An Etymological Dictionary of Astronomy and Astrophysics
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1) Thermodynamics: A process in which a gas, originally at a constant high pressure, passes → adiabatically through a porous wall or a narrow opening into a region of constant lower pressure. The throttling process is → irreversible and is accompanied by an increase of → entropy. The → enthalpy of the gas is the same in the → initial and → final → states. Also called → Joule-Thomson expansion. The change in the temperature of the gas in throttling is known as the → Joule-Thomson effect.
2) A technique implemented in the architecture of computers whereby the frequency produced by the microprocessor is automatically adjusted. Throttling process allows the computer to conserve power and also reduce the heat generated by its chip.

→ throttle; → process.

A chain of nuclear fusion reactions by which three helium nuclei (→ alpha particles) are transformed into → carbon. First two nuclei of helium collide, fuse, and form a nucleus of → beryllium: ⁴He + ⁴He ↔ ⁸Be, which is unstable and will decay back into two helium nuclei within a few 10^-17 seconds. However, due to sufficiently high density and temperature in the stellar core, during a third collision between beryllium and helium, carbon is formed: ⁸Be + ⁴He → ¹²C + γ. The triple-alpha process is possible owing to the existence of the → Hoyle state. It is the main source of energy production in → red giants and → red supergiants in which the core temperature has reached at least 100 million K. Also called → Salpeter process.

→ triple; → alpha particle; → process.

A cycle of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation. Neutrinos carry away energy quickly leading to the cooling of the stellar core and a dramatic drop in the pressure. Due to this process in pre-supernova stars, the core becomes unable to support the weight of the overlying layers. These layers collapse onto the core and subsequently rebound and escape in the supernova explosion.

Named for the de Urca Casino in Rio de Janeiro, by George Gamow and Mario Schönberg, who first studied this process. Gamow is reported to have pointed to the similarity between the disappearance of money in the crowded gambling rooms of the Casino and the disappearance of electron energy into invisible neutrinos; → process.