An Etymological Dictionary of Astronomy and Astrophysics
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An observed correlation between the luminosity of a spiral galaxy and its rate of rotation (measured from its 21 cm hydrogen line). This means that more luminous galaxies have stars that are moving faster. Knowing the rotational velocity of a spiral galaxy, this relation provides its absolute magnitude and then its distance. → Faber-Jackson relation.

Named after R. B. Tully and J. R. Fisher who first derived this relationship (1977, A&A 54, 661); → relation.

1) An act of tumbling or falling.
2a) To fall helplessly down, end over end, as by losing one's footing, support, or equilibrium; plunge headlong.
2b) To roll end over end, as in falling (Dictionary.com).

M.E. tum(b)len "to dance in acrobatic style;" perhaps from O.E. tumbian "dance about, tumble, leap;" maybe related to Du. tuimelen, O.H.G. tumon "to reel."

Kaltâv, from Kermâni keletow, Malâyeri kallatow "wobbling," from kal, kalleh "head" + tâv, tow, tâb "swing, twist," from tâbidan "to twist, to spin."

An asteroid whose rotational motion does not take place about its → principal axis. Such a behavior can be interpreted as a composition of two or more rotational periods, and described mathematically by a two dimensional → Fourier series (Pravec et al. 2005, Icarus, 173, 108).

The term was first used by A. W. Harris, 1994, Icarus, 107, 209. → tumble; → asteroid.

The motion of a solid body whose rotation axis is not fixed in space. For example, that of an asteroid that does not rotate about one of their principal axes. → tumbling asteroid.

→ tumble; → motion.

A very hard, silver-white to steel-gray metal with a body-centered cubic crystalline structure; symbol W. Atomic number 74; atomic weight 183.85; melting point about 3,410°C; boiling point 5,660°C; specific gravity 19.3 at 20°C. The chemical element was discovered by the Swedish chemist Carl-Wilhelm Scheele in 1781. Tungsten metal was first isolated by the Spanish chemists Don Fausto d'Elhuyar and his brother Don Juan Jose d'Elhuyar in 1783.

The name derives from the Swedish ng sten "heavy stone". The chemical symbol, W, is derived from the Ger. wolfram, which was found with tin and interfered with the smelting of tin.

The violent impact of a comet or meteorite in the Tunguska region of Siberia on 30 June 1908. The object exploded in the atmosphere before touching the ground at an estimated height of 5-10 km. Observers reported seeing a fireball as bright as the Sun. The explosion caused a shock wave that shook buildings and caused damage, though there was no loss of human life. The first expedition to the remote area of the explosion took place in 1927. An estimated 80 million trees covering more than 2,150 square km were flattened. The energy of the explosion is estimated to have been equivalent to that of about 15 → megatons of TNT , a thousand times more powerful than the atomic bomb dropped on Hiroshima in 1945.

From the name of the central Siberian region, Russ. Podkamennaya (Lower Stony) Tunguska River, today Krasnoyarsk Krai; → event.

A phenomenon in quantum mechanics whereby a particle can penetrate and cross a potential barrier whose energy is greater than the particle's energy. The tunnel effect, forbidden in classical mechanics, is a direct consequence of the wave nature of material particles. Also called tunneling

M.E. tonel, from M.Fr. tonele, tonnelle "funnel-shaped net," feminine of tonnel,diminutive of tonne "tun, cask for liquids." Sense of "tube, pipe" developed in Eng. and led to sense of "underground passage."

Oskar, → effect; tunel, from Fr. tunnel, as above.

Having sediment or foreign particles stirred up or suspended; obscured, muddy, such as turbid water.

From L. turbidus "muddy, full of confusion," from turbare "to confuse, disturb," from turba "turmoil, crowd," probably from Gk. tyrbe "turmoil;" cf. Pers. târ "dark, obscure, cloudy," Laki tur "balk, refractory, restive."

Târ "obscure, dark," variant târik "dark;" Mid.Pers. târig "dark," târ "darkness;" Av. taθra- "darkness," taθrya- "dark;" cf. Skt. támisrâ- "darkness, dark night," L. tenebrae "darkness;" Hittite taš(u)uant- "blind;" O.H.G. demar "twilight."

1) Meteo.: A measure of vertical extinction of solar radiation in the atmosphere. Turbidity is directly related to aerosol concentrations in the tropospheric and stratospheric layers. → visibility.
2) The cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air.

→ turbid; → -ity.

An engine or motor in which the → kinetic energy of a moving → fluid (water, steam, air, or hot gases) acts on the blades, vanes, or buckets of a → rotor to produce rotational motion that can be converted into electrical or mechanical power. In an impulse turbine the turbine is driven by free jets of fluid striking the blades. In a reaction turbine the turbine is driven by the reactive force of a fluid passing through the rotor blades. Turbines are used in hydroelectric power generators, ship propulsion systems, and jet aircraft engines.

From Fr. turbine, from L. turbinem (nominative turbo) "spinning top, eddy, whirlwind," related to turba "turmoil, crowd."

Turbin, loan from Fr., as above.

A state of hydrodynamic → flow in which the velocity at each point fluctuates rapidly and randomly so that only statistical properties can be recognized and subjected to analysis. Turbulence is the most striking manifestation of the non-linear nature of the laws of hydrodynamics, with the irregularity of flows increasing with the → Reynolds number measuring the strength of non-linear effects. The regime of intermediate Reynolds numbers corresponds to a highly non-universal regime of the onset of turbulence, whereas high Reynolds numbers, common in practical situations, characterize the regime of → developed turbulence. → laminar flow; → chaos.

From L. turbulentia, from turbulentus "full of commotion, restless," from turba "turmoil, crowd;" maybe related to Pers. târ "dark, obscure, cloudy," Laki tur "balk, refractory, restive."

šubnâki, from âšub "turmoil, disturbance," âšoftan "to agitate, disturb;" Mid.Pers. âšôb "confusion, turmoil," âšoftan "to destroy, disturb;" Av. xšuf- "to tremble;" cf. Skt. ksobh- "to stagger, begin to swing, tremble;" Pol. chybac "to rock, move to and fro;" Lith. skubus "hasty, fast;" Goth. afskiuban "to shove;" O.E. scufan "to shove;" PIE base *k^(w)seub^h-, + -nâk state suffix, -i noun suffix.

The process whereby turbulence evolves by exchanging energy, leading to → dissipation.

→ turbulence; → decay.

The quality of a flow that undergoes → turbulence.

Adj. from → turbulence.

The layer in which the Reynolds stresses are much larger than the viscous stresses. When the → Reynolds number is sufficiently high, there is a turbulent layer adjacent to the → laminar boundary layer.

→ turbulent; → boundary; → layer.

A star formation scenario whereby → massive stars form from gravitationally bound → pre-stellar cores, which are supersonically → turbulent and in approximate pressure equilibrium with the surrounding protocluster medium. The high → accretion rates that characterize such media allow accretion to overcome the radiation pressure due to the luminosity of the star. The core is assumed to → collapse via an → accretion disk to form a single star or binary. The core density structure adopted is ρ ∝ r^-k, with k = 1.5 set from observations. This choice affects the evolution of the accretion rate, which increases linearly with time. The high densities in regions of massive-star formation lead to typical time scales for the formation of a massive star of about 10⁵ years (McKee & Tan 2003, ApJ 585, 850).

→ turbulent; → core.

A → flow characterized by → turbulence. In other words, a flow in which the motion at any point varies unpredictably in direction and magnitude. See also → laminar flow; → transitional flow.

→ turbulent; → flow.

The characteristic mass for → cloud fragmentation in a → turbulent medium. While the standard → Jeans mass depends simply on the gas mean → density and → temperature, and fragmentation is purely gravitational, turbulent Jeans mass depends strongly also on the → Mach number (Chabrier et al. 2014, arXiv:1409.8466).

→ turbulent; → Jeans; → mass.

A plasma characterized by a → turbulent flow regime.

→ turbulent; → plasma.

1) To move, or cause to move, around, or partly around a center.
2) The action or an act of turning about a center or axis.

M.E. turnen; O.E. turnian "to rotate, revolve," also from O.Fr. torner "to turn," both from L. tornare "to turn on a lathe," from tornus "lathe," from Gk. tornos "lathe, tool for drawing circles."

Gardidan "to turn; turning," variant gaštan "to turn, to change;" Mid.Pers. vartitan; Av. varət- "to turn, revolve;" Skt. vrt- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wer- " to turn, bend."

The closest point in the path of a sound wave to the center of a star, as studied in → asteroseismology. Starting from the surface, the sound wave first moves into the star almost straight toward the center. Its path then deflects, because of the increasing → sound speed, so that it misses the center of the star. After the turning point, the wave moves out again until it reaches the surface, where it is reflected. If exactly an integer number of wavelengths fits between two reflections at the surface, the sound wave corresponds to a → standing wave with a specific pattern of → node lines on the surface.

→ turn; → point.