An Etymological Dictionary of Astronomy and Astrophysics
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A state characterizing a system which obeys → Cassini's laws.

→ Cassini's law; → state.

Quantum mechanics: A dynamical state whose state vector (or wave function) is an → eigenvector of an → operator corresponding to a specified physical quantity.

→ eigenfunction; → state.

In molecular quantum mechanics, any of → quantum states corresponding to a particular → electron configuration (i.e. an arrangement of the electron(s) in certain → orbitals). The electron configuration with the lowest energy is called the → ground state. All higher energy states are called → excited states.

→ electronic; → state.

Same as → energy level.

→ energy; → state.

In physics and thermodynamics, the equation that describes the relationship between pressure, density, and temperature, e.g. → ideal gas law, → van der Waals equation, → polytropic process, → virial equation of state.

→ equation; → state.

In cosmology, a → dimensionless parameter introduced by the → equation of state representing the ratio of the pressure to the energy density of a fluid, such as the → dark energy: w = p/ρ. The → deceleration or → acceleration of an → expanding Universe depends on this parameter (→ accelerating Universe). A number of numerical values of this parameter are as follows: for the → cosmological constant: w = -1, for → non-relativistic matter (present-day → baryons): w = 0, and for → relativistic matter (photons, neutrinos): w = +1/3. Together with Ω(dark energy) and Ω(matter), w provides a three-parameter description of the dark energy. The simplest parametrization of the dark energy is w = constant, although w might depend on → redshift.

→ equation; → state; → parameter.

A state in which a → thermodynamic system is in → thermodynamic equilibrium.

→ equilibrium; → state.

The condition of a particle or system of particles (especially an atom, nucleus, molecule) after absorbing energy from outside and transiting to a higher → energy level than that of its → ground state. Excited states are transitory as they lose energy through emissions or collisions and return to ground state.

Excited, p.p. of → excite; → state.

The lowest energy state of an atom, molecule, or ion, when all electrons are in their lowest possible energy levels, i.e. not excited.

→ ground; → state.

A proposal regarding the initial state of the → Universe prior to the → Planck era. This → no boundary hypothesis assumes an imaginary time in that epoch. In other words, there was no real time before the → Big Bang, and the Universe did not have a beginning. Moreover, this model treats the Universe like a quantum particle, in an attempt to encompass → quantum mechanics and → general relativity; and attributes a → wave function to the Universe. The wave function has a large value for our own Universe, but small, non-zero values for an infinite number of other possible, parallel Universes.

Hartle, J., Hawking, S., 1983, "Wave function of the Universe," Physical Review D 28; → initial; → state.

An → excited state in the → triple alpha process leading to the production of the most abundant → isotope of → carbon. The existence of this state is of extreme astrophysical importance concerning the → nucleosynthesis of ¹²C in stellar → cores:
⁴He + ⁴He ↔ ⁸Be,
⁸Be + ⁴He ↔ ¹²C^*,
¹²C^*→ ¹²C + γ.
The process proceeds as follows. First the unstable → ground state of ⁸Be is formed in the collision of two → alpha particles. Since ⁸Be exists roughly 7 x 10^-17 sec, it must fuse with an alpha particle before breaking up. However, the probability of three bodies merging simultaneously is extremely low. Hoyle showed that the ¹²C nucleus needs an excited state or resonance at 7.68 MeV to provide for a high reaction probability. The Hoyle state was soon found at 7.65 MeV with the predicted → spin and → parity.

In honor of the British astrophysicist Fred Hoyle (1915-2001), who predicted this state in 1953 (Hoyle et al. 1953, Physical Review 92, 1095); it was discovered by W. A. Fowler in 1957; → state.

Same as → macrostate.

→ macroscopic; → state.

Statistical physics: A state of a physical system that is described in terms of the system's overall or average properties at a macroscopic level (→ temperature, → pressure, → density, → internal energy, etc.). A macrostate will generally consist of many different → microstates. In defining a macrostate we ignore what is going on at the microscopic (atomic/molecular) level. The → probability of a certain macrostate is determined by how many microstates correspond to this macrostate. Therefore, the greater the number of microstates which lead to a particular macrostate, the greater the probability of observing that macrostate. Same as → macroscopic state. See also → entropy, → Boltzmann's entropy formula, → multiplicity.

→ macro-; → state.

An excited state in an atom, which is at the origin of the spectral lines called → forbidden lines. The time duration of the excited state being relatively long, under laboratory conditions the atom cannot pass directly to the ground state by emitting radiation. In the extremely rarefied interstellar medium, however, such highly improbable transitions do occur.

→ meta-; → stable; → state.

Same as → microstate.

→ microscopic; → state.

Statistical physics: For a system made up of a large number of components, a state of the system which is specified by describing the current dynamical variables of each constituting component. For example, for a gas system composed of a large number of molecules, the microstate is defined by the set of quantities which defines the state of each molecule in the system (position, velocity, vibration, etc.). In practice, it is impossible to know perfectly the microstate of a system. The aim of → statistical physics is to relate the macroscopic (average ) observables (→ pressure, → temperature, → internal energy) to the microstate of the system. Also called → microscopic state. See also → macrostate and → multiplicity.

→ micro-; → state.

Of an atom, the same as → ground state.

→ normal; → state.

Same as → oxidation number.

→ oxidation; → state.

In → quantum mechanics, the state of a system as described by a set of → quantum numbers and represented by an → eigenfunction.

→ quantum; → state.

In atomic physics, the electronic state of an atom or molecule for which the total → spin angular momentum is zero.

→ singlet; → state.