An Etymological Dictionary of Astronomy and Astrophysics
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For a radioactive substance, the average number of atoms disintegrating per unit time.

Activity, from → active + -ity.

Žirandegi, noun from žirandé, → active.

The appearance of → gas and → dust features from the rocky-icy nucleus of a comet when approaching the Sun (→ cometary atmosphere, → cometary tail). The → sublimation of → water can explain cometary activity at distances from the Sun up to about 4 → astronomical units. At larger distances, the average temperature of the → comet nucleus' surface is less than 140 K, too low for efficient sublimation of water → ice. However, there are many examples of cometary activity at larger distances. This can probably be due to the sublimation of more → volatile → chemical species. Indeed, radio spectroscopic observations of comets at large distances have revealed an important → outgassing of → carbon monoxide (CO), which can sublimate at temperatures as low as 25 K.

→ cometary; → activity.

The natural variations in the → geomagnetic field due to interactions of the Earth's field and → magnetosphere with energetic particles from the Sun.

→ geomagnetic; → activity.

The property possessed by some substances and their solutions of rotating the plane of vibration of → polarized light. When a beam of → linearly polarized light is sent through an optically active substance, such as crystalline quartz and sugar solution, the direction of vibration of the emerging linearly polarized light is found to be different from the original direction. Those which rotate the → plane of polarization to the right, for an observer looking in the incoming beam, are called → dextrorotatory or right handed; those which rotate it to the left, → levorotatory or left handed. Optical activity may be due to an asymmetry of molecules of a substance (solutions of cane sugar) or it may be a property of a crystal as a whole (crystalline quartz).

→ optical; → activity.

The spontaneous disintegration of certain atomic nuclei, which is accompanied by the emission of either α- or β- particles and/or a γ rays.

→ radio; → activity.

1) General: The quality or condition of being reactive.
2) Physics: A measure of the deviation from the condition at which a → nuclear reactor is critical.
3) Chem.: The relative → capacity of an → atom, → molecule, or → redical to undergo a → chemical reaction with another atom, molecule, or → compound.

From → reactive + → -ity.

The general term for all forms of short-lived phenomena on the Sun, including → solar flares, → sunspots, → prominences, etc., indicating that the Sun is an active star.

→ solar; → activity.

Same as the → solar cycle.

→ solar activity; → cycle.