An Etymological Dictionary of Astronomy and Astrophysics

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.

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 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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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 → 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.

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]

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.

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.

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.

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.

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 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.