An Etymological Dictionary of Astronomy and Astrophysics
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Of a system, the quantity whose decrease gives the maximum amount of external work which is performed when any physical or chemical process is carried out reversibly at constant temperature. It is defined by F = U - TS, where U is the → internal energy, T the → absolute temperature, and S the final → entropy.

After the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → free; → energy.

A → decomposition theorem, whereby a continuous → vector field, F, can be broken down into the sum of a → gradient and a → curl term: F = -∇φ + ∇ xA, where φ is called the → scalar potential and A the → vector potential.

→ Helmholtz free energy; → theorem.

The contraction of a volume of gas under its → gravity, accompanied by the → radiation of the lost → potential energy as → heat.

After the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907) and the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → contraction.

An → instability raised when there is sufficient velocity difference across the interface between two uniformly moving → incompressible fluid layers, or when velocity → shear is present within a continuous fluid.

→ Kelvin-Helmholtz contraction; → instability.

The heating of a body that contracts under its own gravity. For a large body like a planet or star, gravity tries to compress the body. This compression heats the core of the body, which results in internal energy which in turn is radiated as → thermal energy. In this way a star could be heated by its own weight.

William Thomson (Lord Kelvin) and Hermann von Helmholtz proposed that the sun derived its energy from the conversion of gravitational potential energy; → mechanism.

The characteristic time that would be required for a contracting spherical cloud of gas to transform all its → gravitational energy into → thermal energy. It is given by the relation: t_KH ≅ GM²/RL, where G is the → gravitational constant, M is the mass of the cloud, R the initial radius, and L the → luminosity. The Kelvin-Helmholtz time scale determines how quickly a pre-main sequence star contracts before → nuclear fusion starts. For the Sun it is 3 x 10⁷ years, which also represents the time scale on which the Sun would contract if its nuclear source were turned off. Moreover, this time scale indicates that the gravitational energy cannot account for the solar luminosity. For a → massive star of M = 30 Msun, the Kelvin-Helmholtz time is only about 3 x 10⁴ years.

→ Kelvin-Helmholtz contraction; → time.

An older name for the unit of electrical → conductance, which is defined to be the reciprocal of the → ohm. It is now replaced by the → siemens.

Ohm spelt backward.

A hypothetical topological feature, based on → general relativity, that connects two different points like a "tunnel" in → space-time. The most common concept of a wormhole is an → Einstein-Rosen bridge. A trip through the wormhole could take much less time than a journey between the same starting and ending points in normal space. Wormholes have various types, intra-universe wormholes (connecting two distant regions of our Universe with each other) and inter-universe wormholes (that connect our Universe with another universe).

The term was coined by the Princeton physicist John Wheeler (1911-2008), from worm, M.E., O.E. wurm "serpent, dragon;" cf. O.S., O.H.G., Ger. wurm, O.Fris., Du. worm, Goth. waurms "serpent, worm;" akin to Pers. kerm "worm," as below; → hole.

Kerm "worm;" Mid.Pers. kirm "worm, snake, dragon;" cf. Skt. krmi- "worm, maggot;" O.Ir. cruim "worm;" Lith. kirmis "worm;" L. vermis "worm;" E. worm, as above; surâx, → hole.