An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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).

→ dynamical; → friction.

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

→ dynamical; → law.

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

→ 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.

→ 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.

→ 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).

→ dynamical; → stream.

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

→ 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.

→ 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.

→ 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.

→ dynamical; → variable.

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

→ dynamic; → -ics.

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

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

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.

→ 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.

→ 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.

→ dynamo; → theory.

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

Dynamometer, from → dynamo + → -meter.

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.

From Fr., from dynamis "power," → dynamics.

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

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."

As above.

prefix meaning "bad, ill, abnormal."

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."

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.

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