An Etymological Dictionary of Astronomy and Astrophysics
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Decrease in the volume of an astronomical object under the action of a dominant, central gravitational force.

→ gravitational; → contraction.

Mutual attraction between any two bodies that have mass.

→ gravitational; → interaction.

A hypothesized large concentration of mass (about 10¹⁶  → solar masses), some hundred million → light-years from Earth, in the direction of the → Centaurus → supercluster, that seems to be affecting the motions of many nearby galaxies by virtue of its gravity.

→ great; → attractor.

A list of 100 compact groups of galaxies that were identified by a systematic search of the → Palomar Observatory Sky Survey red prints. Each group contains four or more galaxies, has an estimated mean surface brightness brighter than 26.0 magnitude per arcsec² and satisfies an isolation criterion.

Hickson, Paul, 1982, ApJ 255, 382; → compact; → group.

The angular distance of an object below the horizon when it appears to lie on the horizon.

→ horizontal; → refraction.

The method used to detect very brief flashes of → Cherenkov radiation generated by the → cascade shower of → relativistic charged particles produced when a very high-energy → gamma ray (in the range 50 GeV to 50 TeV) strikes the atmosphere at a typical altitude of 10 km. Owing to this technique, it possible to discriminate cosmic gamma rays from the cosmic ray background and to determine their energy and source direction. More specifically, the incoming gamma-ray photon undergoes → pair production in the vicinity of the nucleus of an atmospheric molecule. The electron-positron pairs produced are of extremely high energy and immediately radiate in a → bremsstrahlung process. This radiation is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20 nano-seconds. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes should be large.

→ imaging; → atmospheric; → Cherenkov; → technique.

A collision between two bodies. In the case of solar system objects, when one is much smaller than the other (like a meteoroid colliding with the Earth), a crater may be produced on the larger body.

From L. impactus, p.p. of impingere "to drive into, strike against," from → in- "in" + pangere "to fix, fasten."

Barxord, verbal noun of barxordan "to collide, clash, dash against each other," from bar- "on, upon, up" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + xordan "to hit, strike," originally "to eat, drink, devour," and by extension "to destroy," from Mid.Pers. xvardan "to eat, enjoy (food)," Av. x^var- "to consume, eat;" Laki dialect hovârden "to eat;" Proto-Iranian *huar- "to consume, eat."

A depression produced by the collision of a meteorite, asteroid, or comet with the surface of a planet or a satellite. Impact craters are the most characteristic surface features of solar system rigid bodies. They range in size up to hundreds or thousands of kilometers (where the impacts create giant basins as on the Moon, Mars, and Mercury).

→ impact; → crater.

An → atmospheric escape mechanism that occurs where atmospheric gases are expelled en masse as a result of large body impacts, such as the cumulative effect of asteroids hits (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ impact; → erosion.

A collision between two celestial objects, specially solar system bodies, with considerable consequences. Impact events involve release of large amounts of energy. Some examples are the 1908 Siberian → Tunguska event by a → comet, the → Barringer Crater, and the collision of an → asteroid with Earth 65 million years ago, which is thought to have led to the extinction of the dinosaurs and other species of the → Cretaceous-Paleogene period.

→ impact; → event.

The danger of collision with Earth posed by solar system small bodies that pass near our planet. These objects include → near-Earth asteroids and nuclei of → comets. See also: → near-Earth object, → impact crater, → Torino scale, → Palermo scale, → Space Situational Awareness.

→ impact; → hazard.

The loss of orbital electrons by an atom of a crystal lattice which has undergone a high-energy collision.

→ impact; → ionization.

1) A measure of the distance by which a collision fails being frontal.
2) In → gravitational lensing, the distance of closest approach of the light path to the → lensing object.
3) In → rainbows, the displacement of the → incident from an axis that passes through the center of the water droplet.

→ impact; → parameter

The enormous drop in temperature and the related effects of the shrouding of Earth with soot and dust particles after the planet is struck by a sizable comet or asteroid. Such a phenomenon is believed to have killed off the dinosaurs 65 million years ago.

→ impact; → winter.

A general term used for all rocks affected by, or produced by, the → shock waves and other processes generated by hypervelocity → meteorite → impact events. Impactites occur in and around the → impact crater, typically as individual bodies composed of mixtures of melt and rock fragments, often with traces of meteoritic material.

→ impact; → -ite.

A natural impacting body, such as a comet, asteroid, or planet. It can also be a space probe designed to collide with an astronomical body in the solar system.

Impactor, from → impact + -or a suffix forming agent nouns.

Barxordgar, from barxord, → impact, + -gar agent suffix, from kar-, kardan "to do, to make" (Mid.Pers. kardan, O.Pers./Av. kar- "to do, make, build," Av. kərənaoiti "makes," cf. Skt. kr- "to do, to make," krnoti "makes," karma "act, deed;" PIE base k^wer- "to do, to make").

1) Not → active.
2) Chem.: Not → reactive.

→ in-; → active.

Same as → refractive index.

→ index; → refraction.

A function that converts a → differential equation, which is not exact, into an → exact differential equation. This is done by multiplying all terms of the original equation by the integrating factor.

→ integrate; → factor.

To act upon one another; have a mutual or reciprocal action.

Interact, from → inter- + → act.