An Etymological Dictionary of Astronomy and Astrophysics
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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.

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.

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.

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.

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.

→ kelvin (K); → scale.

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.

→ kelvin; → postulate.

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

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.

→ 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.

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.

→ Kelvin-Helmholtz contraction; → time.

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.

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.

→ 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.

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.

Keplerian, adj. of → Kepler; → equation.

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

→ 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.

→ Kepler; → law.

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

→ 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.

→ 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.

→ Kepler; → third; → law.