An Etymological Dictionary of Astronomy and Astrophysics
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A galaxy that is much smaller than other members of the elliptical class; it is designated as dE. A subtype of dwarf ellipticals is called a → dwarf spheroidal galaxy (dSph). The basic characteristics of the class are low surface brightness and smooth light distribution. They range in luminosity from that of the faintest dSph galaies M_V ~ -9 to about -17. In the → Local Group there are 19 known dEs. They are very common in → galaxy clusters.

→ dwarf; → elliptical; → galaxy.

A small, low luminosity galaxy that is associated with a larger spiral galaxy and may make up part of a galactic halo. There are many of them in the Local Group, and often orbit around larger galaxies such as the Milky Way and the Andromeda Galaxy. There are three main types of them: → dwarf elliptical galaxy (dE), → dwarf irregular galaxy (dI), and → dwarf spiral galaxy (dSA).

→ dwarf; → galaxy.

An irregular galaxy that is much smaller than other irregulars. Dwarf irregulars are generally metal poor and have relatively high fractions of gas. They are thought to be similar to the earliest galaxies that populated the Universe, and are therefore important to understand the overall evolution of galaxies.

→ dwarf; → irregular; → galaxy.

A class of → novae and → cataclysmic variables that have multiple observed → eruptions. Their prototype is → U Geminorum star. Optically, dwarf nova eruptions have amplitudes of 2-6 mag in V, a duration of a few to 20 days and a recurrence time-scale of weeks to years. Dwarf novae are thought to be → semidetached binary stars consisting of a → white dwarf  → primary accreting via → Roche lobe overflow from a → companion which is usually a → late-type, generally → main-sequence star. DN outbursts are usually attributed to the release of gravitational energy resulting from an → instability in the → accretion disk or by sudden mass transfers through the disk.

→ dwarf; → nova.

A new category of → astronomical objects in the → solar system introduced in a resolution by the 26th General Assembly of the → International Astronomical Union (IAU) on August 24, 2006. The characterizing properties are as follows: 1) It is in orbit around the Sun; 2) It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a → hydrostatic equilibrium (nearly round) shape; 3) It has not "cleared the neighbourhood" around its orbit; and 4) It is not a → satellite of a → planet, or other non-stellar body. The property 3 reclassified → Pluto from a planet to a dwarf planet because it has not cleared the neighborhood of its orbit (the → Kuiper Belt). The largest known dwarf planets are: → Eris, → Pluto, → Ceres, → Makemake, and → 2015 RR245.

→ dwarf; → planet.

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.