discrepancy). Age about 18 × 10⁹ yr for a Hubble constant H₀ = 55 km s^-1 Mpc^-1. [H76]
Unstable Equilibrium
Equilibrium such that if the system is disturbed a little, there is a tendency for it to move farther from its original position rather than to return. (see Equilibrium; Stability) [DC99]
Upsilon Particle
(a) A very massive (9 GeV) meson built from a bottom quark and bottom antiquark. Discovered in 1977, it is a member of the most massive family of particles known at present. [D89]
(b) A type of heavy lepton. (see Leptons) [LB90]
Uranium

A toxic radioactive silvery element of the actinoid series of metals. Its three naturally occurring radioisotopes, ²³⁸U (99.283% in abundance), ²³⁵U (0.711%), and ²³⁴U (0.005%), are found in numerous minerals including the uranium oxides pitchblende, uraninite, and carnotite. The readily fissionable ²³⁵U is a major or nuclear fuel and nuclear explosive, while ²³⁸U is a source of fissionable ²³⁹Pu.
Symbol: U; m.p. 1132.5°C; b.p. 3745°C; r.d. 18.95 (20°C); p.n. 92; r.a.m. 238.0289. [DC99]
Uranium-Lead Dating
A method of radioactive dating used for estimating the age of certain rocks. It is based on the decay of ²³⁸U to ²⁰⁶Pb (half-life 4.5 × 10⁹ years) or ²³⁵U to ²⁰⁷Pb (half-life 0.7 × 10⁹ years). The technique is useful for time periods from 10⁷ years ago back to the age of the Earth (about 4.6 × 10⁹ years). [DC99]
Uranus
Seventh planet from the Sun, discovered by Herschel 1781 March 13. Mass 8.78 × 10²⁸ g; radius 25,400 km; oblateness 0.07. Mean density 1.21 g cm^-3. Rotation period 10^h49^m26^s retrograde. Mean distance from Sun 19.18 AU. Orbital-period 84.0 years; orbital velocity 6.8 km s^-1; e = 0.04; i = 0^°.8; obliquity 97^°.9. Escape velocity 22 km s^-1; surface gravity 0.96 Earth's. Synodic period 369.66 days. Albedo 0.66. Maximum brightness +5.7 mag. Surface temperature about 110 K. Atmosphere H₂ and CH₄. Five satellites, all of which orbit in its equatorial plane. [H76]
URCA process
A series of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation and postulated as a cause of stellar collapse. Neutrinos carry away energy quickly and invisibly, so this process was named for the Urca Casino in Rio de Janeiro, which carried away money the same way. [H76]
W Ursae Majoris Stars
A large class of double-lined eclipsing binaries with very short periods (a few hours) whose spectra indicate mass transfer. They are distinguished by the fact that their primary and secondary minima are equal. They are all F or G binaries on or near the main sequence. They may be the progenitors of dwarf novae. [H76]
Ursa Minor
The Little Bear (or Little Dipper), a constellation in the northern sky that contains Polaris, the North Star. [C95]
Ursa Minor Dwarf Galaxy
An intrinsically faint (M_v approx - 9) dwarf elliptical galaxy about 70 pc distant, in the Local Group. [H76]
U Velocity
The component of a star's motion away from the Galactic center. If a star moves away from the Galactic center, the star's U velocity is positive; if a star moves toward the Galactic center, the U velocity is negative; and if the star moves neither toward nor away from the Galactic center, the U velocity is zero. The Sun has a U velocity of -9 kilometers per second, so the Sun is moving toward the Galactic center at 9 kilometers per second. [C95]
UV
Ultraviolet: wavelengths shorter than about 350 nm. [McL97]
UV Stars
see Ultraviolet Stars.
uvby System
A four-color, intermediate-bandwidth, photometric systemdevised by Strömgren consisting of measures in the ultraviolet, violet, blue, and yellow regions of the spectrum. [BFM02]

U(1)	the symmetry group associated with electromagnetic gauge invariance. [D89]
U line	A sodium line at 3302 Å. [H76]
UBV Photometric System	(a) Measurement of the astronomical color index of a star, utilizing the ultraviolet, blue and yellow visual images over two pre-set wavelengths obtained by photoelectric filtering. Other standardized filter wavebands are also used. [A84] (b) A system of stellar magnitudes devised by Johnson and Morgan at Yerkes which consists of measuring an object's apparent magnitude through three color filters: the ultraviolet (U) at 3600 Å; the blue (B) at 4200 Å; and the "visual" (V) in the green-yellow spectral region at 5400 Å. It is defined so that, for A0 stars, B - V = U - B = 0; it is negative for hotter stars and positive for cooler stars. The Stebbins-Whitford-Kron six-color system (U, V, B, G, R, I) is defined so that B + G + R = 0. [H76]
UCD	Ultra-Compact Dwarf galaxy. [BFM03]
UBVRI	Designations for parts of the optical waveband, isolated by means of special glass filters which eliminate the unwanted regions, and used for standard astronomical intensity measurements. [McL97]
UHF	Ultra-High Frequency A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m). [DC99]
*Uhuru*	A satellite devoted entirely to the study of cosmic X-ray sources. It was launched off the coast of Kenya on 1970 December 12. [H76]
ULIRG	Ultra-Luminous InfraRed Galaxy
Ultra-High Frequency	UHF A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m). [DC99]
Ultramicroscopic	Length scales shorter than the Planck length (and also time scales shorter than the Planck time). [G99]
Ultra-Relativistic	Having velocities very nearly equal to the velocity of light (E >> mc²). [H76]
Ultrashort-Period Cepheids	see Delta Scuti Stars. [H76]
Ultraviolet	(a) Part of the electromagnetic spectrum immediately above visible light (but below Gamma-rays and X-rays); it therefore comprises a range of radiation of shorter wavelength and higher frequency than those of visible light. (UV) [A84] (b)A form of electromagnetic radiation, shorter in wavelength than visible light. Ultraviolet wavelengths range between about 1 nm and 400 nm. Ordinary glasses are not transparent to these waves; quartz is a much more effective material for making lenses and prisms for use with ultraviolet. Like light, ultraviolet radiation is produced by electronic transitions between the outer energy levels of atoms. The distinction between the two types of radiation is in fact physiological rather than physical. However, having a higher frequency, ultraviolet photons carry more energy than those of light. [DC99]
Ultraviolet Astronomy	Astronomy carried out in the waveband 300nm to about 10nm. At these wavelengths, the atmosphere is opaque to radiation and hence these astronomies have to be conducted from above the Earth's atmosphere. The wavelength band 300 to 120nm can be successfully explored using telescopes which form a natural extension of optical techniques. At shorter wavelengths, new approaches have to be taken because the mirror materials become nonreflecting. In addition at short ultraviolet wavelengths, < 91.2 nm, the inrerstellar medium is likely to become opaque because of Lyman-continuum absorption. [D89]
Ultraviolet-Bright Stars	Stars that are brighter than the horizontal-branch stars and bluer than the giant-branch stars. [H76]
Ultraviolet Catastrophe	In the late nineteenth century it was realized that the short-wavelength region of black-body radiation could not be explained by the theories of physics of the time (classical physics). The problem - sometimes called the ultraviolet catastrophe - was resolved by the concept of quantization of energy. (see Planck's Radiation Law) [DC99]
Ultraviolet Excess	Property of a star that emits more ultraviolet radiation than one would have expected, based on its visual color. In general, the greater the ultraviolet excess, the lower the star's metallicity, because metals in a star's atmosphere absorb ultraviolet radiation. [C95]
Ultraviolet Excess Screening	Technique devised by Sir Martin Ryle and Allan Sandage to measure the spectral red shift of suspected quasars. It was this process that resulted in the discovery of quasi-stellar objects. [A84]
Ultraviolet Ga Stars	A small group of Bp stars, which in the ultraviolet spectrum exhibit a strong 1414 line of GaII. [JJ95]
Ultraviolet Light	Electromagnetic radiation of a wavelength slightly shorter than that of visible light. [F88]
Ultraviolet Radiation	Electromagnetic radiation "beyond the violet" with wavelengths in the approximate range 100-4000 Å. [H76]
Ultraviolet Stars	Very hot prewhite-dwarf stars; usually the hot central stars of planetary nebulae which are contracting toward the white-dwarf state. [H76]
Umbra	The portion of a shadow cone in which none of the light from an extended light source (ignoring refraction) can be observed. [S92]
Umbriel	A satellite of Uranus about 400 km in diameter (period 4.1 days). Discovered by Lassell in 1851. [H76]
Umklapp Scattering	The contribution to scattering caused when the exchange of momentum crosses the boundary of a Brillouin zone. [H76]
Uncertainty Principle	(a) The principle that the fundamental uncertainty in a variable times that in its canonical conjugate is of the order of Planck's constant: xp = h. Thus the uncertainty in the measurement of the position of an electron varies inversely as the uncertainty in the measurement of its momentum. A corollary is that it is impossible to measure an atomic or nuclear process without at the same time disturbing or altering the process. [H76] (b) Principle of quantum mechanics, discovered by Heisenberg, that there are features of the universe, like the position and velocity of a particle, that cannot be known with complete precision. Such uncertain aspects of the microscopic world become ever more severe as the distance and time scales on which they are considered become ever smaller. Particles and fields undulate and jump between all possible values consistent with the quantum uncertainty. This implies that the microscopic realm is a roiling frenzy, awash in a violent sea of quantum fluctuations. [G99] (c) Also called the Heisenberg Uncertainty Principle, a fundamental result of quantum mechanics stating that the position and speed of a particle cannot be simultaneously known with complete certainty. If one is known with high certainty, the other becomes very uncertain. The product of uncertainty in position and uncertainty in momentum (mass multiplied by speed) is equal to a constant, Planck's constant. Since both initial position and initial speed are required to forecast the future position of a particle, the uncertainty principle prevents completely accurate predictions of the future from the past, even if all the laws of physics are known. [LB90] (d) The uncertainty principle is a consequence of quantum theory. It implies that a pair of observables cannot both be measured simultaneously to arbitrary accuracy. It can often be used to understand quantum theory results in a simple way. [K2000]
Unification	Scientists have sought for centuries to unify the descript ions of apparently different phenomena by showing that they were due to the same underlying natural laws and that complex levels of matter were made of simpler levels. This unification process is a subject of very active research about the forces of nature today. The possible unification of the strong, electromagnetic, and weak forces is called a grand unification. There is a continuing effort to unify these forces with gravity. String theories seem to do that successfully. [K2000]
Unified Field Theory	A single theory to account for the electromagnetic, gravitational, strong, and weak interactions by one set of equations. So far, attempts to find such a theory have been unsuccessful, although there has been some progress in unifying the weak and electromagnetic interactions. [DC99]
Unified Theories	(a) 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. [F88] (b) Any theory that describes all four forces and all of matter within a single, all-encompassing framework. Also called the Unified Field Theory. [G99] (c) Attempts to unite the theories of the strong, electromagnetic, and weak forces of nature. Ultimately it is hoped that gravity will also be incorporated in this scheme. [D89]
Uniform Vibration	The overall motion of a string in which it moves without changes in shape. [G99]
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. [F88]
Unitarity	The principle of conservation of probability. An example would be that if a particle can decay by several modes, the sum of the fractions taking each particular mode should add up to one. [H76]
Unitary Transformation	A transformation (U) whose reciprocal is equal to its Hermitian conjugate. [H76]
Universal Attraction	see Law of Universal Attraction. [A84]
Universal Time (UT)	(a) The local mean time of the prime meridian. It is the same as Greenwich mean time, counted from 0 hours beginning at Greenwich mean midnight. UT0 is uncorrected; UT1 is corrected for the Chandler wobble; UT2 is corrected both for the Chandler wobble and for seasonal changes in Earth's rotation rate. [H76] (b) A measure of time that conforms, within a close approximation, to the mean diurnal motion of the Sun and serves as the basis of all civil timekeeping. UT is formally defined by a mathematical formula as a function of sidereal time. Thus UT is determined from observations of the diurnal motions of the stars. The timescale determined directly from such observations is designated UT0; it is slightly dependent on the place of observation. When UT0 is corrected for the shift in longitude (see Longitude; Terrestrial) of the observing station caused by polar motion, the timescale UT1 is obtained. Whenever the designation UT is used in this volume, UT1 is implied. [S92]
Universality	The phenomenon whereby many microscopically quite different physical systems exhibit critical point behavior in with quantitatively identical features such as critical indices. [D89]
Universality Class	This is a way of classifying the behavior of systems near the critical points of continuous phase transitions. Systems in the same universality class have the same behavior in the critical region, when an appropriate matching is made between the physical variables, and have the same critical exponents. [D89]
UTC	Universal Time Coordinated [LLM96]
Universe	The total celestial cosmos. According to Gott et al. the universe seems to be on a large scale isotropic, homogeneous, matter-dominated, and with negligible pressure. The total proper mass content of about 10²³ M (Sandage derives 10⁵⁶ g from his determination of the deceleration parameter q₀) and radius of about 2 × 10²⁸ cm are the order of magnitude that most cosmologists would accept if the universe is bounded. Total mass contributed by luminous matter, about 3 × 10⁵³ g (see Mass Discrepancy). Age about 18 × 10⁹ yr for a Hubble constant H₀ = 55 km s^-1 Mpc^-1. [H76]
Unstable Equilibrium	Equilibrium such that if the system is disturbed a little, there is a tendency for it to move farther from its original position rather than to return. (see Equilibrium; Stability) [DC99]
Upsilon Particle	(a) A very massive (9 GeV) meson built from a bottom quark and bottom antiquark. Discovered in 1977, it is a member of the most massive family of particles known at present. [D89] (b) A type of heavy lepton. (see Leptons) [LB90]
Uranium	A toxic radioactive silvery element of the actinoid series of metals. Its three naturally occurring radioisotopes, ²³⁸U (99.283% in abundance), ²³⁵U (0.711%), and ²³⁴U (0.005%), are found in numerous minerals including the uranium oxides pitchblende, uraninite, and carnotite. The readily fissionable ²³⁵U is a major or nuclear fuel and nuclear explosive, while ²³⁸U is a source of fissionable ²³⁹Pu. Symbol: U; m.p. 1132.5°C; b.p. 3745°C; r.d. 18.95 (20°C); p.n. 92; r.a.m. 238.0289. [DC99]
Uranium-Lead Dating	A method of radioactive dating used for estimating the age of certain rocks. It is based on the decay of ²³⁸U to ²⁰⁶Pb (half-life 4.5 × 10⁹ years) or ²³⁵U to ²⁰⁷Pb (half-life 0.7 × 10⁹ years). The technique is useful for time periods from 10⁷ years ago back to the age of the Earth (about 4.6 × 10⁹ years). [DC99]
Uranus	Seventh planet from the Sun, discovered by Herschel 1781 March 13. Mass 8.78 × 10²⁸ g; radius 25,400 km; oblateness 0.07. Mean density 1.21 g cm^-3. Rotation period 10^h49^m26^s retrograde. Mean distance from Sun 19.18 AU. Orbital-period 84.0 years; orbital velocity 6.8 km s^-1; e = 0.04; i = 0^°.8; obliquity 97^°.9. Escape velocity 22 km s^-1; surface gravity 0.96 Earth's. Synodic period 369.66 days. Albedo 0.66. Maximum brightness +5.7 mag. Surface temperature about 110 K. Atmosphere H₂ and CH₄. Five satellites, all of which orbit in its equatorial plane. [H76]
URCA process	A series of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation and postulated as a cause of stellar collapse. Neutrinos carry away energy quickly and invisibly, so this process was named for the Urca Casino in Rio de Janeiro, which carried away money the same way. [H76]
W Ursae Majoris Stars	A large class of double-lined eclipsing binaries with very short periods (a few hours) whose spectra indicate mass transfer. They are distinguished by the fact that their primary and secondary minima are equal. They are all F or G binaries on or near the main sequence. They may be the progenitors of dwarf novae. [H76]
Ursa Minor	The Little Bear (or Little Dipper), a constellation in the northern sky that contains Polaris, the North Star. [C95]
Ursa Minor Dwarf Galaxy	An intrinsically faint (M_v - 9) dwarf elliptical galaxy about 70 pc distant, in the Local Group. [H76]
U Velocity	The component of a star's motion away from the Galactic center. If a star moves away from the Galactic center, the star's U velocity is positive; if a star moves toward the Galactic center, the U velocity is negative; and if the star moves neither toward nor away from the Galactic center, the U velocity is zero. The Sun has a U velocity of -9 kilometers per second, so the Sun is moving toward the Galactic center at 9 kilometers per second. [C95]
UV	Ultraviolet: wavelengths shorter than about 350 nm. [McL97]
UV Stars	see Ultraviolet Stars.
uvby System	A four-color, intermediate-bandwidth, photometric systemdevised by Strömgren consisting of measures in the ultraviolet, violet, blue, and yellow regions of the spectrum. [BFM02]