LBV	Luminous Blue Variables -- A variable-star designation for the high-luminosity early type objects. Also called S Dor variables or Hubble-Sandage Variables. [JJ95]
L Component	The part of the Solar corona whose spectrum consists of emission lines. [H76]
L Galaxy	In Morgan's classification, an elongated galaxy of low surface brightness. [H76]
L-magnitude	The magnitude derived from observations at an infrared wavelength of 3.5 microns. [H76]
l-number	The orbital quantum number, which determines the magnitude of an electron's angular momentum. [H76]
BL Lacertae	A highly variable object (the most rapid radio variable known, also an optically violent variable - mv = 12 to 15 mag - and an infrared source). Probably an exceedingly compact nonthermal object, and undoubtedly extragalactic. Its optical spectrum is characterized by an absence of lines, so its redshift cannot be measured. (In 1974 Oke and Gunn infer z = 0.07 from an H absorption line in the surrounding halo and conclude that it lies at the center of a bright (Mv = - 23) elliptical. If true, this would make BL Lac the nearest known quasar.) [H76]
BL Lac Object	A member of a class of astronomical objects with the following characteristics: (1) rapid variations in intensity at radio, infrared, and optical wavelengths; (2) energy distributions such that most of the energy is emitted at infrared wavelengths; (3) absence of discrete features in low-dispersion spectra; and (4) strong and rapidly varying polarization at visual and radio wavelengths. [H76]
Lagoon Nebula	An emission nebula in Sagittarius 2 kpc distant. (M8, NGC 6523) [H76]
Lagrangian	(a) A mathematical expression summarizing the properties and interactions of a physical system. It is essentially the difference between the kinetic energy and potential energy of the system. Moreover, one can derive the system's dynamical equations of motion directly from the Lagrangian. [CD99] (b) A mathematical starting point of most theories in particle physics. The Lagrangian of a theory contains the assumptions of the theory; from the Lagrangian, the laws and consequences of the theory may be derived. [LB90] A Lagrangian is an equation that contains representations o f all of the fundamental particles in the world and specifies how they interact. Given the Lagrangian, the rules of quantum theory specify how to calculate the behavior of the particles, how to build up all of the composite systems they form, and all the consequences of the basic theory. [K2000]
Lagrangian Points	Five points in the orbital plane of two massive particles in circular orbits around a common center of gravity, where a third particle of negligible mass can remain in equilibrium. Three of the points are on the line passing through the centers of mass of the two bodies - L2 beyond the most massive body, L1 (the point through which mass transfer occurs) between the two bodies, and L3 beyond the less massive body. All three of these points are in unstable equilibrium. The other two (L4 and L5) are stable, and are located at the two points in the orbit of the less massive component which are equidistant from the two main components. see Trojans [H76]
Lallemand Camera	A very early form of image tube.
Lamb Shift	(a) A minute correction to the energy levels of atoms (specifically the first excited state of the hydrogen atom) predicted by quantum electrodynamics, and confirmed to great accuracy by Willis Lamb. [D89] (b) The difference in energy levels of the H atom between 2S1/2 and 2P4. [H76] (c) A tiny change in the energy of an electron orbiting the nucleus of an atom, caused by the interaction of the electron with other particles that appear and quickly disappear in "quantum fluctuations". see Quantum Fluctuations [LB90]
Lambda ()	see Cosmological Constant [C95]
Lambda Boo Stars	A-type stars with weak metallic lines, low rotational velocity and low radical velocity. [JJ95]
Lambda Doublet	Two lines in the microwave region of the spectrum of the OH molecule caused by splitting of electronic levels. [H76]
Lambda Term	see Cosmological Constant [LB90]
Lambert's Law	(a) First proposed in 1760; then restricted to visible light, it is now used with all radiations. The law concerns the rate of absorption of radiation as it travels deeper into a medium. It states that equal thicknesses of the medium absorb equal proportions of the incident radiation. In other words, the intensity I of the transmitted radiation falls off exponentially with distance d in the medium: I = I0 exp -d Here I0 is the intensity of the initially incident radiation, and is the linear absorption coefficient of the medium. As well as depending on the medium, varies with wavelength. (b) In photometry, the fact that the luminous intensity of a diffuse surface varies with angle of view: I0 = I0 cos Here, I0 is the intensity along the normal, while I is that along a line at angle to the normal. The principle is often called Lambert's cosine law. [DC99] (c) The intensity of the light emanating in a given direction from a perfectly diffusing surface is proportional to the cosine of the angle of emission measured between the normal to the surface and the emitted ray. [H76]
Laminar Flow	Steady flow in which the fluid moves past a surface in parallel layers of different velocities. Compare turbulent flow. [DC99]
LAN	Local Area Network -- A means of interlinking computers. [McL97]
Landau Damping	Damping caused by electrons that are moving at the phase velocity of the wave. It is analogous to a surfer who will be carried along by a wave if he is already moving at the velocity of the wave when it hits him. [H76]
Landé Factor	The constant of proportionality relating the separations of lines of successive pairs of adjacent components of the levels of a spectral multiplet to the larger of the two J-values for the respective pairs. The interval between two successive components J and J + 1 is proportional to J + 1. [H76]
Lane-Emden Equation	A second-order nonlinear differential equation describing the structure of polytropes. [H76]
Lanthanium
Laplacian Determinism	Clockwork conception of the Universe in which complete knowledge of the state of the Universe at one moment completely determines its state at all future and past moments. [G99]
Laplacian Plane	For planets see Invariable Plane; for a system of satellites, the fixed plane relative to which the vector sum of the disturbing forces has no orthogonal component. [S92]
Large Magellanic Cloud	The nearest and largest of the many galaxies that orbit the Milky Way. It is 160,000 light-years away. [C95]
Large-Scale Structure	The distribution of galaxies and other forms of mass on large distance scales, covering hundreds of millions of light years and larger. A perfectly homogeneous and isotropic Universe would have no large-scale structure; a Universe with all the galaxies lined up in single file would have enormous large-scale structure. [LB90]
Large-Scale Motions	Bulk motions of distant galaxies deviating from the Hubble flow. see Hubble Law [LB90]
Larmor Frequency	The frequency of precession of a charged particle orbiting in a uniform magnetic field. It is equal to eH / 4 me, where e is the electron charge, me is the electron mass, and H is the magnetic field strength. [H76]
Larmor Radius	The radius of the circular orbit that a charged particle describes transverse to a magnetic field. [H76]
Laser	The word laser stands for Light Amplification by Stimulated Emission of Radiation. Proposed by A. Schawlow and C. Townes, the first operating laser was constructed by T. Maiman. Lasers generate intense directional beams of coherent radiation through stimulated emission provided sufficient energy is provided to maintain a large number of radiating atoms in the laser. [D89] (b) A maser which emits radiation at optical wavelengths. [H76] (c) An especially intense beam of light at a specific wavelength. The word is derived from Light Amplification by Stimulated Emission of Radiation. Requires overpopulation of certain energy levels. [McL97]
Last Scattering Surface
Late Type Stars	(a) Stars of spectral type later than the sun (G2). [JJ95] (b) Stars of spectral classes K, M, S, and C. [H76]
Latitude, Terrestrial	(b) On Earth, distance north or south on the equator along a line connecting the poles. [F88] (b) Angular distance on the Earth measured north or south of the equator along the meridian of a geographic location. [S92]
Latitude, Celestial	Angular distance on the celestial sphere measured north or south of the ecliptic along the great circle passing through the poles of the ecliptic and the celestial object. [S92]
Lattice	(a) A regular solids are characterized by the arrangement of the atoms on a set of regularly spaced points known as the lattice sites. [D89]
Lattice Gas	A model of a condensed system in which atoms may be present on or absent from the sites of a lattice, but no movement of the sites or distortion of the lattice is allowed.[D89]
Law	A theory of such wide and invariable application that its violation is thought to be impossible. [F88]
Law of Universal Attraction	Isaac Newton's formulation of the law of gravity. [A84]
Lawrencium	A radioactive transuranic element of the actinoid series, not found naturally on Earth. Several very short-lived isotopes have been synthesized by bombarding 252Cf with boron nuclei or 249Bk with 18O nuclei. Symbol: Lr; p.n. 103; most stable isotope 262Lr (half-life 261 minutes). [DC99]
Laws of Physics
LBG	Lyman Break Galaxy
Lead	A dense, dull, gray, soft metallic element; the end product of radioactive decay series. It occurs in small quantities in a wide variety of minerals but only a few are economically important. Lead is used in (lead-acid) accumulators, alloys, radiation shielding, and water and sound proofing. It is also used in the petrochemical, paint, and glass industries. Symbol: Pb; m.p. 327.5°C; b.p. 1830°C; r.d. 11.35 (20°C); p.n. 82; r.a.m. 207.2. [DC99]
Leap Second	A second (see Second, Système International) added between 60s and 0s at announced times to keep UTC within 0s.90 of UT1. Generally, leap seconds are added at the end of June or December. [S92]
Least Action	see Action [P88]
Least Squares, Principle of	A principle which states that the best estimate of an experimental quantity, deducible from a number of observations, is that for which the sum of the weighted squares of the residuals is least. [H76]
LED	Light-Emitting Diode -- A semiconductor diode, made from certain materials (e.g. Gallium Arsenide), in which light is emitted in response to the forward-bias current. The light results from the recombination of electrons and positive holes, with a transition to a lower energy state. see also Diode [DC99]
LEDA	Lyon/Meudon Extragalactic Database
Lemaître Universe	A big-bang cosmology proposed by the Belgian Abbey Lemaître in 1929 in which the Universe is assumed to have exploded from a primeval "atom". In the Lemaître Universe the rate of expansion steadily decreases. [H76]
Lennard-Jones Potential	An approximation of the interaction between two atoms or molecules. [H76]
Lense-Thirring Effect	The precession of the plane of the geodesic orbit of a test particle around a rotating mass in general relativity. It arises from the coupling of the rotation of the central mass with the orbital angular momentum of the test particle. This precession is described as resulting from the dragging of inertial frames. [H76]
Lensing	see Gravitational Lensing [C95]
Lenz's Law	The current induced by an electromotive force will appear in such a direction that it opposes the charge that produced it. [H76]
Leo I	The most distant galaxy that orbits the Milky Way. A dwarf galaxy, Leo I was discovered in 1950 and lies 890,000 light-years from the Galactic center. It is close to the bright star Regulus, whose glare interferes with the study of the galaxy. [C95]
Leo II	The second most distant galaxy that orbits the Milky Way, lying 720,000 light-years from the Galactic center. Like Leo I, Leo II is a dwarf galaxy that was discovered in 1950.[C95]
Leo Systems	Two dwarf elliptical galaxies (about 220-250 kpc distant) belonging to the Local Group. Leo I (dE3), Mv - 11, diameter 1.8 kpc; Leo II, Mv = -9.5, diameter 1.3 kpc. [H76]
Leonid Meteor Shower	Shower of meteors emanating from an apparent point in Leo every 33 years; the most recent one having occurred in 1999.
17 Leporis	A close binary system (A0 V, M1 III) with a shell-like spectrum indicating that mass transfer may be occurring from the late-type companion onto the A0 primary. [H76]
Lepton	(a) A fermion which is not made of quarks. [C97] (b) The generic name for any spin-1/2 particle which does not feel the strong nuclear force. The six known leptons are the electron, the muon, the tau lepton, and their respective neutrinos. The name was originally coined to refer to light particles. [CD99] (c) The generic name for certain particles of matter. The presently known ones are electrons, muons, tau particles and neutrinos. They experience electromagnetic and weak forces, but unlike the quarks, not the strong QCD forces (leptons have no `color' quantum numbers). Leptons belong to the class of particles known as fermions. [D89] (d) Any fermion that does not participate in the strong interactions. Leptons include the electron family and the muon family. [H76] (e) Elementary particles that do not participate in the strong interactions, including electrons, muons, and neutrinos. [Silk90] (f) Elementary particles that have no measurable size and are not influenced by the strong nuclear force. Electrons, muons, and neutrinos are leptons. [F88] (g) A class of non-strongly-interacting particles that includes the electron, muon, tau, and their associated neutrinos. [G97] (h) Fundamental particles of nature, which may interact via all of the fundamental forces except the strong nuclear force. The electron is an example of a lepton. [LB90] (i) Elementary particles like the electron and neutrino that do not experience the strong nuclear force. Unlike the strongly interacting hadrons, the leptons have small masses. [P88] (j) A class of particles defined by certain properties: leptons are fermions with spin 1/2 that do not carry color charge and that have another property called lepton number that is different for each family. The known leptons are the electron, the muon, the tau, and their associated neutrinos. [K2000]
Lepton Era	The era following the hadron era, when the temperature had dropped to about 1012 K and when the Universe consisted mainly of leptons and photons. It started about 10-4 s after the big bang and lasted until about 10 s after the big bang; it was followed by the radiation era. [H76]
Leptonic Era	The era following the Hadronic Era, when the Universe consisted mainly of leptons and photons. It began when the temperature dropped below 1012 K some 10-4 seconds after the Big Bang, and it lasted until the temperature fell below 1010 K, at an era of about 1 second. At this stage, the characteristic photon energy fell below the rest mass energy of an electron, and the abundance of electron-positron pairs fell by many orders of magnitude. Only one electron survived for every 108 photons. The Universe was subsequently radiation-dominated (substantial numbers of neutrinos were also present, but they did not interact directly with the matter or the radiation). [Silk90]
LERG	Low-Excitation Radio Galaxy
LHe	The symbol for Liquid Helium. The temperature of liquid helium is normally 4 K, that is, four degrees above absolute zero. [McL97]
LHEA	Laboratory for High Energy Astrophysics
Li-rich stars	A subgroup of C stars, with a very strong LiI 6078 line. [JJ95]
Libration	(a) The "turning" of the Moon so that although the same face is presented to Earth at all times, the overall surface of the Moon visible is 59% of the total. Libration is described as latitudinal, longitudinal and diurnal. [A84] (b) Any of several oscillations in the apparent aspect of the Moon as seen from Earth, which, when combined, enable Earth-based observers over a period of time to see altogether about 59 percent of the Moon's surface. Physical librations are angular motions about the center of mass due to gravitational torques on the Moon. Optical librations are the apparent rotations of the Moon, caused by our observing it from somewhat different directions at different times. [H76]
Libration Orbits	see Lagrangian Points [H76]
Librations	Variations in the orientation of the Moon's surface with respect to an observer on the Earth. Physical librations are due to variations in the rate at which the Moon rotates on its axis. The much larger optical librations are due to variations in the rate of the Moon's orbital motion, the obliquity of the Moon's equator to its orbital plane, and the diurnal changes of geometric perspective of an observer on the Earth's surface. [S92]
LIDAR	LIght Detection And Ranging [LLM96]
Life Zone	The region around a star where a planet can have liquid water and so may support life. [C95]
Light	Electromagnetic radiation with wavelengths of or close to those detectable by the eye. [F88]
Light Clock	A hypothetical clock that measures elapsed time by counting the number of round-trip journeys completed by a single photon between two mirrors. [G99]
Light Cone	(a) The history, in space-time, of a light flash. [D89] (b) A cone representing the transmission, at the speed of light, of an event's existence on a spacetime diagram. [C97] (c) The set of all directions in which a light signal can travel toward an event (past light cone) or from an event (future light cone). [H76] (d) Any event, such as an explosion, affects other regions of the Universe by sending out light and heat and other forms of energy in all directions. None of these signals can travel faster than the speed of light. Therefore, at each moment after the event, the most distant region of space that can have been affected by the event lies at the distance light has traveled since the event. This most distant affected region, at each moment of time, is called the "light cone" of the event. The light cone spreads out in time, just like ripples spreading out from a stone thrown in a pond. [LB90] (e) Light emanating from a source forms, in three-dimensional space, a sphere that expands in time. In space-time, however, it produces a conical structure. Points within this light cone can be causally connected together. The light cone structure is of key importance in relativity. The light cone is unchanged by conformal transformations. [P88]
Light-Emitting Diode	LED -- A semiconductor diode, made from certain materials (e.g. gallium arsenide), in which light is emitted in response to the forward-bias current. The light results from the recombination of electrons and positive holes, with a transition to a lower energy state. see also Diode [DC99]
Lightest Superpartner	The superpartner with the least mass. The LSP may have seve ral important roles. In particular, it may be the cold dark matter of the universe, and its properties are crucial for identifying the events of superpartner production at colliders, because all of the heavier superpartners decay into the lightest one. [K2000]
Lifetime	The time it takes for a sample of identical particles to decay to 1/e of its initial population (e 2.718). A related concept is "half-life", being the time it takes for the number of particles to halve. Half-life, 1/2, is related to lifetime, , by 1/2 = (ln 2). [CD99]
Light Curve	A plot of magnitude or intensity versus time for a variable star. [H76]
Light Cylinder	The cylinder whose radius is that at which the rotational velocity of a neutron star would equal the speed of light. RL = cT / 2, where RL is the radius of the light cylinder and T is the period. [H76]
Light, Deflection of	The bending of the beam of light due to gravity. It is observable when the light from a star or planet passes a massive object such as the Sun. [S92]
Light Elements	(a) Usually, hydrogen, helium, and lithium, which have atomic numbers of one, two, and three; sometimes also beryllium and boron, which have atomic numbers of four and five. [C95] (b) In astrophysics, usually Li, Be, and B. [H76]
Light Pressure	see Radiation Pressure [H76]
Light-Time	The interval of time required for light to travel from a celestial body to the Earth. During this interval the motion of the body in space causes an angular displacement of its apparent place from its geometric place. see Geometric Position; Aberration, Planetary [S92]
Light-Year	(a) Distance traveled at the speed of light after one Earth-year: 9.46 million million km. [A84] (b) The distance light travels in one year: 5.88 trillion miles, or 9.46 trillion kilometers. The nearest star system to the Sun is 4.3 light-years away. [C95] (c) The distance light travels in a vacuum in 1 year. 1 lt-yr = 9.4605 × 1012 km = 0.307 pc (c = 299,792.46 km s-1 = 186,274 miles s-1). (1 lt-min 0.13 AU.) [H76]
LIGO	Large Interferometric Gravitational Wave Observatory
Limb	(a) Apparent edge of the disk of a Solar System body as projected on the sky. see Terminator [H76] (b) The apparent edge of the Sun, Moon, or a planet or any other celestial body with a detectable disc. [S92]
Limb Brightening	Increase in the intensity of radio or X-ray brightness of the Sun or other star from its center to its limb. [H76]
Limb Correction	Correction that must be made to the distance between the center of mass of the Moon and its limb. These corrections are due to the irregular surface of the Moon and are a function of the librations in longitude (see Longitude, Celestial) and latitude (see Latitude, Celestial) and the position angle from the central meridian. [S92]
Limb Darkening	Decrease in, and reddening of, the optical brightness of the Sun or other star from its center to its limb. [H76]
Limit Cycle	The attractor describing a time-periodic regime of a dissipative dynamical system. In the phase space a limit cycle is represented by a closed curve. [D89]
Lindblad Resonance	A resonance hypothesized by Lindblad in the 1920s in his attempt to explain the existence of spiral arms (see Density Wave Theory). It is a resonance which occurs when the frequency at which a star encounters the galactic spiral wave is a multiple of its epicyclic frequency. The inner Lindblad resonance occurs whenever the ratio of the frequency of the radial oscillation to that of the rotational motion around the center of the galaxy (in a frame of reference rotating together with the spiral pattern) is 2:1. [H76]
Linear Electron Collider	Particles traveling in a curved path continuously radiate p hotons that carry away some of the particles' energy. The fraction of energy radiated increases with the energy of the particle, and the radiation happens with greater probability for lighter particles than for heavier ones. For electrons this loss of energy is a large effect at the circular CERN LEP collide r, and it would be worse at a higher-energy collider, so it is unlikely that any future electron collider will be circular. The next electron collider built is expected to be a linear one, where the radiated energy loss is greatly decreased (NLC, for Next Linear Collider), modeled on the first linear collider, the SLC at SLAC. [K2000]
Line Blanketing	The combined effects of spectral lines upon the emergent energy distribution from and the temperature distribution in a stellar or planetary atmosphere. [H76]
Line Broadening	Increase in the range of wavelengths in which some characteristic emission or absorption occurs, due to a number of causes (e.g., Doppler broadening, the effects of perturbers, etc.). [H76]
Line Profile	A plot of intensity versus wavelength across a spectral line. [H76]
Line Spectrum	(a) A spectrum composed of a number of discrete lines corresponding to single wavelengths of emitted or absorbed radiation. Line spectra are produced by atoms or simple (monatomic) ions in gases. Each line corresponds to a change in electron orbit, with emission or absorption of radiation. [DC99] (b) A pattern of lines, each corresponding to an image of the entrance slit of the spectrometer, seen when light is either emitted by or interrupted by a hot rarefied gas such as hydrogen. The pattern is characteristic of the gas and the wavelength at which the features are observed to occur is indicative of the velocity of the object. [McL97]
Line Width	The width of a spectral line in wavelength terms. The usual measure is the half-width of the line. see Monochromatic Radiation [DC99]
Line Wings	Broad "wings" that appear on either side of a spectral line when the number of atoms producing the line is very great. [H76]
LINER	Low-Ionization Nuclear Emission-line Region
Liouville's Theorem	The Equation of Continuity: For a general Hamiltonian system the volume of an element of phase space is invariant with respect to the equations of motion. As applied to astronomy, the difference between the number of the stars moving into a volume of six-dimensional phase space at a given time and those moving out of it at the same time must be equal to the increase in the distribution function for those stars. [H76]
Liquid Crystal	substances intermediate in their properties between liquids and crystals. There is considerable variety in the type of structure that they can have, but in general they have the anisotropy of a complex crystalline solid but no crystalline long range order, and they can flow like a liquid. [D89]
LISA	Large Interferometric Space Antenna
Lithium	(a) Lightest of all solid elements, third in the periodic table after hydrogen and helium. Its atom comprises one proton and three electrons. One of the electrons is at a higher energy level than the other two. [A84] (b) Element with atomic number three. Some lithium formed in the big bang, along with huge amounts of hydrogen and helium. Lithium has two stable isotopes: the rarer, lithium-6 (three protons and three neutrons); and the more common, lithium-7 (three protons and four neutrons). [C95] (c) A light silvery moderately reactive metal; it is a rare element accounting for 0.0065% of the Earth's crust. Symbol: Li; m.p. 180.54°C; b.p. 1347°C; r.d. 0.534 (20°C); p.n. 3; r.a.m. 6.941. [DC99] (d) The chemical element with atomic number 3, meaning that there are three protons in the nucleus. The most common isotope is lithium-7 (92.5% of naturally occurring lithium), which has four neutrons in its nucleus. [G97]
Lithium Stars	Peculiar giant stars (spectral types G-M) whose spectra show high abundances of lithium. They are primarily S stars and carbon stars, although Li is also found in T Tauri stars, and is sometimes observed in normal late-type giants. Interstellar Li / H 2 x 10-10. (Solar System Li / H 10-9.) Lithium is destroyed rapidly (in about 7500 years) at typical nuclear burning temperatures. Spallation is the only production mechanism known for 6Li, but 7Li can be transported from the core in the form of 7Be and converted in the envelope to 7Li by electron capture. Observed 7Li / 6Li > 10. [H76]
Little Bangs	Hypothetical explosions of supermassive stars shortly after the big bang, with the release of processed elements into the interstellar medium, postulated by Wagoner to account for the anomalously high abundance of Helium in the Universe, and for the fact that even the oldest stars have some metals. [H76]
Littrow	The configuration of a diffraction grating spectrograph in which the diffracted ray returns along the same direction as the incident ray. [McL97]
LN2	The symbol for Liquid Nitrogen. The temperature of liquid nitrogen is normally 77 K, that is, 77 degrees above absolute zero. [McL97]
Lobes	In radio astronomy, regions of sensitivity in an antenna pattern. Lobes are analogous to fringes in optical astronomy. [H76]
Local Arm	see Orion Arm [H76]
Local Bubble	The region of the Galaxy near the Sun which has little neutral hydrogen gas. It extends about a hundred light-years in most directions but up to a thousand in some. The local bubble may have been produced by supernovae. [C95]
Local Group	(a) The gravitationally bound collection of nearby galaxies ruled by the Andromeda Galaxy and the Milky Way, which are its largest members. The Local Group has about 30 known galaxies. [C95] (b) The association of galaxies to which the Milky Way galaxy belongs. [F88] (c) A conglomeration of about twenty galaxies including our own Milky Way. [G97] (d) The cluster of galaxies to which our Galaxy belongs. It is a poor, irregular cluster with some 20 certain members: three spirals (the Galaxy, M31, and M33); four irregulars (LMC, SMC, IC 1613, and NGC 6822); and about 13 intermediate or dwarf ellipticals (NGC 147; NGC 185; NGC 205; M32; the Sculptor, Fornax, Leo I and II, UMi, and Draco systems; and three companions to M31 discovered by van den Bergh in 1972). It may also include several other dwarf galaxies as well as the giant elliptical Maffei 1. The total mass of the Local Group is less than 1.5 times the combined masses of the Galaxy and M31. [H76] (d) The system of galaxies to which our Milky Way galaxy belongs is a small group, containing only two large spirals (our galaxy and the Andromeda galaxy, Messier 31) and twenty or more smaller systems. [Silk90] (e) The cluster of galaxies of which our own, the Milky Way, is one. Its radius is estimated at 106 parsecs. Largest of the Group is the Andromeda spiral galaxy. [A84]
Local Hypothesis (of quasars)	The hypothesis that quasars are not at the distances inferred from their redshifts. [H76]
Local Inertial Frame	A coordinate system or frame of reference defined in the vicinity of the earth in which Newtons first law of motion is valid; that is, a nonrotating and nonaccelerating reference frame. [Silk90]
Local Sidereal Time	The local hour angle of a catalog equinox. [S92]
Local Standard of Rest	(a) LSR -- An imaginary point, located at the Sun's distance from the Galactic center, that revolves clockwise around the Galaxy on a circular orbit. Astronomers measure a star's U, V, and W velocities with respect to the Local Standard of Rest rather than with respect to the Sun, because the Sun has a slightly noncircular orbit. The orbital velocity of the Local Standard of Rest around the Galaxy is about 220 kilometers per second. [C95] (b) A frame of reference in which the mean motion of stars in the immediate neighborhood is zero. In such a reference system, the motions of stars in the Solar neighborhood (a volume of space about 100 pc in diameter) average out to zero (see Solar Motion). It is a coordinate system in which the origin is a point in the Galactic plane moving in a circular orbit around the Galactic center with a period equal to that of the Sun, and in which the three velocity components are , in the direction from the galactic center to the origin; , in the direction of Galactic rotation; and Z, perpendicular to the galactic plane. [H76]
Local Supercluster	(a) The supercluster to which the Local Group belongs. It is shaped like a cigar, with the Virgo cluster of galaxies at its center and the Local Group near one end. [C95] (b) A loosely knit assemblage of some 100 clusters of galaxies, including the Local Group. [G97] (c) The supercluster of galaxies to which the Local Group may belong (see Virgo Supercluster). de Vaucouleurs lists 54 groups of galaxies in the LSC (R 17 Mpc). [H76]
Local Thermodynamic Equilibrium	LTE -- The assumption that all distribution functions characterizing the material and its interaction with the radiation field at a point in the star are given by thermodynamic equilibrium relations at local values of the temperature and density. [H76]
Localisation	the wave-function of an electron is said to be localized if it is confined to a small region of a large system rather than being extended through the system. [D89]
Logistic Equation	models the growth of a population as a competition between self-reproduction on the one side and inhibition arising from density-dependent effects on the other side. [D89]
Lommel-Seeliger Surface	A surface with large-scale roughness where shadowing effects are important. [H76]
Long Distance Scale	The cosmological distance scale which uses a Hubble constant of approximately 50 km/s/Mpc. [C97]
Longitude, Celestial	Angular distance on the celestial sphere measured eastward along the ecliptic from the dynamical equinox to the great circle passing through the poles of the ecliptic and the celestial object. [S92]
Longitude, Terrestrial	(a) On Earth, distance east or west of Greenwich, England, measured along lines drawn parallel to the equator. [F88] (b) Angular distance measured along the Earth's equator from the Greenwich meridian to the meridian of a geographic location. [S92]
Longitude of the Perihelion	For a Solar System body, the longitude of the ascending node plus the angle along the orbit from the node to the perihelion point. () [H76]
Longitudinal Wave	A wave vibrating along the direction of propagation - e.g., a sound wave. [H76]
Long-Period Variables	LPV -- Pulsating disk-population red giants or supergiants with periods of 100 to 1000 days (Population I typically have periods greater than 200 days; Population II, periods less than 200 days). Typical is Mira (o Ceti), which has a period of 331 days. Long-period variables may vary by as much as 9 magnitudes in the visible, but in the integrated spectrum (most of their radiation is in the infrared) they vary by only 2 or 3 magnitudes. They are usually of spectral type M, R, or N. (sometimes called Red Variables) [H76]
Look-Back Time	(a) 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. [F88] (b) The time in the past at which the light we now receive from a distant object was emitted. Galaxies of a certain type (redshift and luminosity) can be seen only at a certain distance. [H76]
Loop Nebula	see 30 Doradus [H76]
Lorentz Contraction	(a) 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. [F88] (b) Feature emerging from special relativity, in which a moving object appears shortened along its direction of motion. [G99]
Lorentz Force Equation	The equation relating the force on a charged particle to its motion in an electromagnetic field. [H76]
Lorentz Invariance	(a) The principle that physics in an inertial frame is independent of the velocity of the frame relative to any other frame. [D89] (b) The assumption that the laws of physics are identical for all observers moving at constant velocity relative to each other. Lorentz invariance is a basic tenet of Special Relativity and, consequently, of all of modem physics. see Special Relativity [LB90]
Lorentz Invariant	Invariant with respect to Lorentz transformations. [H76]
Lorentz Transformation	(a) The transformation which keeps the speed of light invariant between relativistic frames of reference. [C97] (b) A transformation which enables one to relate the physical parameters describing an object when viewed in one frame of reference to those which are appropriate to an observer moving with a uniform velocity in that frame. [H76]
Loschmidt Number (constant)	(a) Number of molecules of an ideal gas per unit volume (2.687 × 1019 molecules per cm3). [H76] (b) This is the number of molecules in a cubic metre of an ideal gas at s.t.p. It is equal to the Avogadro constant divided by the molar volume and its value is 2.6868 × 1025 m-3. The number was first derived by J. Loschmidt (1821-1895) in Vienna in 1865. In Germany it is sometimes called Avogadro's constant. [JM92]
Lossy	Subject to absorption of light. If a material is not lossy, it means that light would be scattered or reflected off. [H76]
Love Number	A measure of the rotational deformation of a rotating body in hydrostatic equilibrium. [H76]
Low Surface Brightness Galaxy	LSBG -- A galaxy which is very faint because it contains a very limited number of stars. [C97]
Low-Temperature Physics	usually defined as the physics of matter below about 20 degrees absolute (-253 degrees Celsius). [D89]
Low-Velocity Star	A star whose U, V, and W velocities are all near zero. Such stars have nearly circular orbits around the Galaxy. [C95]
Lowell's Band	A dark border sometimes found on the Martian polar cap. [H76]
LPV	Long-Period Variables -- Pulsating disk-population red giants or supergiants with periods of 100 to 1000 days (Population I typically have periods greater than 200 days; Population II, periods less than 200 days). Typical is Mira (o Ceti), which has a period of 331 days. Long-period variables may vary by as much as 9 magnitudes in the visible, but in the integrated spectrum (most of their radiation is in the infrared) they vary by only 2 or 3 magnitudes. They are usually of spectral type M, R, or N. (sometimes called Red Variables) [H76]
LS Coupling	A condition in an atom in which the spins couple to spins and orbital angular momenta couple to orbital angular momenta to form total spin and total orbital angular-momentum vectors which then couple to form the total angular momentum of the atom. In this case spin-spin and orbit-orbit torques exceed spin-orbit torques; the opposite extreme results in j-j coupling. (also called Russell-Saunders coupling)[H76]
LSB	Least Significant Bit see also DN [McL97]
LSBG	Low Surface Brightness Galaxy
LSR	(a) Local Standard of Rest -- An imaginary point, located at the Sun's distance from the Galactic center, that revolves clockwise around the Galaxy on a circular orbit. Astronomers measure a star's U, V, and W velocities with respect to the Local Standard of Rest rather than with respect to the Sun, because the Sun has a slightly noncircular orbit. The orbital velocity of the Local Standard of Rest around the Galaxy is about 220 kilometers per second. [C95] (b) A frame of reference in which the mean motion of stars in the immediate neighborhood is zero. In such a reference system, the motions of stars in the Solar neighborhood (a volume of space about 100 pc in diameter) average out to zero (see Solar Motion). It is a coordinate system in which the origin is a point in the Galactic plane moving in a circular orbit around the Galactic center with a period equal to that of the Sun, and in which the three velocity components are , in the direction from the galactic center to the origin; , in the direction of Galactic rotation; and Z, perpendicular to the galactic plane. [H76]
LTE	Local Thermodynamic Equilibrium -- The assumption that all distribution functions characterizing the material and its interaction with the radiation field at a point in the star are given by thermodynamic equilibrium relations at local values of the temperature and density. [H76]
Lumen	Symbol: lm -- The SI unit of luminous flux, equal to the luminous flux emitted by a point source of one candela in a solid angle of one steradian. 1 lm = 1 cd sr.[DC99]
Luminance	Symbol: Lv A measure of the brightness of an extended source (one that cannot be considered a point). In a given direction, it is the luminous intensity per unit area projected at right angles to the direction. The unit is the candela per square meter (cd m-2). [DC99]
Luminescence	(a) The emission of radiation from a substance in which the particles have absorbed energy and gone into excited states. They then return to lower energy states with the emission of electromagnetic radiation. If the luminescence persists after the source of excitation is removed it is called phosphorescence; if not, it is called fluorescence. The excitation of the particles may occur by a variety of mechanisms. Absorption of other electromagnetic radiation gives photoluminescence. If the original excited states are produced by bombardment with electrons the phenomenon is electroluminescence. Chemiluminescence is luminescence produced by chemical reactions. Bioluminescence occurs in natural systems, e.g. glowworms and fireflies. Radioluminescence occurs in radioactive materials. Luminescence produced in materials by friction is called Triboluminescence. see also Fluorescence [DC99] (b) In CCDs and infrared arrays-light emission caused by currents through faulty parts of the structure which behave like a light-emitting diode. [McL97]
Luminiferous Ether	A material that, prior to special relativity, was believed to permeate all of space, allowing the propagation of light. With the advent of relativity, we now believe that light can propagate without a medium. [G97]
Luminosity	(a) Brightness of a celestial body, measured in terms of (apparent) magnitude, absolute magnitude, or using the Sun's brightness as 1.0 on a Solar scale. The luminosity of a star corresponds with its internal radiation pressure, which in turn depends on its mass. [A84] (b) The total amount of energy radiated by a star - that is, its true, or intrinsic, brightness. [C95] (c) The intrinsic brightness of a star. Usually defined in terms of absolute magnitude. [F88] (d) Total radiant energy output per second (absolute brightness, usually expressed in ergs s-1 or in magnitudes). L = 4R2T4 see Stefan's Law [H76] (e)Any particle physics collider has two basic figures of merit: the maximum en ergy it can supply to the collisions and how often it can cause collisions to occur. The number of events at a collider over some period of time is the product of two factors: the probability that something will happen if two particles actually collide, and the number of collisions. The latter is a property of the collider, not of the physics that governs the collision. It is called the luminosity. It depends on how many particles can be accelerated, how tightly bunches of them can be packed, and so forth. Loosely speaking we can refer to the luminosity as the intensity. [K2000]
Luminosity Class	(a) A measure of a star's intrinsic brightness, as determined from the star's spectrum. Supergiants have luminosity class I, bright giants have luminosity class II, giants have luminosity class III, subgiants have luminosity class IV, and main-sequence stars have luminosity class V. [C95] (b) A classification of stellar spectra according to luminosity for a given spectral type. The luminosity class is an indication of a star's surface gravity and tells whether it is a dwarf, a giant, or supergiant. Luminosity classes (MKK system): 0, very brightest; Ia, supergiants of high luminosity; Ib, supergiants of low luminosity: II, luminous giants: III, normal giants; IV, subgiants; V, main-sequence stars. [H76]
Luminosity Distance	(a) Any distance to a celestial object which has been calculated using a standard candle. [C97] (b) If the intrinsic luminosity of a distant object is known, then measurement of its apparent brightness and application of the inverse square law enables the luminosity distance to be calculated. For very distant galaxies, spatial curvature is important, and the luminosity distance differs from other measures of distance. [Silk90]
Luminosity Function	(a) The number of stars in the Galaxy with a particular absolute magnitude. The luminosity function reveals that luminous stars are rare and intrinsically faint stars common. (b) The distribution of galaxies by absolute magnitude. Luminous galaxies are rare and intrinsically faint ones are common. [C95] (c) Number distribution of stars or galaxies with respect to their absolute magnitudes. The luminosity function shows the number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given volume of space. The stellar density in the Solar neighborhood is about 0.16 M per cubic parsec. [H76] (d) A quantitative description of the variation of luminosities among galaxies. The luminosity function gives the number of galaxies shining at each level of luminosity in each volume of space. [LB90] (e) The distribution of galaxies with respect to their luminosities. [Silk90]
Luminous	Intrinsically bright, as opposed to being just apparently bright. [C95]
Luminous Blue Variables	LBV -- A variable-star designation for the high-luminosity early type objects. Also called S Dor variables or Hubble-Sandage Variables. [JJ95]
Luminous Flux	Symbol: v The rate of flow of energy of visible radiation. It is the radiant flux corrected for the fact that the sensitivity of the eye is different for different wavelengths. The unit is the lumen (lm). [DC99]
Luminous Intensity	Symbol: Iv The luminous flux from a point source per unit solid angle. The unit is the candela (cd). [DC99]
Luminous Mass	The mass contributed by luminous matter in galaxies (see Missing Mass). Luminous mass density, 5 × 10-32 g cm-3 for H0 = 50 km s-1 Mpc-1. [H76]
Lumogen	A material used as a down-converter. [McL97]
Lunar	Of the Moon. [A84]
Lunar Phases	Cyclically recurring apparent forms of the Moon. New Moon, First Quarter, Full Moon, and Last Quarter are defined as the times at which the excess of the apparent celestial longitude (see Longitude, Celestial) of the Moon over that of the Sun is 0°, 90°,180°, and 270°, respectively. [S92]
Lunation	(a) The period of time between two successive new Moons (cf. synodic month). [H76] (b) The period of time between two consecutive New Moons. [S92]
Lunisolar Precession	That component of general precession that is caused by the gravitational coupling between the Moon and the Earth and between the Sun and the Earth. Lunisolar precession causes the equinox to move westward along the ecliptic about 50" per year (cf. planetary precession). [H76]
Lupus Loop	A radio source, a large broken shell 4°.5 in diameter, identified as a prehistoric supernova remnant. [H76]
LUT	Look-Up Table [McL97]
Lutetium	A silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids. Lutetium is a very rare lanthanoid and has few uses. Symbol: Lu; m.p. 1663°C; b.p. 3395°C; r.d. 9.84 (25°C); p.n. 71; r.a.m. 174.967. [DC99]
Luyten 726-8	A binary (M5.5e V, M6e V) (component B is UV Cet) with a very small mass (total mass of system [1974] 0.3 M ). It is about 2.7 pc distant. [H76]
Lyapounov Exponent	measures the rate of exponential separation of initially nearby states of a dynamical system. In a system undergoing chaotic dynamics there is at least one positive Lyapounov exponent. [D89]
Lyman Alpha	The spectral line at 1216 Å in the far ultraviolet that corresponds to the transition of an electron in the hydrogen atom between the two lowest energy levels. [McL97]
Lyman Alpha Clouds	Gas lying between us and quasars that absorbs some of the radiation from those quasars. see Quasars [LB90]
Lyman Alpha Line	The characteristic spectral line of atomic hydrogen associated with its lowest excited state. The corresponding wavelength is 1216 Å in the far ultraviolet, and so the Lyman alpha line can only be studied from spacecraft or in the spectra of highly redshifted quasars. [Silk90]
Lyman Forest	The appearance of many differentially redshifted Lyman- absorption lines in a quasar's spectrum, caused by intervening hydrogen clouds along our line of sight to the quasar. [C97]
Lyman Series	(a) A series of lines in the ultraviolet spectrum emitted by excited hydrogen atoms. They correspond to the atomic electrons falling into the lowest energy level and emitting energy as radiation. The wavelength () of the radiation in the Lyman series is given by 1/ = R(1/12 - 1/n2), where n is an integer and R is the Rydberg constant. see also Spectral Series [DC99] (b) The spectral series of the hydrogen atom associated with the first energy level or ground state. The series lies in the ultraviolet (L at 1215.67 Å; L at 1026 Å). The series limit is at 912 Å. (He II L, 303.78 Å; He II Lyman limit, 227 Å.) [H76]
Lyot Division	In Saturn's rings, the gap between rings B and C. [H76]
Lyrae	see Vega [H76]
Lyrae Stars	A class of eclipsing binaries whose secondary minima are intermediate between those of Persei and those of W UMa. The prototype Lyr (B8.5 II, F V) is a complex eclipsing system and is presently in a state of rapid mass transfer. The spectrum of one companion is invisible; it may be a black hole. Beta Lyrae is also a weak radio source. [H76]
RR Lyrae Stars	A large class of pulsating (amplitude variation about 1 mag) blue giants of anomalous spectral type (A2-F6) with periods of less than 1 day. Their average absolute magnitude is about +0.8. which makes them almost 50 times more luminous than the Sun. They are Population II objects often (but not always) present in globular clusters. RR Lyrae stars are valid distance indicators out to more than 200 kpc. (also called Cluster Variables) [H76]