An Etymological Dictionary of Astronomy and Astrophysics
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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.

1) Avoiding waste or extravagance; thrifty.
2) → economic.

→ economic; → -al.

Of or relating to → geochemistry.

→ geochemistry; → -al.

An orbiting infrared telescope (60 cm mirror) which successfully operated from launch in January 1983 until the supply of coolant ran out in November 1983. It was a collaborative mission between NASA, the Netherlands, and the UK, and mapped 95% of the whole sky in the wavelength bands 12, 25, 60, and 100 microns.

→ infrared; → astronomical; → satellite.

An astronomical association of astronomers that is the controlling body of world astronomy. It was founded in Brussels in 1919.

→ international; → astronomical; → union.

A shiny silicate mineral composed of varying amounts of → aluminum, → potassium, → magnesium, and → iron. Mica appears as thin, flexible layers in → granite and other rocks, or as → crystals. It is used as a thermal or electrical → insulator.

From L. mica "crumb, grain."

A → dynamical variable which does not have an → imaginary number part.

→ real; → dynamical; → variable.

A data center dedicated to the collection and worldwide distribution of astronomical data and related information. It is located at the Strasbourg Astronomical Observatory, France. The CDS has several goals, mainly: collecting all of the useful information regarding astronomical objects in computerized form, including observational data produced by observatories on the ground or in space; upgrading these data by critical evaluations and comparisons; and distributing the results to the astronomical community. Currently the CDS services include: → SIMBAD, Aladin interactive sky atlas, and VizieR catalogues.

CDS, short for Centre de Données astronomiques de Strasbourg.

A uniform atomic time scale for apparent geocentric ephemerides defined by a 1979 IAU resolution, which replaced Ephemeris Time. TDT is independent of the variable rotation of the Earth, and the length of the tropical year is defined in days of 86,400 seconds of international atomic time. In 1991 it was replaced by Terrestrial Time.

→ terrestrial; → dynamical; → time.