An Etymological Dictionary of Astronomy and Astrophysics

1. Involving or relating to force related to motion.
   
2. Pertaining to the science of → dynamics.

Etymology (EN): From Fr. dynamique, from Ger. dynamisch, introduced by Leibnitz in 1691, from Gk. dynamikos “powerful,” from dynamis “power,” from dynasthai “be able to have power” + → -ic.

Etymology (PE): Tavânik, from tavân “power, strength,” tavânestan “to be powerful, able,” + Pers. suffix -ik; → -ics. The first component from Mid.Pers. tuwan “power, might,” from O.Pers./Av. base tav- “to have power, to be strong, to be able,” Av. tavah- “power,” təviši- “strength,” Mod.Pers. tuš, tâb “power, ability,” O.Pers. tauman- “power, strength,” tunuvant- “powerful,” Skt. tu- “to be strong, to have authority,” tavas-, tavisa- “strong, energetic,” tavisi- “power, strength” + -ik→ -ic.

Mechanics: The condition of a moving mechanical system when the accelerating force is balanced by an imaginary kinetic reaction according to → d’Alembert’s principle. See also → static equilibrium; → thermodynamic equilibrium.

See also: Dynamic, adj. from → dynamics; → equilibrium.

A measure of the extent to which mass has been shifted from the polar regions of a (spinning) body toward its equator (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → dynamic; → oblateness.

A property of a moving → fluid defined by (1/2)ρv² in → Bernoulli’s law, where ρ is → density of fluid and v is → velocity. Dynamic pressure is the difference between → total pressure and → static pressure. Also called → velocity pressure. → ram pressure.

See also: → dynamic; → pressure.

The ratio of the maximum to minimum signal levels present in an image. For instance, a true 12-bit digital camera is capable of providing a dynamic range of 4096 to 1.

See also: Adj. of → dynamics; → range.

Same as → viscosity and → absolute viscosity.

See also: → dynamic; → viscosity.

Of or pertaining to force or power; of or pertaining to force related to motion.
Pertaining to the science of dynamics. Same as dynamic.

See also: Adj. from → dynamics.

Age based on dynamical properties of a system. For example, the time derived for a system to evolve from an initial state to its present state, based on velocity and dimension (size) measurements.

See also: → dynamical; → age.

The process whereby a → bound system, such as a → binary system or a → globular cluster, is broken apart.

See also: → dynamical; → disruption.

Of a physical system, a condition in which the parts of the system are in continuous motion, but they move in opposing directions at equal rates so that the system as a whole remains in equilibrium.

See also: → dynamical; → equilibrium.

The gravitational interaction between a relatively massive body and a field of much less massive bodies through which the massive body travels. As a result, the moving body loses → momentum and → kinetic energy. An example of dynamical friction is the sinking of massive stars to the center of a → star cluster, a process called → mass segregation. Dynamical friction plays an important role in → stellar dynamics. It was first quantified by Chandrasekhar (1943).

See also: → dynamical; → friction.

A law that describes the motion of individual particles in a system, in contrast to → statistical laws.

See also: → dynamical; → law.

The mass of an object derived indirectly from theoretical formulae based on the laws governing the behavior of a → dynamical system.

See also: → dynamical; → mass.

A method for deriving the distance to a binary star. The angular diameter of the orbit of the stars around each other and their apparent brightness are observed. By applying Kepler’s laws and the mass-luminosity relation, the distance of the binary star can be calculated.

See also: → dynamical; → parallax.

The evolution over time of a gravitationally → bound system consisting of N components because of encounters between the components, as studied in → stellar dynamics. Due to this process, in a → star cluster,
→ low-mass stars may acquire larger random velocities, and consequently occupy a larger volume than → high-mass stars. As a result, massive stars sink to the cluster centre on a time-scale that is inversely proportional to their mass. See also → mass segregation.

See also: → dynamical; → relaxation.

A group of stars pervading the Solar neighbourhood and travelling in the → Galaxy with a similar spatial velocity, such as the → Ursa Major star cluster, The term dynamical stream is more appropriate than the traditional term supercluster since it involves stars of di fferent ages, not born at the same place nor at the same time. A possible explanation for the presence of young groups in the same area as those streams is that they have been put there by the → spiral wave associated with their formation place, while kinematics of the older stars of the sample have also been disturbed by the same wave. The seemingly peculiar chemical composition of the Hyades-Pleiades stream suggests that this stream originates from a specific galactocentric distance and that it was perturbed by a spiral wave at a certain moment and radially pushed by the wave in the solar neighbourhood. This would explain why this stream is composed of stars sharing a common metallicity but not a common age (Famaey et al. 2005, A&A 430, 165).

See also: → dynamical; → stream.

A system composed of one or more entities in which one state develops into another state over the course of time.

See also: → dynamical; → system.

The independent variable in the theories which describe the motions of bodies in the solar system. The most widely used form of it, known as Terrestrial Time (TT) or Terrestrial Dynamical Time (TDT) uses a fundamental 86,400 Systeme Internationale seconds (one day) as its fundamental unit. → Terrestrial Time; → Terrestrial Dynamical Time;
→ Barycentric Dynamical Time.

See also: → dynamical; → time.

1. The characteristic time it takes a protostellar cloud to collapse
   if the pressure supporting it against gravity were suddenly removed;
   also known as the → free-fall time.
   

2. → crossing time for a stellar system like a galaxy.

See also: → dynamical; → time-scale.

Mechanics: One of the variables used to describe a system in classical mechanics, such as coordinates (of a particle),
components of velocity, momentum, angular momentum, and functions of these quantities.

See also: → dynamical; → variable.

The branch of → mechanics that explains how particles and systems move under the influence of forces.

See also: → dynamic; → -ics.

An electric generator, i.e. a machine that converts mechanical energy into electrical energy by virtue of the → electromagnetic induction.

Etymology (EN): From Ger. dynamoelektrischemaschine, coined (1867) by the German inventor Werner von Siemens (1816-1892), from Gk. dynamis “power,” → dynamics.

Etymology (PE): Tavânzâ, from tavân “power,” → dynamics + -zâ “generator,” from zâdan “to give birth,” Mid.Pers. zâtan, Av. zan- “to bear, give birth to a child, be born,” infinitive zazâite, zâta- “born,” cf. Skt. janati “begets, bears,” L. gignere “to beget,” PIE base *gen- “to give birth, beget.”

The generation of magnetic fields by movements within a
→ plasma, such as the → convective cores and → convective envelopes of stars. The magnetic field is intensified by the motion of the plasma in much the same way as in a dynamo. The generated magnetic field is not static, but evolves over time.

See also: → dynamo; → effect.

A theory for the generation of a star’s or planet’s magnetic field by the circulation of conducting fluids inside it. → solar dynamo.

See also: → dynamo; → model.

Branch of magnetohydrodynamics concerned with self-excitation of magnetic fields in any large rotating mass of conducting fluid in motion (usually turbulent). Self-exciting dynamo action is believed to account for magnetic fields at the planetary, stellar, and galactic scales.

See also: → dynamo; → theory.

A device for measuring mechanical force; specifically, one that measures the output or driving torque of a rotating machine.

Etymology (EN): Dynamometer, from → dynamo + → -meter.

Etymology (PE): Tavânsanj, from tavân “power,” → dynamics

• -sanj, → -meter.

The centimeter-gram-second (cgs) unit of force (symbol dyn) that imparts an acceleration of 1 cm s^-2 to a mass of 1 gram. 1 dyn = 10^-5 → newton.

See also: From Fr., from dynamis “power,” → dynamics.

An electrode that performs electron multiplication by means of secondary emission.

Etymology (EN): From dyn(a)- a combining form meaning “power,” → dynamics

• -ode a combining form meaning “way, road,” used in the formation of compound words (anode; electrode), from Gk. hodos “way.”

Etymology (PE): As above.

prefix meaning “bad, ill, abnormal.”

Etymology (EN): From Gk. dys- “bad, hard, unlucky,” cognate with O.Pers. duš-, Av. duž- (see below), Skt. dus- “bad, wrong, difficult, un-, -less,” PIE *dus- “bad, ill.”

Etymology (PE): Doš-, dož- “bad, ill, abnormal,” from Mid.Pers. duž-, duš-,
O.Pers. duš- (dušiyâr- “bad year, famine”), Av. duž-, duš- “bad” (duž-mainnyav- “evil-minded, enemy,” Mod.Pers. došman “enemy”); PIE *dus-, as above.

A → satellite of the
→ dwarf planet → Eris.

See also: Dysnomia in Gk. mythology is the daughter of Eris and the goddess of lawlessness.

→ Dyson sphere.

See also: → Dyson; → shell.

A hypothetical structure built around a → star by an advanced → civilization to utilize most or all of the → energy radiated by their star.
The idea of such a sphere was first formalized and popularized by theoretical physicist Freeman Dyson in 1960, though it was originally put forward by a 1945 science fiction novel. Dyson assumed that the power needs of → intelligent civilizations never stops increasing. He also proposed that searching for the existence of such structures might lead to the discovery of advanced civilizations elsewhere in the Galaxy. Sometimes referred to as a → Dyson shell or → megastructure.

See also: Freeman John Dyson (1923-). His article, entitled “Search for Artificial Stellar Sources of Infrared Radiation,” appeared in the 1960 issue of Science, 131 (3414), 1667-1668; → sphere.