An Etymological Dictionary of Astronomy and Astrophysics
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→ Spectral lines intermingled.

→ blend; → line.

One of the pair of yellow lines in emission spectra of neutral sodium (Na I). D1 has a wavelength of 5895.94 Å and D2 is 5889.97 Å. This sodium doublet is one of the strongest absorption features in the spectra of late-type stars.

Labelled D in a sequence of alphabetical letters first used by Joseph von Fraunhofer to designate spectral features in the solar spectrum, → Fraunhofer line.

A → magnetic field line in a → fluid when the motion of the fluid carries the magnetic field along with it.

Frozen, p.p. of → freeze; → magnetic; → line.

An imaginary line used for representing the strength and direction of a magnetic field. Charged particles move freely along magnetic field lines, but are inhibited by the magnetic force from moving across field lines.

→ magnetic; → field; → line.

In the context of solar physics, a → magnetic field line when it crosses the solar surface only once, i.e., when it goes from surface to infinity. This is the case at a sufficiently large scale in → coronal holes. This is mostly not the case in → active regions.

→ open; → magnetic; → field; → line.

An ordinary spectral line emitted by atoms undergoing energy transitions that are allowed by the selection rules of quantum mechanics. → forbidden lines.

→ permitted; → line.

A spectral line whose wavelength does not coincide with its theoretical value and is shifted toward longer wavelengths.

→ red; → shift; → line.

A → spectral line that is not contaminated by other nearby lines.

→ resolved; → line.

A collisionally excited spectral line formed in high density condition well above the → critical density. At such densities the → excitation temperature is at (or very near) the → kinetic temperature of the gas. At low densities, below the critical density, the excitation temperature will be only slightly above the radiation temperature and the emission line will be practically invisible.

Thermalized, p.p. of → thermalize; → line.

A spectral line whose origin is not clearly established. → line identification.

→ unidentified; → line.

In relativity, the path traced out in four-dimensional → space-time that represents a continuous sequence of events relating to a given particle. A point on a world line is called an → event. Any straight world line corresponds to an → inertial motion. Curved world lines represent → accelerated motion. A world line that curves corresponds to an accelerated observer. World lines are shown on space-time diagrams.

→ world; → line.