An Etymological Dictionary of Astronomy and Astrophysics
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A change in the → energy level or → state of an → atom in which a → quantum of energy is either gained or lost. See also → forbidden transition; → permitted transition; → semiforbidden transition.

→ atomic; → transition.

A transition between two energy levels of an electron bound to a nucleus. The electron remains tied to the nucleus before and after the transition. → bound-free transition; → free-free emission.

Bound, p.p. of → bind; → transition.

A transition in which a bound electron is liberated. → free-bound emission; → free-free emission.

Bound, p.p. of → bind; → free.

A photon generation mechanism in an atom in which a transition initiates a series of secondary transitions from lower electronic levels.

→ cascade; → transition.

A transition between two quantum-mechanical energy levels. See also → discrete spectrum.

→ discrete; → transition.

The → transfer of an → electron from one → energy level to another.

→ electronic; → transition.

A transition between two quantum mechanical → states that → violates the quantum mechanical → selection rules.

→ forbidden; → transition.

An → atomic transition involving a → hyperfine structure.

→ hyperfine; → transition.

A change in the → energy level or state of an atomic → nucleus involving a → quantum of energy.

→ nuclear; → transition.

A transition between two quantum mechanical states that does not violate the quantum mechanical selection rules.

→ permitted; → transition.

The changing of a substance from one phase to another, by → freezing, → melting, → boiling, → condensation, or → sublimation. Also known as phase transformation. A well known phase transition is the transition from → water to → ice. Phase transitions are often associated with → symmetry breaking. In water there is a complete symmetry under rotations with no preferred direction. Ice has a crystal structure, in which certain orientations in space are preferred. Therefore, in transition from water to ice the continuous rotational symmetry is lost.

→ phase; → transition.

A phase transitions that occurs at zero temperature as a function of a non-thermal parameter like → pressure, → magnetic field, or → chemical composition. In contrast to ordinary → phase transitions, which are associated with passage through a critical temperature, quantum phase transitions are associated with → quantum fluctuations, a consequence of → Heisenberg's uncertainty principle. For example, see → Bose-Einstein condensation.

→ quantum; → phase; → transition.

A phase transition, predicted by cosmological models, to have occurred at approximately 10^-5 seconds after the Big Bang to convert a plasma of free quarks and gluons into hadron.

→ quark; → hadron; → phase; → transition.

A transition between two states of an atomic or molecular entity, the energy difference being emitted or absorbed as photons.

→ radiative; → transition.

A slight change in the energy level of a molecule due to the rotation of its constituent atoms about their center of mass.

→ rotational; → transition.

An → atomic transition whose probability is reduced by a factor of the order of 10⁶ because of → selection rules. Same as → interconnection line.

→ semi-; → forbidden; → transition.

An → atomic transition that gives rise to a → spontaneous emission.

→ spontaneous; → transition.

1) Passage from one position, state, stage, subject, concept, etc., to another.
2) A change between phases such as solid to liquid or liquid to gas.
3) A change in the → energy level or state of an atom or nucleus. → atomic transition, → bound-bound transition, → bound-free transition, → cascade transition, → discrete transition, → forbidden transition, → hyperfine transition, → nuclear transition, → permitted transition, → phase transition, → quark-hadron phase transition, → radiative transition, → rotational transition, → semi-forbidden transition, → transition disk, → transition function, → transition probability, → vibrational transition.

From L. transitionem (nominative transition) "a going across or over," noun of action from transire "go or cross over," from → trans- "across" + ire "to go."

Gozareš, verbal noun of gozaštan "to transit, pass," → transit, + -eš, → -tion.

The → accretion disk of a → T Tauri star that displays very weak or no → infrared excess at → near infrared wavelengths, but shows strong excess at → intermediate infrared and longer wavelengths. This happens when the hot inner dust disk has dissipated whereas the cooler outer disk is still intact.

→ transition; → disk.

The → probability of finding the → Universe in a → state labelled X₂ at a time t₂, if it was in a state X₁ at an earlier time t₁.

→ transition; → function.