An Etymological Dictionary of Astronomy and Astrophysics
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A subtype of dwarf ellipticals (→ dwarf elliptical galaxy), which are companion to the → Milky Way and other similar galaxies. The first example of such objects was discovered by Harlow Shapley (1938) in the constellation → Sculptor. 22 such galaxies are known currently to orbit the Milky Way and at least 36 exist in the → Local Group of galaxies. Nearby → galaxy clusters such as the → Virgo, → Fornax, → Centaurus, and → Coma clusters contain hundreds to thousands of individual dSph galaxies. These galaxies have very low → surface brightnesses, as low as only 1% that of the → sky background. They are also among the smallest, least luminous galaxies known. Most of the radiation from dSph galaxies is emitted by stars in the optical portion of the → electromagnetic spectrum. The lack of strong → emission lines, → infrared, or → radio emission suggests that these galaxies are generally devoid of → interstellar medium. The velocities of stars within dSph galaxies are so high that them must be disrupting. However, the bulk of mass in these galaxies might be undetected. Dynamical models that include → dark matter do adequately explain the → velocity dispersion of the stars in all dSph systems. In the most extreme cases, only 1% of the mass of the galaxy is visible. Many of the Local Group dSph galaxies show evidence for → star formation more recent than 10 Gyr.

→ dwarf; → spheroidal; → galaxy.

A galaxy that belongs to the spiral class but is significantly smaller.

→ dwarf; → spiral; → galaxy.

A star that burns its hydrogen content to produce its energy and therefore belongs to the main-sequence luminosity class.

→ dwarf; → star.

A → starburst galaxy that is a → dwarf galaxy. Examples include: the → dwarf irregular galaxy IC 10, the compact dwarf irregular Henize 2-10, and the dwarf irregular galaxy NGC 5253.

→ dwarf; → starburst; → galaxy.

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

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.

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.

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).

→ 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.

→ 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.

Adj. of → dynamics; → range.

Same as → viscosity and → absolute viscosity.

→ 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.

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.

→ dynamical; → age.

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

→ 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.

→ 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).

→ 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.