An Etymological Dictionary of Astronomy and Astrophysics

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.

See also: K from Ger. Kontinuum, → continuum; → corona.

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.

See also: 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.

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.

See also: K the letter of alphabet; → star.

Same as the → Cretaceous-Tertiary event.

See also: K, representing the “→ Cretaceous period,” and T the “→ Tertiary;” → event.

A follow-up mission of the → Kepler satellite funded by → NASA. K2 provides an opportunity to continue Kepler’s observations in the field of → exoplanets and expand its role into new astrophysical observations by assigning to Kepler new mission.

See also: K, short for → Kepler spacecraft; 2, for second → mission.

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.

See also: Kaiser, N., 1987, MNRAS 227, 1; → effect.

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).

See also: 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).

See also: Kaon, from ka (for the letter K) + (mes)on, → meson.

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.

See also: κ, the Gk. letter which denotes opacity; → mechanism.

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.

See also: The scale was originally designed in 1964 by the Russian astrophysicist Nikolai Kardashev (1932-); → scale.

In the system of → Saturn’s rings, the gap near the outer edge of the → A ring. It has a width of 35 km and lies at a distance of 136,530 km from the center of → Saturn.

See also: After James A. Keeler (1857-1908); → gap.

The first achromatic eyepiece consisting of a convex lens and a plano-convex lens. The convex surfaces are turned toward one another.

See also: Named after the inventor Carl Kellner (1826-1855), a German engineer and optician; → eyepiece

The → SI unit of → thermodynamic temperature; symbol K. It is defined by taking the fixed numerical value of the → Boltzmann constant, k, to be 1.380 649 × 10^-23 when expressed in the unit J K^-1, which is equal to kg m² s^-2 K^-1 , where the kilogram, meter and second are defined in terms of → Planck’s constant (h), → velocity of light (c), and Δν_Cs.

See also: Named after the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907), one of the most influential scientists of the 19th century.

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.

See also: → kelvin (K); → scale.

The contraction of a volume of gas under its → gravity, accompanied by the → radiation of the lost
→ potential energy as → heat.

See also: 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.

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.

See also: → Kelvin-Helmholtz contraction; → instability.

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.

See also: William Thomson (Lord Kelvin) and Hermann von Helmholtz proposed that the sun derived its energy from the conversion of gravitational potential energy; → mechanism.

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: t_KH ≅ 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.

See also: → Kelvin-Helmholtz contraction; → time.

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.

See also: → kelvin; → postulate.

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.

See also: 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.

1. Given the trajectory of a particle moving in a → central force field, determine the law governing the central force.
   

   2. Inversely, considering a central force -k/r², determine the trajectory a particle moving in the field will take.

See also: → Kepler; → problem.

A → NASA space telescope launched in March 2009 to discover Earth-size planets using the → transit method. The telescope has a diameter of 0.95 m and its
only instrument is a → photometer that continuously monitors the brightness of over 145,000 → main sequence stars in a fixed field of view of 115 deg² (about 12° diameter). The expected
mission lifetime is 3.5 years extendible to at least 6 years.

See also: In honor of Johannes → Kepler; → spacecraft.

An equation that enables the position of a body in an elliptical orbit to be calculated at any given time from its orbital elements. It relates the → mean anomaly of the body to its → eccentric anomaly.

See also: Keplerian, adj. of → Kepler; → equation.

Planets move in elliptical paths, with the Sun at one focus of the ellipse (year 1609).

See also: → Kepler; → first; → law.

1. The planets move about the Sun in ellipses, at one focus of which the Sun is situated.
   

2. The → radius vector joining each planet with the Sun describes equal areas in equal times.
   

3. The ratio of the square of the planet’s period of revolution to the cube of the planet’s mean distance from the Sun is the same for all planets.

See also: → Kepler; → law.

A line joining a planet to the Sun sweeps out equal areas in equal intervals of time (year 1609).

See also: → Kepler; → second; → law.

A → supernova in → Ophiuchus, first observed on 1604 October 9, and described by Johannes Kepler in his book De stella nova (1606). It reached a maximum → apparent magnitude of -3 in late October. The star remained visible for almost a year. The → light curve is that of a → Type Ia supernova. The → supernova remnant consists of a few filaments and brighter knots at a distance of about 30,000 → light-years. It is the radio source 3C 358. Also known as SN 1604 and Kepler’s supernova.

See also: → Kepler; → star.

The ratio between the square of a planet’s → orbital period (P) to the cube of the mean distance from the Sun (a) is the same for all planets: P²∝ a³ (year 1618). More accurately, P² = (4π²a³) / [G(M₁ + M₂)], where M₁ and M₂ are the masses of the two orbiting objects in → solar masses and G is the → gravitational constant. In our solar system M₁ = 1. The → semi-major axis size (a is expressed in → astronomical units and the period (P) is measured in years.

See also: → Kepler; → third; → law.

Of or pertaining to Johannes Kepler or to his works or discoveries.

See also: From → Kepler + -ian a suffix forming adjectives.

The angular velocity of a point in a circular orbit around a central mass. It is given by:
Ω_K = (GM/r³)^1/2,
where G is the → gravitational constant, M is the mass of the gravitating object, and r is the radius of the orbit of the point around the object.

See also: → Keplerian; → angular; → velocity.

A circumstellar disk (such as an → accretion disk or a → protoplanetary disk) in which the → angular velocity at each radius is equal to the angular velocity
of a circular → Keplerian orbit at the same radius. The
main characteristic of the Keplerian disk is that
→ orbital velocity
varies as r^-1/2. This means that an object on an orbit closer to the central mass turns more rapidly than that on a farther orbit. This velocity difference is at the origin of internal friction or kinematic viscous forces
between disk particles, which heats up the material.

See also: → Keplerian; → disk.

The orbit of a spherical object of a finite mass around another spherical object, also of finite mass, governed by their mutual → gravitational forces only.

See also: → Keplerian; → orbit.

The velocity of an object orbiting another object according to → Kepler’s laws.

See also: → Keplerian; → orbital; → velocity.

A → rotation curve in which the speed of the orbiting body is inversely proportional to the → square root of its distance from the mass concentrated at the center of the system.

See also: → Keplerian; → rotation; → curve.

Shearing motion of an ensemble of particles, each on a nearly circular, → Keplerian orbit. → Orbital velocity decreases as orbital radius increases, yielding shear. Viscous drag on such shear, due to ring-particle collisions, plays a key role in ring processes (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → Keplerian; → shear.

A → refracting telescope which has simple → convex lenses for both → objective and → eyepiece. It suffers from → chromatic aberration, which can be reduced by increasing the → focal ratio. It was first devised by Kepler in 1615.

See also: → Keplerian; → telescope.

The fourth → natural satellite of → Pluto discovered in 2011 using the → Hubble Space Telescope. Also called Pluto IV (P4). It has an estimated diameter of 14-44 km, which makes it the second smallest known moon of Pluto after → Styx. Kerberos revolves around Pluto in the region between → Nix and → Hydra at a distance of about 58,000 km and makes a complete orbit roughly every 32.1 days.

See also: Named after the three-headed dog of Greek mythology.

1. Chemistry: The remainder of an atom after the valence electrons have been removed.
   

2a) Math.: 1) The set of elements that a given function from one set to a second set maps into the identity of the second set.

2b) Let A be a linear transformation of the vector space U into the vector space V . The collection of all those vectors x in U such that Ax = 0 is called the kernel of A and is denoted by ker(A).

3. Computers: The set of functions that make up the operating system, the core that provides basic services for all other parts of the operating system. → core = maqzé (مغزه); → nucleus = hasté (هسته).

Etymology (EN): Kernel, from M.E. kirnel, from O.E. cyrnel, from P.Gmc. *kurnilo- (cf. M.H.G. kornel, M.Du. cornel), from *kernan- (root of corn “seed, grain”), akin to L. granium, + -el, diminutive suffix, variant of → -al.

Etymology (PE): Astel, from asté “kernel, fruit stone,” variants hasté, ostoxân “bone,”
from Mid.Pers. astak “fruit stone, bone,” ast “bone;” Av. ast- “bone;” cf. Skt. asthi- “bone;” Gk. osteon; L. os; Hittite hashtai-; PIE base *os-

• Pers. diminutive suffix -el→ -al.

A → black hole that possesses only mass (not electric charge) and
rotates about a central axis. It has an → ergosphere and a → stationary limit.

See also: Named after the New Zealand mathematician Roy P. Kerr (1934-) who, in 1963, was the
first to solve the → field equationss of Einstein’s theory of → general relativity
for a situation of this kind; → black hole.

A rotating charged black hole. Compare with the → Kerr black hole and the → Reissner-Nordstrom black hole.

See also: Named after Roy P. Kerr and Ezra T. Newman (1929-) who in 1963 independently found this solution to Einstein’s → field equations; → black; → hole.

The largest → impact cratrer on → Ceres, which has a diameter of about 280 km. It is distinctly shallow for its size.

See also: Named for The crater is named after the Hopi spirit of sprouting maize, Kerwan. The name was approved by the IAU on July 3, 2015.[1]

In Dirac’s notation, a vector which describes the state of a quantum system, whether it is in a space of finite or infinite dimensions. A ket vector,
written as | A >, is the dual of the → bra. Like the bra, it appears as an incomplete → bracket expression.

See also: From -ket the second syllable in → bracket.

Kilo (thousand) → electron volt. A unit of → energy used to describe the total energy carried by a → particle or → photon.

Etymology (EN): → kilo- + → electron volt.

A usually metal instrument used to operate a lock’s mechanism.

Etymology (EN): M.E. key(e), kay(e), O.E. cæg “key,” of unknown origin,

Etymology (PE): Kelid, variants (Tabari) kali, (Lori) kelil, (Laki) kalil “key; lock,” (Kurd) kilil, kolun “latch, bolt;” Mid.Pers. kilêl “key.” See also → include.

1. The hole in which a key of a lock is inserted.
   

2. → Keyhole Nebula.
   

3. A small, about 600 m wide, region of space close to the Earth where the Earth’s gravity would perturb the trajectory of a passing → Near-Earth Object. The object will receive a gravitational push that will bring it back for a collision in the future. Also called resonance keyhole.

See also: → key; → hole.

A relatively small and dark cloud of molecules and dust seen silhouetted against the much brighter → Carina Nebula. It contains bright filaments of emitting hot gas and is roughly 7 → light-years in size.

See also: → keyhole; → nebula. The name was given by the English astronomer Sir John Herschel in the 19th century, because of the appearance of the nebula in low-resolution telescopes of that epoch.

A version of the → H-R diagram displaying stellar gravities (→ gravity, log g) against the corresponding → effective temperatures (T_eff).

See also: Named after the group of astrophysicists (W.-R. Hamann, W. Schmutz, U. Wessolowski) working at Kiel University (Germany), who introduced the diagram in 1980s; → diagram.

A → vector field on a → Riemannian manifold (or → pseudo-Riemannian manifold) that preserves the → metric. In other words, the → derivative of the metric with respect to this vector field is null.

See also: Named after the German mathematician Wilhelm Killing (1847-1923); → vector.

A prefix meaning 10³.

See also: Introduced in France in 1795, when the → metric system was officially adopted, from Gk. khilioi “thousand,” of unknown origin.

The basic unit of mass in the
→ International System of Units (SI) and → MKS versions of the → metric system, equal to 1,000 → grams. The kilogram was until 2019 defined as the mass of the standard (international prototype) kilogram, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM), at Sèvre, near Paris, France. Copies of this prototype are kept by the standards agencies of all the major industrial nations.
A kilogram is equal to the mass of 1,000 cubic cm of water at 4°C (→ maximum density).

According to the new (2019) definition, the kilogram is defined by taking the fixed numerical value of the → Planck constant (h) to be 6.62607015 × 10^-34 when expressed in the unit J.s, which is equal to kg m² s^-1, where the meter and the second are defined in terms of c and Δν Cs.

See also: → kilo-; → gram.

A metric unit of force which is equal to a mass of one kilogram multiplied by the standard acceleration due to gravity on Earth (9.80665 m sec^-2). Therefore one (1) kilogram-force is equal to 1 kg × 9.80665 m sec^-2 = 9.80665 → newtons.

See also: → kilogram; → force.

A unit of → frequency, equal to 10³ Hz.

See also: → kilo-; → hertz.

A unit of length, equal to 1000 meters.,

See also: → kilo-; → meter.

A fast-evolving → supernova-like phenomenon resulting from the → merger of compact, binary objects such as two → neutron stars or a neutron star and a → black hole. A kilonova represents an → electromagnetic counterpart to → gravitational waves. Also called → macronova. A simple model of the phenomenon was put forward by Li and Paczynski (1998, ApJL 507, L59). The kilonova phenomenon can last between days and weeks following the merger.

Within the small volume of space where a merger occurs, the combination of a huge amount of energy, and a large number of neutrons, is the instigator for the → r-process. The high density favors this rapid → neutron capture by nuclei, leading to the formation of new → chemical elements with high → atomic numbers
and high → atomic weights. Many elements heavier than → iron form in these environments, including many rare elements, most notably → platinum (atomic number 78) and → gold (atomic number 79).

The decay of heavy atomic nuclei leads to the radioactive heating and a release of electromagnetic radiation. The heat cannot easily escape as radiation, because of the high opacity of the ejected material. The heat is radiated thermally, heating up the nearby matter, which can be then seen in the → near-infrared.

It was long thought that the r-process could also occur during core-collapse supernovae, but the density of neutrons within supernovae appears to be too low.

The first indication of a kilonova following a short GRB 
came from the extensive follow-up of GRB 130603B, which was one of the

nearest and brightest short GRBs ever detected, and also the first short GRB with an optical afterglow spectrum.

The first kilonova found to be associated with a gravitational waves was detected in the study of → GW170817.

See also: The term kilonova was introduced by Metzger et al. (2010, MNRAS 406, 2650), who argued that the peak luminosities of neutron star merger transients are typically ~ few × 10⁴¹ erg s^-1, or a factor of ~ 10³ larger than the → Eddington luminosity for a solar mass object. They therefore dubbed these events kilonovae; from → kilo-; → nova.

A unit of distance equal to 1,000 → parsec (pc)s, or 3,260 → light-years.

See also: → kilo-; → parsec.

A unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1 h) of time. The kilowatt-hour is not a standard unit in any formal system, but it is commonly used to measure the consumption of electrical energy. To convert to → joules, use:
1 kWh = 3.6 × 10⁶ J = 3.6 × 10¹³→ ergs.

See also: → kilo-; → watt-hour.

Of or relating to → kinematics. Same as kinematical.

See also: → kinematics.

A systematic error introduced in a sample of stellar → proper motion data by higher velocity stars that are easier to measure.

See also: → kinematic; → bias.

The ratio of the → dynamic viscosity (η)
to the density (ρ) of a fluid: ν = η/ρ. The unit of kinematic viscosity in the → SI system is m²s^-1. In the → cgs system, cm²s^-1, equal to 10^-4 m²s^-1, is called the → stokes (st).

See also: → kinematic; → viscosity.

Of or relating to → kinematics. → kinematic.

See also: → kinematic; → -al.

A central, tightly bound stellar subsystem observed in some elliptical galaxies which rotates in the opposite direction with respect to the main body of the → elliptical galaxy. Elliptical galaxies are thought to be the result of the → merger of two or more sizable galaxies. A plausible scenario for how counter-rotating cores could form in such a merger is as follows. If at least one of the galaxies has a core region that is fairly tightly bound by the galaxy’s gravity, and the direction in which the two galaxies orbit each other before merging is opposite to the direction of rotation of stars in that tightly bound core,
it is likely that, after the merger, the tightly bound core will end up as the core of the new, larger galaxy, while retaining its original sense of rotation. The surrounding stars, on the other hand, will rotate in a different way dictated by the orbital motion of the galaxies around each other, before the merger.

While this is a plausible scenario, it can only explain some of the counter-rotating cores.

Recently A. Tsatsi et al. (2015, ApJ 802, L3) have shown that although the two → progenitor galaxies are initially following a → prograde orbit, strong reactive forces during the merger can cause a short-lived change of their orbital spin; the two progenitors follow a → retrograde orbit right before their final coalescence. This results in a central kinematic decoupling and the formation of a large-scale (~2 kpc radius) counter-rotating core at the center of the final elliptical-like merger remnant, while its outer parts keep the rotation direction of the initial orbital spin.

See also: → kinematical; → decouple; → core.

The branch of mechanics dealing with the description of the motion of bodies or fluids without reference to the forces producing the motion.

Etymology (EN): From Gk. kinetikos “moving, putting in motion,” from kinetos “moved,” verbal adj. of kinein “to move;”
PIE base *kei- “to move to and fro” (cf. Mod.Pers. šodan, šow- “to go; to become;” Av. šiyav-, š(ii)auu- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Goth. haitan “call, be called;” O.E. hatan “command, call”).

Etymology (PE): Jonbešik, from jonbeš “motion” + -ik→ -ics. The first element from Mid.Pers. jumbidan, jumb- “to move,” Lori, Laki jem “motion,” related to gâm “step, pace;”
O.Pers. gam- “to come; to go,” Av. gam- “to come; to go,” jamaiti “goes,” gāman- “step, pace”
(Mod.Pers. âmadan “to come”); Skt. gamati “goes;” Gk. bainein “to go, walk, step,” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come.”

Of or relating to motion; caused by motion; characterized by movement.

Etymology (EN): From Gk. kinetikos “moving, putting in motion,” from kinein “to move,” → kinematics.

Etymology (PE): Jonbeši, adj. of jonbeš, verbal noun of jonbidan, → move.

The energy which a body possesses as a consequence of its motion, defined as one-half the product of its mass m and the square of its speed v, i.e. 1/2 mv².

See also: → kinetic; → energy.

In fluid mechanics, a quantity that describes helical flow. It is defined by the integrated scalar product of the velocity field and the → vorticity: K_K = ∫ dVu . (∇ x u). In the absence of magnetic field, this quantity is conserved by the → Euler equation. See also → magnetic helicity.

See also: → kinetic; → helicity.

Same as → Lagrangian function.

See also: → kinetic; → potential.

The temperature of a gas defined in terms of the average kinetic energy of its atoms or molecules.

See also: → kinetic; → temperature.

A theory that explains macroscopic properties of gases, such as pressure, temperature, or volume, by considering their molecular composition and motion.

See also: → kinetic; → theory;
→ gas.

A plot representing the evolution of the internal structure of a star as a function of time. The x-axis indicates the time, the y-axis the mass, and a color or
shading specifies convective regions. A vertical line through the graph corresponds to a model at a particular time.

See also: Named after Rudolf Kippenhahn (1926-), a German astrophysicist;
→ diagram

The radiation law which states that at thermal equilibrium the ratio of the energy emitted by a body to the energy absorbed by it depends only on the temperature of the body.

See also: Gustav Robert Kirchhoff (1824-1887), a German physicist who made major contributions to the understanding of electric circuits, spectroscopy, and the emission of black-body radiation from heated objects; → law.

Regions in the asteroid belt within which few asteroids are found. The Kirkwood gaps are due to the perturbing effects of Jupiter through resonances with Jupiter’s orbital period.

See also: Named for the American astronomer Daniel Kirkwood (1814-1895), who Discovered them in 1866; → gap.

A → dwarf galaxy with a → kiloparsec size → starburst at one end, giving the system a → tadpole or → cometary shape. Also called LEDA-36252, KUG 1138+327. Its distance is 24.5 → megaparsecs (Mpc). The rotation speed of ~ 35 km s^-1 combined with a radius of 1.2 kpc in the bright part of the disk implies that the corresponding → dynamical mass is 3 × 10⁸/ sin²i Msun. This estimate
is a factor of ~ 6 larger than the → stellar mass of 5 × 10⁷ Msun from the → Sloan Digital Sky Survey photometry, but is comparable to the total → neutral hydrogen (H I) mass of ~ 3 × 10⁸ Msun. The metallicity in the → starburst “head” appears to be less than in the rest of the galaxy (the “tail”). This peculiar pattern of metallicity, seen also in several other comparable galaxies, suggests that the starbursts in these systems were triggered by accreting a gas with lower metallicity than in the rest of the galaxy.

The → Hubble Space Telescope observations of Kiso 5639 in six UV-optical and Hα filters were used to resolve the head and derive the star formation properties. The head contains 14 young → star clusters more massive than 10⁴Msun and an overall clustering fraction for star formation of 25-40%. The Hα luminosity of the core region of the head is 8.8 ± 0.16 × 10³⁹ erg s^-1 inside an area of 3.6 × 3.6 square arcsec. The corresponding → star formation rate is ~ 0.04 Msun yr^-1

(Elmegreen et al., 2018, arxiv/1805.08253, and references).

See also: Kiso Survey of UV Bright Galaxies (Miyauchi-Isobe et al., 2010, Pub.Nat.Astro.Ob.Japan, 13, 9).

Any of several small birds of the hawk family Accipitridae that have long, pointed wings, feed on insects, carrion, reptiles, rodents, and birds, and are noted for their graceful, gliding flight (Dictionary.com).

Etymology (EN): M.E. kyte, O.E. cyta, cognate with Ger. Kauz “owl.”

Etymology (PE): Zaqan “kite,” of uknown origin.

An strong, extended infrared source in the Orion Nebula, about 1 arcminute NW of the Trapezium and about 12 arcseconds south of the → Becklin-Neugebauer object. It dominates the infrared emission at wavelengths above 20 microns. It probably represents a cluster of young and forming stars embedded in a dusty molecular cloud.

See also: Named after Douglas E. Kleinmann (1942-) and Frank J. Low (1933-), who first studied this object in 1967; → nebula.

A → main belt asteroid (97) discovered by the German astronomer Ernst W. Temple in 1868 working at Marseille Observatory.

See also: Named after Klotho (literally “spinner”) the Gk. goddess of fate who spins the thread of life, from klothein “to spin.”

An electron tube for converting direct-current energy into radio frequency energy by alternately speeding up and slowing down the electrons. It is used as a microwave amplifier or oscillator in radar and high-frequency radio work.

See also: From Gk. kluzein, klus- “to wash, break over” + -tron.

1. The joint of the leg that allows for movement between the femur and tibia and is protected by the patella; the central area of the leg between the thigh and the lower leg.
   

2. Something resembling a bent knee, especially a rigid or braced angle between two framing members (Dictionary.com). → alpha element knee

Etymology (EN): M.E. kne; O.E. cneo, cneow “knee” (cognates: O.Norse kne, O.Sax. kneo, M.Du. cnie, Dutch knie, O.H.G. kniu, Ger. Knie; cf. Pers. zânu, as below.

Etymology (PE): Zânu “knee,” Mid.Pers. šnûg “knee;” Av. žnu- “knee;” cognates: Skt. jānu-, Hittite genu “knee;” Gk. gonu “knee,” gonia “corner, angle;” L. genu “knee;” O.E. cneo, as above; PIE *gnéwo-.

1. An instrument for cutting, consisting essentially of a thin, sharp-edged, metal blade fitted with a handle.
   

2. Any blade for cutting, as in a tool or machine (Dictionary.com).

Etymology (EN): M.E. knif; O.E. cnif, probably from O.N. knifr;
cf. M.L.G. knif, M.Du. cnijf, Ger. Kneif; of uncertain origin.

Etymology (PE): Kârd “knife,” from Mid.Pers. kârt “knife;” Av. karət- “to cut;” cf. Skt. kart- “to cut,”
karəta- “knife;” Proto-Ir. *kart- “to cut.”

The same as → Foucault knife-edge test.

See also: → knife; → edge; → test.

1. To perceive or understand as fact or truth; to apprehend clearly and with certainty.
   

2. To have established or fixed in the mind or memory (Dictionary.com).

Etymology (EN): M.E. knowen, knawen, from O.E. cnâwan, akin to O.H.G. bichnâan “to recognize,” L. gnoscere, noscere “to come to know,” Gk. gignoskein, Pers. šenâxtan, dânestan, as below.

Etymology (PE): Dânestan “to know;” Mid.Pers.
dânistan “to know;” variant šenâxtan, šenâs- “to recognize, to know” (Mid.Pers. šnâxtan, šnâs- “to know, recognize”);
O.Pers./Av. xšnā- “to know, learn, come to know, recognize;” cf. Skt. jñā- “to recognize, know,” jānāti “he knows;” Gk. gignoskein “to know, think, judge,” cognate with L. gnoscere, noscere “to come to know” (Fr. connaître; Sp. conocer); P.Gmc. *knoeanan; O.E. cnawan, E. know, as above; Rus. znat “to know;” PIE base *gno- “to know.”

1. Acquaintance with facts, truths, or principles, as from study or investigation.
   

2. All the information, facts, truths, and principles learned throughout time.
   

Etymology (EN): M.E. cnawlece, from O.E. cnawan, cf. O.H.G. bi-chnaan, ir-chnaan “to know;” cognate with Pers. šenâxt, as below.

Etymology (PE): 1) Šenâxt, past stem of šenâxtan, šenâsidan “to know, discern, distinguish, be acquainted with;”
Mid.Pers. šnâxtan, šnâs- “to know, recognize,” dânestan “to know;” O.Pers./Av. xšnā- “to know, learn, come to know, recognize;”
cf. Skt. jñā- “to recognize, know,” jānāti “he knows;” Gk. gignoskein “to know, think, judge;” L. gnoscere, noscere “to come to know” (Fr. connaître; Sp. conocer); O.E. cnawan; E. know; Rus. znat “to know;” PIE base *gno- “to know.”

2. Dânestgân, literally “body of (what is) known,” from dânest, short for dâneste “known,” p.p. of dânestan variant of šenâxtan, as above,

• -gân suffix forming plural entities.

The thin layer of → vapor immediately adjacent to an irradiated surface. The thickness of the Knudsen layer is generally recognized to be in the order of a few → mean free paths from the surface.

See also: Named after Danish physicist Martin Knudsen (1871-1949); → layer.

The second brightest star in the constellation → Ursa Minor.
It is a reddish, evolved → giant of → spectral type K4 with a visual magnitude of 2.1. It is almost 500 times more luminous than the Sun and lies at a distance of 126 light years. Also called Kocab, Kochah.

See also: Kochab, from Ar. al-Kaukab (الکوکب) “star,” shortened from al-Kaukab al-shemali (الکوکب الشمالی) “North Star.”

The proportionality constant C in Kolmogorov’s mathematical analysis of → turbulence which states that the spectral energy E(k) in the range of turbulent scales is E(k) =C ε^2/3 k^-5/3, where k represents the → wave number (inversely proportional to the wavelength or → eddy size), and ε is the average energy dissipation per unit mass in the fluid. Experimental measurements give C close to 1.5.

See also: Andrei Nikolaevich Kolmogorov (1903-1987), a prominent Soviet mathematician, who made major advances in different scientific fields, mainly probability theory, topology, turbulence, classical mechanics, and computational complexity; → constant.

Length scale of → turbulent flow below which the effects of molecular → viscosity are non-negligible.

See also: → Kolmogorov constant; → scale.

The distribution of energy over different scales in a → turbulent flow where → energy cascade occurs. Let E be the energy per unit → wave number (k)
and ε the energy → dissipation parameter, E = E(k,ε). → Dimensional analysis yields: E = Cε^2/3k^-5/3, where C is the → Kolmogorov constant.

See also: A. N. Kolmogorov, 1941, Local structure of turbulence in an incompressible fluid for very large Reynolds numbers, Doklady Acad Sci. USSR 31, 301;
→ spectrum.

An ice-filled → impact crater located in the northern lowlands of Mars at 73° north latitude and 165° east longitude, south of the large Olympia Undae dune field that partly surrounds Mars’ north polar cap.

Korolev crater is 82 km across with its centre hosting a mound of water ice some 1.8 kilometres thick all year round. The reason for the permanently stable water ice in the crater is because its deepest part acts as a natural cold trap. The air above the ice cools and is thus heavier compared to the surrounding air: since air is a poor conductor of heat, the water ice mound is effectively shielded from heating and sublimation.

See also: The crater is named after chief rocket engineer and spacecraft designer Sergei Korolev (1907-1966), dubbed the father of Soviet space technology.

In the → three-body problem, the → perturbation of the orbit of a → secondary body by the garvity of a third body located at a distance much larger than the separation between the → primary body and the secondary. The secondary’s orbit oscillates about a constant value involving a periodic exchange between the extreme values of its → inclination and orbital → eccentricity. The Kozai-Lidov mechanism results from the conservation of the quantity (1 - e²)^1/2.cos i for each component, where e is eccentricity and i is inclination. The total → angular momentum of the system remains constant while the angular momentum is exchanged betwwen the components. It has been suggested that the Kozai mechanism is responsible for the high eccentricities observed in the orbits of → extrasolar planets. If the parent star has a massive yet unseen substellar companion, orbiting at a great distance, and in an orbit highly inclined to the plane of the planets’ orbits, the mechanism should induce high eccentricities into the orbits of the planets. Similarly, this mechanism may be responsible for the high eccentricities observed in the orbits of many → Kuiper Belt Objects such as 2003 UB313.

See also: Named for the japanese Yoshihide Kozai (1962, Astronomical J. 67, 591), and the Russian Michael Lidov (1962, Planetary & Space Science 9, 719).

An approximate expression for deriving the → opacity that depends upon temperature with a power law: κ ∝ ρT^-3.5, where ρ represents the density. In → partial ionization zones, a part of the energy released during a layer’s compression can be used for further ionization, rather than raising the temperature of the gas. As the temperature of the compressed layer has not substantially increased, the increase in density produces a corresponding increase in the opacity of the layer. Likewise, during the expansion phase, the temperature does not decrease significantly since the ions release energy when they recombine with electrons.

See also: Derived in 1923 by the Dutch physicist Henrik Kramers (1894-1952); → law.

Same as → Kramers’ law.

See also: Named after Henrik Kramers (1894-1952); → law.

The function δⁱ_k of two variables i and j defined by δⁱ_k = 1 if i = j, and δⁱ_k = 0 if i ≠ j.

See also: Leopold Kronecker (1823-1891), a German mathematician; delta, Gk. letter of alphabet.

A diagram used to plot trajectories in → space-time near a → black hole. The vertical and horizontal axes are two complicated functions of time and distance from the black hole. Lines of constant time radiate from the origin of the diagram, with steeper slopes corresponding to later times. Lines of constant distance are hyperbolas, lines of constant time pass through the origin; photons always travel along diagonal lines at ±45° to the vertical. The trajectory of an object falling into the black hole is shown as a curving line moving upward on the diagram at less than 45° to the vertical.

See also: Named after the American physicist Martin David Kruskal (1925-2006); → diagram.

A colorless, odorless, highly un-reactive gaseous chemical element and a member of the inert gas family. Symbol Kr; atomic number 36; atomic weight 83.80; melting point -156.6°C; boiling point -152.3°C.

See also: Krypton, from Gk. kryptos “concealed, hidden”. It was discovered in liquefied atmospheric air by the Scottish chemist William Ramsay and the English chemist Morris William Travers in 1898.

A region of the → Solar System extending roughly from the orbit of → Neptune, or 30 → astronomical units (AU),
to 50 AU from the Sun that contains many small icy bodies. The Kuiper belt is now considered to be the source of the → short-period comets.

See also: Named after Gerard Peter Kuiper (1905-1973), a Dutch-born American astronomer, who predicted the belt in 1951. He is also considered the father of modern planetary science for his contributions to the study of our solar system; → belt.

A → Solar System object belonging to the → Kuiper belt. The largest known objects of this type are → Pluto and its moon → Charon, → Quaoar, → Sedna, and → Orcus. See also → trans-Neptunian object.

See also: → Kuiper belt; → object.

The measure of “peakedness” of the curve describing a frequency distribution in the region about its mode. The kurtosis of the normal distribution is 0.

Etymology (EN): From Gk. kurtosis “bulging, curvature,” from kurtos “convex,” kirkos “a ring;” cf. Av. skarəna- “round;”
L. circus “circle, ring;” PIE base *sker- “to turn, bend.”

Etymology (PE): Afrâštegi condition, state adj. of afrâšté “elevated, erect, upright,” p.p. of afrâštan “to raise, exalt, extole,” from Mid.Pers. abrâstan, abrâz- “to lift, raise,”
from ab-, from O.Pers./Av. abiy-/aiwi- “to, upon, against;” cf. Skt. abhi-, Gk. amphi- + râst “straight, direct, true;”
from O.Pers. rāsta- “straight, true,” rās-
“to be right, straight, true;” Av. rāz- “to direct, put in line, set,” razan- “order;” cf. Skt. raj- “to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Ger. recht; E. right; PIE base *reg- “move in a straight line,” hence, “to direct, rule.”