Fr.: couronne K
The inner part of the → solar corona which extends to about two solar radii. It is due to the → Thomson scattering of light from the → photosphere by the free electrons in the corona. The K corona exhibits a → linearly polarized continuous spectrum. The high speeds of the scattering electrons (on the average 10,000 km s-1 for a temperature of 2 million K) smear out the → Fraunhofer lines except the → H and K lines.
Fr.: correction K
A → color index correction applied to the photometric magnitudes and colors of a distant galaxy to compensate for the "reddening" of the galaxy due to → cosmological redshift. K correction is intended to derive the magnitudes in the → rest frame of the galaxy. Typically it is given as K(z) = az + bz2, where a and b depend on galaxy types. Conversely, one may deduce the redshift of a galaxy by its colors and a K-correction model.
The term K correction, probably stems from the K-term used by C. W. Wirtz (1918, Astron. Nachr. 206, 109), where K stands for Konstante, the German word for constant. The K-term was a constant offset in the redshift applied to diffuse nebulae in that epoch (source: A. L. Kinney, 1996, ApJ 467, 38); → correction.
setâre-ye K (#)
Fr.: étoile de type K
An orange-red star of → spectral type K with a surface temperature of about 3600-5000 K. The spectra of K stars are dominated by the H and K lines of calcium and lines of neutral iron and titanium, with molecular bands due to cyanogen (CN) and titanium dioxide (TiO). Examples are → Arcturus and → Aldebaran.
K the letter of alphabet; → star.
ruydâd-e K-T (#)
Fr.: événement K-T
Same as the → Cretaceous-Tertiary event.
Fr.: effet de Kaiser
The observed peculiar velocities of galaxies in the → redshift space of → galaxy clusters when the galaxies undergo → infall toward a central mass. This → redshift space distortion differs from the → fingers of God in that the peculiar velocities are not random, but correspond to the coherent falling of galaxies toward the central mass. See also → peculiar velocity.
Kaiser, N., 1987, MNRAS 227, 1; → effect.
Fr.: hypothèse de Kant-Laplace
The hypothesis of the origin of the solar system proposed first by Kant (1755) and later by Laplace (1796). According to this hypothesis, the solar system began as a nebula of tenuous gas. Particles collided and gradually, under the influence of gravitation, the condensing gas took the form of a disk. Larger bodies formed, moving in circular orbits around the central condensation (the Sun).
Named after the German prominent philosopher Immanuel Kant (1724-1804) and the French great mathematician, physicist, and astronomer Pierre-Simon Marquis de Laplace (1749-1827); → hypothesis.
Any of a group of four short-lived → mesons distinguished by a → quantum number called → strangeness. Also called K meson and denoted K. They are positive, negative, or neutral and have a mass of about 495 MeV/c (about 970 times that of an → electron).
Kaon, from ka (for the letter K) + (mes)on, → meson.
Fr.: mécanisme κ
A process based on the effects of → opacity (κ) that drives the → pulsations of many types of variable stars. Consider a layer of material within a star and suppose that it undergoes inward contraction. This inward motion tends to compress the layer and increase the density. Therefore the layer becomes more opaque (See also → partial ionization zone). If a certain amount of flux comes from the deeper layers it gets stuck in the high κ region. The energy accumulates and heat builds up beneath it. The pressure rises below the layer, pushing it outward. The layer expands as it moves outward, cools and becomes more transparent to radiation. Energy can now escape from below the layer, and pressure beneath the layer diminishes. The layer falls inward and the cycle repeats. The κ mechanism is believed to account for the pulsations of several star families, including → Delta Scuti stars, → Beta Cephei variables, → Cepheids, and → RR Lyrae stars (See Baker & Kippenhahn, 1962, Zeitschrift für Astrophysik 54, 114). Same as κ effect and → valve mechanism. See also → gamma mechanism.
κ, the Gk. letter which denotes opacity; → mechanism.
Fr.: échelle de Kardashev
A way of measuring a civilization's technological advancement based upon how much usable energy it has at its disposal. The scale was originally designed in 1964 by the Russian astrophysicist Nikolai Kardashev (who was looking for signs of extraterrestrial life within cosmic signals). It has three base classes, each with an energy disposal level: Type I, Type II, and Type III. Type I designates a civilization that is capable of controlling the total energy of its home planet (1016 watts). Type II is an interstellar civilization, capable of harnessing the total energy output of a star (1026 W). And Type III represents a galactic civilization, capable of inhabiting and harnessing the energy of an entire galaxy (1036 W). The scale has since been expanded by another four. Type 0 is civilization that harnesses the energy of its home planet, but not to its full potential. The Earth civilization is currently at about 0.73 on the Kardashev scale.
The scale was originally designed in 1964 by the Russian astrophysicist Nikolai Kardashev (1932-); → scale.
Fr.: lacune de Keeler
After James A. Keeler (1857-1908); → gap.
cešmi-ye Kellner (#)
Fr.: oculaire de Kellner
The first achromatic eyepiece consisting of a convex lens and a plano-convex lens. The convex surfaces are turned toward one another.
Named after the inventor Carl Kellner (1826-1855), a German engineer and optician; → eyepiece
The basic unit of thermodynamic temperature in the international system of units (→ SI units). The kelvin is also the fundamental unit of the → Kelvin scale. One kelvin represents the same temperature difference as one degree → Celsius. It was previously called the degree Kelvin (°K).
Named after the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907), one of the most influential scientists of the 19th century.
Fr.: échelle de Kelvin
A temperature scale, redefined in 1954, in which the zero point is equivalent to -273.16 °C. This fundamental fixed point, based on the → triple point of water, is considered to be the lowest possible temperature of anything in the Universe. Also known as the absolute temperature scale.
Fr.: postulat de Kelvin
A transformation whose only final result is to transform into work heat extracted from a source which is at the same temperature is impossible. Kelvin's postulate is a statement of the → second law of thermodynamics and is equivalent to → Clausius's postulate.
Fr.: contraction de Kelvin-Helmholtz
After the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907) and the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → contraction.
nâpâydâri-ye Kelvin-Helmholtz (#)
Fr.: instabilité de Kelvin-Helmholtz
An → instability raised when there is sufficient velocity difference across the interface between two uniformly moving → incompressible fluid layers, or when velocity → shear is present within a continuous fluid.
Fr.: mécanisme Kelvin-Helmholtz
The heating of a body that contracts under its own gravity. For a large body like a planet or star, gravity tries to compress the body. This compression heats the core of the body, which results in internal energy which in turn is radiated as → thermal energy. In this way a star could be heated by its own weight.
William Thomson (Lord Kelvin) and Hermann von Helmholtz proposed that the sun derived its energy from the conversion of gravitational potential energy; → mechanism.
Fr.: échelle de temps de Kelvin-Helmholtz
The characteristic time that would be required for a contracting spherical cloud of gas to transform all its → gravitational energy into → thermal energy. It is given by the relation: tKH ≅ GM2/RL, where G is the → gravitational constant, M is the mass of the cloud, R the initial radius, and L the → luminosity. The Kelvin-Helmholtz time scale determines how quickly a pre-main sequence star contracts before → nuclear fusion starts. For the Sun it is 3 x 107 years, which also represents the time scale on which the Sun would contract if its nuclear source were turned off. Moreover, this time scale indicates that the gravitational energy cannot account for the solar luminosity. For a → massive star of M = 30 Msun, the Kelvin-Helmholtz time is only about 3 x 104 years.
lâye-ye Kennelly-Heaviside (#)
Fr.: couche de Kennelly-Heaviside
One of several layers in the Earth's ionosphere occurring at 90-150 km above the ground. It reflects medium-frequency radio waves whereby radio waves can be propagated beyond the horizon.
Named after the American electrical engineer Arthur Edwin Kennelly (1861-1939) and the English physicist Oliver Heaviside (1850-1925), who independently predicted the existence of the reflecting layer in 1902; → layer.
Johannes Kepler (1571-1630), a German mathematician and astronomer and a key figure in the 17th century astronomical revolution. He discovered that the Earth and planets travel about the Sun in elliptical orbits; gave three fundamental laws of planetary motion, and also did important work in optics and geometry.