An Etymological Dictionary of Astronomy and Astrophysics
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In a → rotating star, the sum of the → gravity and the → centrifugal acceleration. The effective gravity is a function of the rotation velocity (Ω) and the → colatitude (θ). At the pole (θ = 0°) and the equator (θ = 90°) the effective gravity is radial. See also → total gravity.

→ effective; → gravity.

Of a galaxy, the distance from its center within which half of the total luminosity is included.

→ effective; → radius.

A measure of the surface temperature of a star derived from the total emitted energy, assuming that the star is a → blackbody emitter (→ Stefan-Boltzmann law, → Planck's radiation law). See also → brightness temperature; → color temperature.

→ effective; → temperature.

The degree to which goals are achieved and the extent to which posed problems are solved. Compare → efficiency.

→ effective; → -ness.

Same as → geodetic precession.

→ Einstein-de Sitter Universe; → effect.

The → photoelectric effect in solids where free electrons are emitted from the surface of a substance (e.g., → semiconductor) when radiation of appropriate frequency falls on it. Also called → photoemissive effect.

→ external; → photoelectric; → effect.

Same as → Faraday rotation.

→ farad; → effect.

Increased reddening and monochromaticity of light as the path length in the air increases.

After the Scottish physicist James David Forbes (1809-1868); → effect.

An increase in → temperature caused when incoming → solar radiation is passed but outgoing → thermal radiation is trapped by the → atmosphere. The major factors for this effect are → carbon dioxide and → water vapor. The greenhouse effect is very important on Venus and Earth but very weak on Mars. On average, about one third of the solar radiation that hits the Earth is reflected back to space. The Earth's surface becomes warm and emits → infrared radiation. The → greenhouse gases trap the infrared radiation, thus warming the atmosphere. Without the greenhouse effect the Earth's average global temperature would be -18° Celsius, rather than the present 15° Celsius. However, human activities are causing greenhouse gas levels in the atmosphere to increase.

→ greenhouse; → effect.

The continuum trough observed in the spectra of high redshift quasars (z> 6) at the blue wing of their Lyman-alpha emission line (1216 Å). It is explained by the scattering of the radiation of the quasar by intergalactic neutral hydrogen on the line of sight. Because of the cosmological expansion, the quasar line is redshifted with respect to the continuum trough. The Gunn-Peterson opacity increases rapidly with redshift. It is interpreted as a strong evidence for the reionization of the Universe around z = 6.

After James E. Gunn and Bruce A. Peterson who predicted the effect in 1965; → effect.

The → polarization arising from line scattering in the presence of "weak" magnetic fields. The effect occurs when precession around magnetic field depolarizes and rotates polarization of the scattered light. The Hanle effect is sensitive to ~10³ times smaller field strengths than the → Zeeman effect. It is in particular used to measure the weak magnetic field of the solar → prominences, which is 10^-3 tesla and over 10^-2 tesla for the active prominences.

Named for the German physicist Wilhelm Hanle (1901-1993), who published his his discovery in 1923 (Naturwissenschaften 11, 690); → effect.

The → photoelectric effect whereby photons absorbed by a solid (→ semiconductor) raise electrons from a lower to a higher → energy level (from → valence band to → conduction band). See also → external photoelectric effect.

→ internal; → photoelectric; → effect.

A → scattering process by which fast-moving, energetic particles transfer energy to photons, decreasing the wavelength of the radiation. This is a particularly important effect in astrophysics and cosmology since it explains the → Sunyaev-Zel'dovich effect.

→ inverse; → Compton effect.

The → Zeeman effect obtained in absorption. The phenomenon is observed by sending white light through an absorbing vapor when the latter is subjected to a uniform magnetic field. The laws governing the inverse effect are similar to those for the direct effect.

→ inverse; → Zeeman effect.

A quantum mechanical → tunnel effect allowing the flow of a continuous current across two weakly coupled → superconductors which are separated by a very thin insulating barrier.

Named after the British physicist Brian David Josephson, who predicted the existence of the effect in 1962; → effect.

A → conductor becomes heated by the passage of an electric current through it due to the → resistance of the conductor. Same as → Ohmic dissipation.

→ joule; → effect.

The change in the temperature of a gas in the → throttling process.

→ Joule; → Thomson; → effect.

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.

An effect predicted by → general relativity whereby a rotating body alters the → space-time around it. This effect can be thought of as a kind of "dragging of inertial frames," as first named by Einstein himself. A massive spinning object pulls nearby objects out of position compared to predictions for a non-rotating object. The effect is important for rapidly rotating → neutron stars and → black holes, but that near Earth is extraordinarily small: 39 milli-arc second per year, about the width of a human hair seen from 400 meters away.

Named after Austrian physicists Joseph Lense (1890-1985) and Hans Thirring (1888-1976), who first discovered this phenomenon in 1918; → effect.

Effect created by a → gravitational lens.

→ lensing; → effect.