An Etymological Dictionary of Astronomy and Astrophysics
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Any → transformation preserving → collinearity.

→ affine; → transformation.

The theory that bright galaxies form preferentially from anomalously overdense perturbations in the → early Universe.

→ biased; → galaxy; → formation.

A concept of → star formation in which → high-mass stars and → low-mass stars form in different physical conditions involving different → molecular clouds. Following the pioneering suggestion of Herbig (1962), successive investigations have generally supported the idea that star formation proceeds bimodally with respect to stellar mass. The star formation rate appears to differ both spatially and temporally for low mass and → massive stars. This is of considerable importance for galactic evolution, since the low-mass stars lock up mass and are long-lived, low luminosity survivors to the present epoch, whereas massive stars are short-lived, recycle and enrich interstellar gas, and leave dark remnants while producing a high luminosity per unit of mass (Silk, J., 1988, in Galactic and Extragalactic Star Formation, p. 503, eds. R. E. Pudritz and M. Fich).

→ bimodal; → star; → formation.

A → structure formation scenario in which small galaxies form first, and larger structures are then formed in due course. Contrary to → top-down structure formation.

→ bottom; → up; → structure; → formation; → galaxy.

An intense → star formation activity in a region of → interstellar medium or, more globally, in a → galaxy. It is characterized by a → star formation rate which is much higher than the corresponding average. Same as → starburst.

→ burst; → star; → formation.

The fraction of → star formation which happens in → bound clusters. It is defined as the ratio between the → cluster formation rate and → star formation rate (Bastian, 2008, MNRAS 390, 759, arxiv/0807.4687).

→ cluster; → formation; → efficiency.

A parameter used in star formation models representing the ratio of the total mass in → star clusters to the corresponding age range (Bastian, 2008, MNRAS 390, 759, arxiv/0807.4687).

→ cluster; → formation; → rate.

The chemical reaction that gives rise to → carbon monoxide in the → interstellar medium. According to models, several processes may lead to CO formation. For example, HCO⁺ + e → CO + H. The molecule HCO⁺ is itself produced through several paths, for example: H₃⁺ + C → CH₂⁺ + H, CH₂⁺ + H₂ → CH₃⁺ + H, CH₃⁺ + O → HCO⁺ + H. Alternatively: C⁺ + H₂O → HCO⁺ + H. Another possibility: C⁺ + OH → CO⁺ + H, CO⁺ + H₂→ HCO⁺ + H.

→ carbon monoxide; → formation.

A theory according to which the Earth and Moon formed from the → protoplanetary disk at the same time. See also → giant impact hypothesis, → capture theory, → fission theory.

→ co-; → formation; → theory.

Formation of stars, especially → massive stars, in group as opposed to individual formation.

→ collective; → star; → formation.

Empirical mathematical transformation applied to the observed magnitudes in order to convert them into a standard system, or into a different system.

→ color; → transformation.

A crucial period in the history of the → Universe, when the bulk of stars in massive galaxies were likely formed. Observations of young stars in distant galaxies at different times in the past have indicated that the → star formation rate peaked at the → redshift of z ~ 2, some 10 billion years ago, before declining by a factor of around ten to its present value (P. Madau & Dickinson, 2014, arXiv:1403.0007).

→ cosmic; → star; → formation; → peak.

Altering in the size or shape of a body. See also → deformable.

Verbal noun of → deform.

A deformation of a → solid body in which the change (→ strain) in the relative position of points in the body disappears when the deforming stress is removed. See also → elastic limit.

→ elastic; → deformation.

1) The act or process of forming or the state of being formed, such as → star formation.
2) Geology: A laterally continuous rock unit with a distinctive set of characteristics that make it possible to recognize and map from one → outcrop or well to another. The basic rock unit of stratigraphy.

Verbal noun of → form, + -ate + -ion.

Diseš, verbal noun of disidan, → from dis-, → form, + -eš.

The study dealing with the processes that gave rise to galaxies in a remarkably → early Universe. See also → structure formation, → protogalaxy

→ galaxy; → formation.

The method of relating a measurement in one → reference frame to another moving with a constant velocity with respect to the first within the → Newtonian mechanics. The Galilean transformation between the coordinate systems (x,y,z,t) and (x',y',z',t') is expressed by the relations: x' = x - vt, y' = y, z' = z. Galilean transformations break down at high velocities and for electromagnetic phenomena and is superseded by the → Lorentz transformations.

→ Galilean; → transformation.

A change of the fields of a gauge theory that does not change the value of any measurable quantity.

→ gauge; → transformation.

The process by which dust grains are assembled or produced.

→ grain; → formation.

A cosmological → structure formation model in which the smallest gravitationally bound structures (→ quasars and galaxies) form first, followed by → groups, → galaxy clusters, and → superclusters of galaxies.

→ hierarchical; → structure; → formation.