An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Of a telescope, the smallest angle betwwen two → point sources that produces distinct images. It depends on both the wavelength at which observations are made and on the diameter of the telescope. Same as → spatial resolution.

→ angular; → resolution.

That condition of a → galactic disk at an orbital radius in which the → angular velocity of the disk equals the → pattern speed. It is significant that the spiral wave pattern rotates as a rigid body (Ω_P = const), whereas the galactic disk rotates differentially (Ω is a function of galactocentric distance r). The distance r_C at which the two angular velocities coincide (Ω(r_C) = Ω_P) is referred to as the → corotation radius. The corotation resonance and its position within the galaxy is one of the fundamental properties of a spiral galaxy.

→ corotation; → resonance.

Verbal form of → desorption.

Desorb, from → de- + -sorb, modeled on → absorb.

vâšamidan, from vâ-, → de-, + šamidan, modeled on baršamidan, → absorb.

A physical or chemical process by which a substance that has been sorbed (adsorbed or absorbed) by a liquid or solid material is removed from the material. Reverse of → sorption. → absorption; → adsorption.

Verbal noun of → desorb.

An major intergovernmental research organisation in astronomy supported by 14 European countries. ESO was founded in 1962 as a consortium among Belgium, Denmark, France, Germany, the Netherlands, and Sweden. The ESO Headquarters are located in Garching near Munich, Germany. The organization operates three outstanding observing sites in the Atacama Desert region of Chile: → La Silla, → Paranal, and Chajnantor. The → Very Large Telescope (VLT), the world's most advanced visible-light astronomical facility, is located on the 2600 m high mountain of Paranal, which also hosts the → VLT Interferometer (VLTI). The Chajnantor site, 5000 m above sea level, near San Pedro de Atacama, operates a submillimeter telescope (APEX). Moreover, a giant array of 12 m submillimeter antennas, called → ALMA, is being constructed in collaboration with North America, East Asia and Chile. ESO is currently planning a 42 m European Extremely Large optical/near-infrared Telescope, the → E-ELT.

→ European; → southern; → observatory.

An observation that provides a particularly narrow, peaked image of a point source. → point spread function.

→ high; → resolution; → observation.

The separation between two detached but adjacent points in an image.

→ image; → resolution.

A → Lindblad resonance expressed by: Ω_p = Ω - κ/m.

→ inner; → Lindblad resonance.

For parallel propagating → electrostatic waves in a → plasma, the → resonance which occurs when the particle velocity equals the parallel phase velocity of the wave.

→ Landau damping; → damping.

An → orbital resonance that makes a 4:2:1 period ratio among three bodies in orbit. The → Galilean satellites → Io, → Europa, → Ganymede are in the Laplace resonance that keeps their orbits elliptical. This interaction prevents the orbits of the satellites from becoming perfectly circular (due to tidal interactions with Jupiter), and therefore permits → tidal heating of Io and Europa. For every four orbits of Io, Europa orbits twice and Ganymede orbits once. Io cannot keep one side exactly facing Jupiter and with the varying strengths of the tides because of its elliptical orbit, Io is stretched and twisted over short time periods.

This commensurability was first pointed out by Pierre-Simon Laplace, → Laplace; → resonance.

A kinematic resonance hypothesized to explain the existence of galactic → spiral arms. It occurs when the frequency at which a star encounters the spiral → density wave is a multiple of its → epicyclic frequency. Orbital resonances occur at the location in the disk where Ω_p = Ω ± κ/m, where Ω_p is → pattern speed, κ → epicyclic frequency, and m an integer representing the number of spiral arms. The minus sign corresponds to the inner Lindblad resonance (ILR) and the plus sign to the outer Lindblad resonance (OLR). The corotation resonance corresponds to Ω_p = Ω. In general, the Lindblad resonances are defined for two spiral arms (m = 2), and low order. There are other less important resonances corresponding to higher m values. These resonances tend to increase the object's orbital eccentricity and to cause its longitude of periapse to line up in phase with the perturbing force. Lindblad resonances drive spiral density waves both in galaxies (where stars are subject to forcing by the spiral arms themselves) and in Saturn's rings (where ring particles are subject to forcing by Saturn's moons).

After the originator of the model, Bertil Lindblad (1895-1965), a Swedish astronomer, who made important contributions to the study of the rotation of the Galaxy; → resonance.

A repeated electromagnetic force on an electrically charged ring particle, nudging the particle in the same direction and at the same point in its orbit. Lorentz resonances are especially important for tiny ring particles whose charge-to-mass ratio is high and whose orbit periods are a simple integer fraction of the rotational period of the planet's magnetic field (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ Lorentz; → resonance.

The quality of an instrument that lacks sufficient resolution for a specific observation. This is a relative quality, but presently a resolution below about 1 arcsecond.

→ low; → resolution.

A phenomenon exhibited by certain atoms whereby they absorb energy at specific (resonant) frequencies when subjected to alternating magnetic fields.

→ magnetic; → resonance.

A combining form meaning "middle," used in the formation of compound words; e.g. → meson; → mesosphere.

From Gk. mesos "middle, in the middle;" akin to L. medius, Pers. miyân, → middle; → medium.

Meso-, loan from Gk.

A nuclear particle with a mass intermediate between that of a proton and an electron, which is believed to be responsible for the strong nuclear force. In contrast to the case of baryons or leptons, meson number is not conserved: like photons, mesons can be created or destroyed in arbitrary numbers. Their charge can be positive, negative, or zero.

From mes-, meso-, from Gk. mesos "middle," akin to L. medius, Pers. miyân, → medium, → middle, + -on a suffix used in the names of subatomic particles.

A class of → meteorites that is → brecciated→ stony-iron with nearly equal amounts of → metal and → silicates.

→ meso-; → siderite.

The layer of the atmosphere located between the → stratosphere and the → ionosphere, where temperature drops rapidly with increasing height. It extends between 17 to 80 kilometers above the Earth's surface.

→ meso-; → sphere.

A resonance such that the period of the driving force is the same as the natural period of the system.

→ natural; → resonance.

The outermost natural satellite of → Neptune, discovered in 2002. Also known as Neptune XIII, it follows a highly inclined and highly eccentric orbit at about 48 million km from Neptune. According to preliminary estimates, Neso is about 60 km in diameter.

In Gk. mythology, one of the Nereids, the fifty daughters of Nereus and Doris.