An Etymological Dictionary of Astronomy and Astrophysics
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The wavelength at which the radiant intensity of a source is maximum.

→ peak; → wavelength.

The size limit, l_P = √ (ħ G/ c³), about 10^-33 cm, at which Einstein's notions of space-time are supposed to break down, and space is predicted to become "foam like."

→ Planck; → length.

The mean distance traveled by a photon or particle in a given medium before its energy is reduced by a factor e due to its interaction with matter.

→ radiation; → length.

The → electromagnetic radiation with a wavelength ranging from 1 mm to several 100 km. See so → radio wave.

→ radio; → wavelength.

In the → Sobolev approximation, the length over which the conditions of the gas do not change and the approximation is valid. It is expressed by: l_s = v_th/(dv/dr), where v_th is the thermal line width and (dv/dr) the velocity gradient. In other words, the length over which the profile function of a line is shifted through a distance equal to its own width by the macroscopic velocity gradients that exist in the moving medium.

→ Sobolev approximation; → length.

The scale beyond which for a thin, rotating disk, rotation stabilizes self-gravitational contraction. The Toomre length is given by: λ_T = 4π²GΣ / κ², where G is the → gravitational constant, Σ is the mass → surface density, and κ is the → epicyclic frequency (Toomre 1964, ApJ 139, 1217).

→ Toomre parameter; → length.

The distance between two successive points in the wave that are characterized by the same phase of oscillation; e.g. → de Broglie wavelength; → Compton wavelength; → blaze wavelength; → peak wavelength; → center wavelength; → central wavelength; → cutoff wavelength.

→ wave; → length.