An Etymological Dictionary of Astronomy and Astrophysics
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The forces of jets from a comet's nucleus that can cause a rocket-like effect and alter a comet's direction of motion slightly.

→ non-; → gravitational; → force.

The force of → attraction between molecules of different substances; for example, the force between the molecules of a solid and a liquid. When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the surface. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules.

→ adhesive; → force.

The force exerted by a gaseous fluid upon a body completely immersed in it caused by their relative motion. The components of aerodynamic force are: → lift and → drag.

→ aerodynamic; → force.

Any → transformation preserving → collinearity.

→ affine; → transformation.

Any of the four main → proposition forms treated in Aristotle's → syllogism:
The A form (universal affirmative): All P's are Q's,
The E form (universal negative): No P's are Q's,
The I form (particular affirmative): Some P's are Q's, and
The O form (particular negative) Some P's are not Q's.

Greek philosopher Aristotle (384-322 BC); → form.

A physical force (→ gravitational, → electric, → magnetic, etc.) by which a body attracts another.

→ attractive; → force.

A special solution of the mathematical formula which represents the wavelengths of the various spectral series of hydrogen in which the lower energy level is n = 2.

→ Balmer; → formula.

A system for estimating and reporting wind speeds which has 13 standardized categories and associated descriptions. The Beaufort scale ranges from 0 for complete calm to 12 for a cyclone. In this scale, the wind speed (in km/h) equals 3B^1.5, where B is the Beaufort number of the wind. The scale was originally devised for use at sea but has subsequently been modified for use over land.

Named after Admiral Sir Francis Beaufort (1774-1857), who introduced the first version of the system in 1805; → scale.

The mathematical expression giving the → entropy, S, of a → black hole as a function of the area of its → event horizon, A: S = (kc³A)/(4Għ), where k is → Boltzmann's constant, ħ is the → reduced Planck's constant, and G the → gravitational constant. It can also be expressed by S = (kA)/(4l_P²), where l_P is the → Planck length. The existence of this entropy led to the prediction of the → Hawking radiation, because an entropy is associated with a temperature and a temperature to a → thermal radiation. The entropy of a black hole increases continuously because the fall of material into it increases its area.

For Jacob D. Bekenstein (1947-), an Israeli theoretical physicist, who contributed to the foundation of black hole thermodynamics; → formula.

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.

The retrieval and analysis of biochemical and biological data using mathematics and computer science, as in the study of genomes (Dictionary.com).

→ bio-; → informatics.

A mechanism for the extraction of energy from a rotating → Kerr black hole. It relies on the assumption that the material → accreted by a → black hole would probably be → magnetized and increasingly so as the material gets closer to the → event horizon. Since all black holes of current astrophysical interest are probably accreting from magnetized disks, this has led to suggestions that the Blandford-Znajek process plays a vital role in → active galactic nuclei (AGN) and other accreting black hole systems. The power, P, generated is given by: P = (4π/μ₀) B²R_S²c, where B is the → magnetic field of the → accretion disk, and R_S is the → Schwarzschild radius of the black hole. As an example, for a 10⁸ solar mass black hole with a 1 T magnetic field, the power generated is approximately 2.7 × 10³⁸ W. In perspective, the annual energy consumption of the world is estimated around 5 × 10²⁰ J. The example case presented produces more energy in a single second than the entire globe consumes in a year. While this is a bold claim to make, it is only an example case where not all the energy produced is extractable as useable energy. However, at that point, even a system which is less that < 10-15 % efficient would be sufficient to supply enough energy to power the world for a full year. Of course, the system itself is limited in its lifetime due to the extraction of energy by slowing down the rotation of the black hole. Hence, the system can only exist as long as the black hole has angular momentum, continuing to rotate. At some point, the rotation will cease and the energy source will be unusable (D. Nagasawa, PH240, Stanford University, Fall 2011).

Blandford, R. D., & Znajek, R. L., 1977, MNRAS 179, 433; → process.

In → statistical thermodynamics, a probability equation relating the → entropy S of an → ideal gas to the quantity Ω, which is the number of → microstates corresponding to a given → macrostate: S = k. ln Ω. Same as → Boltzmann's relation.

→ Boltzmann's constant; → entropy; → formula.

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.

The force that causes immersed bodies to float or rise to the surface of a liquid or upward in a gas. Buoyant force is produced by → gravity and density differences. Same as → buoyancy.

From buoy (current meaning) "a float moored in water to mark a location," from M.E. boye, from O.Fr. buie or M.Du. boeye, from L. boia "fetter, chain" + suffix -ant; → force.

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 simplest expression of an equation, statement, or rule.

→ canonical; → form.

A → force that is always directed toward a fixed point and whose → magnitude depends only on the distance from that point. Mathematically, F is a central force if and only if: F = f(r)r₁ = f(r)r/r, where r₁ = r/r is a unit → vector in the direction of r. If f(r) < 0 the force is said to be → attractive toward the source. If f(r) > 0 the force is said to be → repulsive from the source. In other words, a central force is one whose → potential, V(r), depends only on the → distance from the source. → Gravitational force and → electrostatic force are central, with V(r)∝ 1/r.

→ central; → force.

A force in a rotating reference frame directed outward from the axis of rotation.

→ centrifugal; → force.