An Etymological Dictionary of Astronomy and Astrophysics
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In plasma physics, the theory that considers the interactions between waves and particles are of first order only. It ignores all terms of second order in the fluctuating quantities.

→ quasi-; → linear; → theory.

A → recombination line whose wavelength lies in the radio range of the electromagnetic spectrum. Radio recombination lines are the result of electronic transitions between high energy levels (n > 50) in an atom or ion.

→ radio; → recombination line.

1) A straight line that connects the points corresponding to the initial and final states on a graph of pressure versus specific volume for a substance subjected to a → shock wave. The slope of the Rayleigh line is proportional to the square of shock speed. Steeper Rayleigh lines correspond to higher shock speeds. See also → Hugoniot curve.
2) In → Raman scattering, the spectral line in scattered radiation which has the same frequency as the corresponding incident monochromatic radiation.

→ rayleigh; → line.

An → emission line in a spectrum produced in an → H II region when a free electron combines with an ionized atom to form a neutral atom or an ion of lower → ionization stage. Same as → free-bound emission.

→ recombination; → line.

In a straight line; consisting of straight lines. → curvilinear

From recti- combining form of rectus, → right, + → line + -ar, variant of the adjective-forming suffix → -al.

Râst-xatt, from râst, → right, + xatt→ line.

The motion of light in the first approximation, as evidenced from the formation of shadows and other every day experience. However, → diffraction

→ rectilinear; → propagation; → light.

An optical system that is corrected for → distortion and → spherical aberration and therefore forms the image of a straight line as a straight line.

→ rectilinear; → system.

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

→ red; → shift; → line.

The curve representing a → linear regression. It is a line drawn through a set of data that summarizes the relationship between the → variables being studied.

→ regression, → line.

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

→ resolved; → line.

For a particular atom, the spectral line corresponding to the longest wavelength arising from a transition between the ground state and an excited state.

→ resonance; → line.

Radio astro.: Of an OH source, which emits at 1665 and 1667 MHz as the main frequencies, one of the lines arising from transitions at 1612 and 1730 MHz.

→ satellite; → line.

A → spectral line for which the upper and lower → energy levels have different values of S, the total → spin angular momentum. These lines violate the quantum mechanical → selection rule under → LS coupling, ΔS = 0. For example, the Ca I λ6573 line results from transition between the upper → triplet state (³P₁) with a total spin angular momentum S = 1 and the → ground state, a → singlet state (¹S₀, total spin angular momentum S = 0). A semi-forbidden line is marked by a right bracket following the atom name, i.e. Ca I], in the above-mentioned case. Same as → interconnection line and → intersystem line.

→ semi-; → forbidden; → line.

→ line of sight.

→ sight; → line.

A → spectroscopic binary in which only one set of → spectral lines is detectable. The binary nature of the system is deduced from the fact that the spectral lines exhibit periodic → Doppler shifts due to orbital motions in the system. Same as → SB1 binary. See also: → double-lined binary.

→ single; → line; → binary.

In a → protoplanetary disk, the limit between the regions where water is gaseous and the region where it is cold enough for water to become ice. The core accretion theory predicts that → giant planets form just outside the snow line where they can accrete enough rock and ice to generate a core. Subsequently the core grows into a gas giant like → Jupiter or → Saturn via the → accretion of hydrogen and helium. The snow line location depends on the → luminosity of the central star. For solar system it is about 5 AU, the position of Jupiter. Also known as ice line.

→ snow; → line.

Marz, → frontier; yax, → ice.

A dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow wavelength range, compared with the nearby wavelengths.

→ spectral; → line.

A function consisting of several segments, usually → polynomials, joined smoothly together at specific points with an explicitly stated degree of accuracy. Spline functions are used to approximate a given function on an interval.

From East Anglian dialect, maybe related to O.E. splin and to modern splint. A spline was originally a slat or a thin strip of wood. A later meaning was "a long, thin, flexible strip used as a guide for drawing arcs of curves;" → function.

A line without curvature or angles. A line whose → slope is → constant.

→ straight; → line.

In → fluid mechanics, the curve defined by the positions of all particles which have passed through a given point. In laboratory experiments, streak line may be displayed by the stream of color resulting from injection of a dye into the flow.

→ streak; → line.