An Etymological Dictionary of Astronomy and Astrophysics
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A → supercluster of galaxies that includes our → Local Group and about 300 to 500 known → galaxy clusters and groups. Also called → Local Supercluster. If approximated as round, it has a diameter of 12,000 km s^-1 in units of the → cosmic expansion or 160 megaparsecs, and encompasses about 10¹⁷ → solar masses. Our Local Group lies toward the outer regions of Laniakea. Its main components are the four previously known superclusters: → Virgo supercluster (the part where the → Milky Way resides), Hydra-Centaurus Supercluster (including the → Great Attractor, Antlia Wall, known as Hydra Supercluster, → Centaurus supercluster), Pavo-Indus Supercluster, and Southern Supercluster (including Fornax Cluster, Dorado and Eridanus clouds). The most massive galaxy clusters of Laniakea are Virgo, Hydra, Centaurus, Abell 3565, Abell 3574, Abell 3521, Fornax, Eridanus, and Norma. The Laniakea supercluster was discovered by Tully et al. (2014, Nature 513, 71).

From the Hawaiian words lani "heaven," and akea "spacious, immeasurable;" → supercluster.

Any of the series of 15 consecutive → chemical elements in the → periodic table from → lanthanum to lutetium (→ atomic numbers 57 to 71 inclusive). The atoms of these metals have similar configurations and similar physical and chemical properties. They are grouped apart from the rest of the elements in the → Periodic Table because they all behave in a similar way in chemical reactions. Also called → rare-earth element. International Union of Pure and Applied Chemistry currently recommends the name lanthanoid rather than lanthanide.

From the chemical element → lanthanum.

A soft, malleable, ductile, silver-white metallic → chemical element; symbol La. → atomic number 57; → atomic weight 138.9055; → melting point about 920°C; → boiling point about 3,460°C; → specific gravity 6.19 at 25°C; → valence +3. Lanthanum is a member of the → lanthanide group, also called → rare-earth elements. Two naturally occurring → isotopes of lanthanum are known, ¹³⁹La (more than 99%) and ¹³⁸La (less than 0.1%). The → half-life of ¹³⁸La is 1.1 x 10¹¹ years.

From lanthan- + suffix -um, variant of → -ium. The first component from Gk. lanthanein for "to lie hidden, to escape notice" because it hid in cerium ore and was difficult to separate from that rare-earth mineral. It was discovered in the form lanthanium oxide, called lanthana, by the Swedish surgeon and chemist Carl-Gustav Mosander (1797-1858) in 1839. Subsequently, in 1842, Mosander separated his lanthanium sample into two oxides; for one of these he retained the name lanthanum and for the other he gave the name didymium (or twin).

The French great mathematician, physicist, and astronomer Pierre-Simon Marquis de Laplace (1749-1827). → Laplace operator; → Laplace plane; → Laplace resonance; → Laplace transform; → Laplace's demon ; → Laplace's equation ; → Kant-Laplace hypothesis

Same as → Laplacian.

→ Laplace; → operator.

The plane normal to the axis about which the pole of a satellite's orbit → precesses. In his study of Jupiter's satellites, Laplace (1805) recognized that the combined effects of the solar tide and the planet's oblateness induced a "proper" inclination in satellite orbits with respect to Jupiter's equator. He remarked that this proper inclination increases with the distance to the planet, and defined an orbital plane (currently called Laplace plane) for circular orbits that lies between the orbital plane of the planet's motion around the Sun and its equator plane (Tremaine et al., 2009, AJ, 137, 3706).

→ Laplace; → plane.

An → orbital resonance that makes a 4:2:1 period ratio among three bodies in orbit. The → Galilean satellites → Io, → Europa, → Ganymede are in the Laplace resonance that keeps their orbits elliptical. This interaction prevents the orbits of the satellites from becoming perfectly circular (due to tidal interactions with Jupiter), and therefore permits → tidal heating of Io and Europa. For every four orbits of Io, Europa orbits twice and Ganymede orbits once. Io cannot keep one side exactly facing Jupiter and with the varying strengths of the tides because of its elliptical orbit, Io is stretched and twisted over short time periods.

This commensurability was first pointed out by Pierre-Simon Laplace, → Laplace; → resonance.

An integral transform of a function obtained by multiplying the given function f(t) by e^-pt, where p is a new variable, and integrating with respect to t from t = 0 to t = ∞.

→ Laplace; → transform.

An imaginary super-intelligent being who knows all the laws of nature and all the parameters describing the state of the Universe at a given moment can predict all subsequent events by virtue of using physical laws. In the introduction to his 1814 Essai philosophique sur les probabilités, Pierre-Simon Laplace puts forward this concept to uphold → determinism, namely the belief that the past completely determines the future. The relevance of this statement, however, has been called into question by quantum physics laws and the discovery of → chaotic systems.

→ Laplace; → demon.

A → linear differential equation of the second order the solutions of which are important in many fields of science, mainly in electromagnetism, fluid dynamics, and is often used in astronomy. It is expressed by: ∂²V/ ∂x² + ∂²V/ ∂y² + ∂²V/ ∂z² = 0. Laplace's equation can more concisely expressed by: ∇²V = 0. The function V may, for example, be the potential at any point in the electric field where there is no free charge. The general theory of solutions to Laplace's equation is known as potential theory.

→ Laplace; → equation.

A differential → operator, denoted ∇² = ∇.∇, which is the sum of all second partial derivatives of a dependant variable: ∇²≡ ∂²/∂x² + ∂²/∂y² + ∂²/∂z², in Cartesian coordinates. It has numerous applications in several fields of physics and mathematics. Also called Laplace operator.

Named after → Laplace.

Of more than average size, quantity, degree, etc.; of great scope or range.

From O.Fr. large "broad, wide," from L. largus "abundant, copious, plentiful," of unknown origin.

Bozorg "great, large, immense, grand, magnificient;" Mid.Pers. vazurg "great, big, high, lofty;" O.Pers. vazarka- "great;" Av. vazra- "club, mace" (Mod.Pers. gorz "mace"); cf. Skt. vájra- "(Indra's) thunderbolt," vaja- "strength, speed;" L. vigere "be lively, thrive," velox "fast, lively," vegere "to enliven," vigil "watchful, awake;" P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen "to be awake;" E. wake); PIE base *weg- "to be strong, be lively."

The larger of the two Magellanic Cloud galaxies visible in the southern hemisphere at about 22 degrees from the South Celestial Pole. It is approximately on the border between the constellations → Dorado and → Mensa in a region of faint stars. The center of the LMC is approximately RA: 5h 23m 35s, dec: -69° 45' 22''. The LMC shines with a total → apparent visual magnitude of approximately zero. It spans an area of the sky about 9 by 11 degrees, corresponding to about 30,000 → light-years across in the longest dimension, for a distance of some 162,000 light-years. It has a visible mass of about one-tenth that of our own Galaxy (10¹⁰ Msun). The LMC and its twin, the → Small Magellanic Cloud, are two of our most prominent Galactic neighbors. The LMC is classified as a disrupted → barred spiral galaxy of type SBm, the prototype of a class of → Magellanic spirals. The galaxy is characterized by a prominent offset → stellar bar located near its center with the dominant → spiral arm to the north with two "embryonic" arms situated to the south. The → metallicity in the LMC is known to be lower than in the solar neighborhood by a factor 2 or more. Based on 20 → eclipsing binary systems, the distance to the LMC is measured to one percent precision to be 49.59±0.09 (statistical) ±0.54 (systematic) kpc (Pietrzynski et al., 2019, Nature 567, 200).

→ large; → Magellanic; → cloud.

A → dimensionless number representing the ratio of various → physical constants. For example:
1) The ratio of the → Coulomb force to the → gravitational force for a proton-electron pair (e²/Gm_pm_e), which is of the order of 10⁴⁰.
2) The age of Universe (T = 10-20 x 10⁹ years) in units of the → elementary time: T/t_e≅ 10⁴⁰.
3) The square root of the total number of particles in the Universe ≅ 10⁴⁰.

→ large; → number.

The idea whereby the coincidence of various → large numbers would bear a profound sense as to the nature of physical laws and the Universe. Dirac suggested that the coincidence seen among various large numbers of different nature is not accidental but must point to a hitherto unknown theory linking the quantum mechanical origin of the Universe to the various cosmological parameters. As a consequence, some of the → fundamental constants cannot remain unchanged for ever. According to Dirac's hypothesis, atomic parameters cannot change with time and hence the → gravitational constant should vary inversely with time (G∝ 1/t). Dirac, P. A. M., 1937, Nature 139, 323; 1938, Proc. R. Soc. A165, 199.

→ large; → number; → hypothesis.

A turbulent flow in which viscous forces are negligible compared to nonlinear advection terms, which characterize the variation of fluid quantities. The dynamics becomes generally turbulent when the Reynolds number is high enough. However, the critical Reynolds number for that is not universal, and depends in particular on boundary conditions.

→ large; → Reynolds number; → flow.

1) A scale representing measures that significantly override the usual ones of the same kind.
2) In meteorology, a scale in which the curvature of the earth is not negligible.

→ large; → scale.

Initial name given to → Vera C. Rubin Observatory.

→ large; → synoptic; → survey; → telescope.

The distribution of galaxies and other forms of mass on large distance scales, covering hundreds of millions of → light-years.

→ large; → scale; → structure.

The fifth of Neptune's known satellites. It orbits 73,600 km from Neptune and is a non spherical object about 208 × 178 km in size. It was discovered using NASA's Voyager 2 mission in 1989.

In Gk. mythology, Larissa is a princess of Argos (in central Greece) who, according to some, bore Poseidon three sons: Akhaios, Pelasgos and Pythios (though others gave these eponymous heroes different parents).