An Etymological Dictionary of Astronomy and Astrophysics
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The formula of a chemical compound, showing the kind and arrangement of atoms.

→ molecular; → formula.

Any of several theories about how the → Moon originated, among which: → fission theory, → capture theory, → co-formation theory, and → giant impact hypothesis. The model that is best supported by all the available data is the giant impact hypothesis. See also → canonical model.

→ lunar; → formation.

The formula expressing the value of a → definite integral of a given function over an interval as the difference of the values at the end points of the interval of any → antiderivative of the function: ∫f(x)dx = F(b) - F(a), summed from x = a to x = b.

Named after Isaac → Newton and Gottfried Wilhelm Leibniz (1646-1716), who both knew the rule, although it was published later; → formula.

The mean square noise voltage across a resistance in thermal equilibrium is four times the product of the resistance, Boltzmann's constant, the absolute temperature, and the frequency range within which the voltage is measured. → Johnson-Nyquist noise.

Named after Harry Nyquist (1889-1976), a Swedish-born American physicist, who made important contributions to information theory. → Johnson-Nyquist noise; → formula.

1) To carry out; → execute; do.
2) To go through or execute in the proper, customary, or established manner (Dictionary.com).

M.E. parformen, from Anglo-Fr. performer, from O.Fr. parfornir "to do, carry out, finish, accomplish," from par- "completely," → per-, + fornir "to provide."

Pergâlidan, from Kurd. (Sanandaj) pergâl "work, doing; order, command," ultimately from Proto-Ir. *parikar-, from *pari- "through, throughout; thoroughly" (O.Pers. pariy "around, about;" Av. pairi "around, over") + *kar- "to do;" Pers. kar-, variants kâr, gar, gâr, → work.

1) The act of performing a ceremony, play, piece of music, etc.
2) The execution or accomplishment of work, acts, feats, etc.
3) A musical, dramatic, or other entertainment presented before an audience (Dictionary.com).

→ perform; → -ance.

1) Relating to or of → performance, especially of dramatic or artistic performance.
2) A → performative utterance.

→ perform; → -ive.

A sentence or expression which is not only describing a given reality, but actually does or accomplishes something. For example "I now declare you husband and wife" (when uttered by the authorized officiator during a marriage ceremony).

→ performative; → utterance.

1) Someone who performs in front of an audience, for example an actor or musician.
2) Someone who does something with a particular amount of success (Macmillan Dictionary).

→ perform; → -er.

A formula that determines the distribution of intensity of radiation that prevails under conditions of thermal equilibrium at a temperature T: B_v = (2hν³ / c²)[exp(hν / kT) - 1]^-1 where h is Planck's constant and ν is the frequency.

→ Planck; → blackbody; → formula.

Permanent → deformation of a → solid subjected to a → stress.

→ plastic; → deformation.

An approximate version of → general relativity that applies when the → gravitational field is → weak, and the matter → velocity is → small. Post-Newtonian formalism successfully describes the gravitational field of the solar system. It can also be applied to situations involving compact bodies with strong internal gravity, provided that the mutual gravity between bodies is weak. It also provides a foundation to calculate the → gravitational waves emitted by → compact binary star systems, as well as their orbital evolution under radiative losses. The formalism proceeds from the Newtonian description and then, step by step, adds correction terms that take into account the effects of general relativity. The correction terms are ordered in a systematic way (from the largest effects to the smallest ones), and the progression of ever smaller corrections is called the → post-Newtonian expansion.

→ post-; → Newtonian; → formalism.

A mathematical analysis, based on → self-similarity, used to predict the → mass function of spherically collapsing → dark matter halos. The formalism assumes that the fraction of mass in halos more massive than M is related to the fraction of the volume in which the smoothed initial density field is above some threshold δ_cρ, where ρ is the average density of the Universe, with the volume encompassing a mass larger than M. A variety of smoothing → window functions and thresholds have been argued, but the most common is a top-hat window in real space and δ_c≅ 1.69. The Press-Schechter formalism provides a relatively good fit to the results of numerical simulations in cold dark matter theories.

First described by William H. Press and Paul Schechter's paper (1974, ApJ 187, 425); → formalism.

A formula relating the unknown part of a → quadratic equation (the roots of the equation, x) to the known parts (a, b, and c): x = (-b± (b² - 4ac)^½) / 2a.

→ quadratic; → formula.

The science concerned with the transmission, storage, and processing of information using quantum mechanical systems. It exploits the notion of → quantum entanglement between systems and joins several fields of knowledge, mainly quantum physics, information, computation, and probability.

→ quantum; → information.

1) (n.) The improvement, amendment, or reorganization of something that is considered to be wrong, ineffective, or unsatisfactory; e.g. calendar reform.
2) (v.tr.) To change and improve by correction of faults, alteration, substitution, abolition, etc.

From M.E. reformen, from M.Fr. reformer, from O.Fr., from L. reformare "to form again, change, alter," from → re- "again" + formare "to form," from forma "form, mold, shape, case," origin unknown.

1) Bâzdisi, from bâz- "again," → re-, + dis, disé "form, appearance," (variants -diz, -diš (tandis "body form, like a body; effigy; statute;" mâhdis "moon-like;" šabdiz "night color; a horse of dark rusty color;" andiš- "to think, contemplate"); Mid.Pers. dêsag "form, appearance," dêsidan "to form, build;" Av. daēs- "to show," daēsa- "sign, omen;" cf. Skt. deś- "to show, point out;" PIE *deik- "to show" (cf. Gk. deiknumi "to show," dike "manner, custom;" L. dicere "to utter, say;" O.H.G. zeigon, Ger. zeigen "to show;" O.E. teon "to accuse," tæcan "to teach") + -i noun suffix.
2) Bâzdisidan infinitive from bâzdisi, as above.

A formula, used in atomic physics, which describes the wavelengths or frequencies of light in various series of related spectral lines, such as those emitted by hydrogen atoms.

→ rydberg; → formula.

Same as → Weizsacker formula.

→ semiempirical; → binding; → energy; → formula.

The formation of second-generation stars in a → molecular cloud, as triggered by the presence of → massive stars. The observation that some nearby → OB associations contain distinct, spatially separate subgroups of → OB stars in a sequence of monotonically changing age led Blaauw (1964, ARA&A 2, 213) to suggest that star formation in fact occurs in sequential bursts during the lifetimes of the corresponding molecular clouds. The first quantitative model of this mechanism was presented by Elmegreen and Lada (1977, ApJ 214, 725), who showed that the powerful ultraviolet photons of the massive star create an → ionization front which advances in the molecular cloud and is preceded by a → shock front. The compressed neutral gas lying between the ionization and shock fronts is gravitationally unstable and collapses in time-scales of a few million years to form a new generation of massive stars. The propagation of successive births of OB groups would produce a chain of associations presenting a gradient of age. Elmegreen and Lada estimated the propagation velocity to be 5 km s^-1. For a region with a length larger than 100 pc, this would imply an age difference of the order of 20 million years between the extremities. See also → stimulated star formation, → triggered star formation; → collect and collapse model.

→ sequential; → star formation.

1) A transformation that preserves angles and changes all distances in the same ratio.
2) A transformation of the form B = X^-1AX relating two → square matrices A and B.

→ similarity; → transformation.