An Etymological Dictionary of Astronomy and Astrophysics
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Of a stellar → nuclear fusion, the equation describing the → energy generation rate as a function of → density and → temperature.

→ energy; → generation; → equation.

Of a stellar → nuclear fusion, the energy produced per unit mass per unit time, usually denoted ε (erg g^-1s^-1). The general form of the energy generation equation is: ε = ε₀ρ^λT^ν, where ε₀, ρ, and λ are constants over some efficiently restricted range of → temperature T, → density ρ, and → chemical composition. The temperature exponent ν is about 4, 15, and 40 for → proton-proton chain, → CNO cycle, and → triple alpha process, respectively.

→ energy; → generation; → rate.

Any of the several discrete states of energy which a particle, atom, or molecule can adopt under conditions where the possible values are restricted by quantum mechanical laws.

→ energy; → level.

Of cosmic rays, the plot representing the number of particles as a function of their energy.

→ energy; → spectrum.

Same as → energy level.

→ energy; → state.

The → conversion of one → form of energy into another, or the movement of energy from one place or system to another.

→ energy; → transfer.

A tensor (T_μν) related to the → Einstein tensor through → Einstein's field equations. The energy-momentum tensor depends upon the distribution of the → energy and → matter in the space.

→ energy; → momentum; → tensor.

1) General: Equal sharing of the → total energy among all → components of a → system.
2) In the → kinetic theory of gases, the → theorem according to which → molecules in → thermal equilibrium have the same average energy (¹/₂ kT) associated with each independent → degree of freedom of their motion.

→ equi-; → partition.

The → CGS unit of → energy; the → work done by a → force of 1 → dyne acting over a distance of 1 → centimeter. 1 erg = 10^-7 → joules = 6.242 × 10¹¹ → electron-volts.

From Gk. ergon "work," from PIE base *werg- "to work" (cf. Av. varəz- "to work, do, perform, exercise;" Mod.Pers. varz-, varzidan "to labor, exercise, practise;" Arm. gorc "work;" Lith. verziu "tie, fasten, squeeze," vargas "need, distress;" Goth. waurkjan; O.E. wyrcan "work," wrecan "to drive, hunt, pursue").

The property of a dynamical system such that in an interval of sufficient duration, it will return to states that are closely similar to previous ones.

From → erg + Gk. (h)od(os) "way, road" + → ic.

The study of the relationship between people and their working environment, in particular its effect on a person's efficiency. Ergonomics is applied in designing equipment and office systems to maximize productivity by reducing discomfort and fatigue of people in their workplace.

From Gk. ergon "work," → erg, + -nomics, → -nomy, → -ics.

Varzdâtik, from varz "work, " cognate with Gk. ergon, → erg, + dâtik "law, rule," → -nomy.

The region between the → event horizon and the → stationary limit of a rotating → Kerr black hole. It is possible for a particle falling inside the ergosphere to break into two parts, one of which will fall into the black hole and the other will come out.

→ erg + → sphere.

Amount of energy (usually measured in → electron-volts) required to bring an electron from its → ground state to a given → excited state.

→ excitation; → energy.

The energy of the highest occupied quantum state in a system of fermions at absolute zero temperature. See also → Fermi sea.

→ fermi; → energy.

The total energy needed to create a thermodynamic system minus the energy provided the environment. It is defined by G = U + PV -TS, where U is the → internal energy, T the → absolute temperature, S the → entropy, P the → pressure, and V is the final → volume. Same as the → Gibbs function and → thermodynamic potential.

Named after Josiah Willard Gibbs (1839-1903), an American physicist who played an important part in the foundation of analytical thermodynamics; → free; → energy.

Same as → gravitational potential energy.

→ gravitational; → energy.

1) The energy that an object possesses because of its position in a → gravitational field, especially an object near the surface of the Earth where the → gravitational acceleration can be assumed to be constant, at about 9.8 m s^-2.
2) In a two body system. It is the amount of work done in bringing the mass m to the distance R from M: E_P = -GMm/R, where G is the → gravitational constant.
3) For a uniform sphere. It is E_P = -(3/5)GM²/R, where G is the gravitational constant and M is the mass contained in the sphere of radius R.

→ gravitational; → potential; → energy.

The uncertainty in the measurement of the position and momentum of an elementary particle. The more precisely one quantity is known, the less certain the precision of the other. A similarly linked pair of quantities is the time and energy content in a volume of space.

Named after Werner Heisenberg (1901-1976), the German physicist who in 1927 derived the uncertainty principle. In 1932 he was awarded the Nobel Prize in Physics; uncertainty, from → un- "not" + → certainty; → principle.

Of a system, the quantity whose decrease gives the maximum amount of external work which is performed when any physical or chemical process is carried out reversibly at constant temperature. It is defined by F = U - TS, where U is the → internal energy, T the → absolute temperature, and S the final → entropy.

After the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → free; → energy.

An array of → IACT telescopes for studying cosmic → gamma rays in the 100 GeV to 100 TeV energy range. The HESS observatory is located in Namibia, southern Africa, at an altitude of 1800 m, and the project is an international collaboration of more than 100 scientists from nine countries. In its Phase I, HESS used four telescopes each consisting of a light collector with a diameter of 13 m and a focal length of 15 m placed at the corners of a square 120 m apart. Each telescope is segmented into 380 round mirror facets of 60 cm diameter and uses a camera consisting of 960 closely packed → photomultiplier tubes. The first of the telescopes went into operation in Summer 2002. Phase II includes a fifth telescope, called Large Cherenkov Telescope (LCT), of 27 m diameter, located in the centre of the initial array. This upgrade lowers the triggering threshold of the HESS array to about 20 GeV, thus broadening the energy window in which gamma-ray astronomy can be done, opening up more opportunities in astrophysical research (see, e.g., Bernlöhr et al. 2003, Astroparticle Physics 20, 111).

H.E.S.S., short for High Energy Stereoscopic System, is also intended to pay homage to Victor F. Hess (1883-1964), an Austrian-American physicist who received the Nobel Prize in Physics in 1936 for his discovery of → cosmic rays.