spectral line

A spectral line (or just line) is a dark or bright line within a continuous spectrum. For example, a spectral energy distribution (graph of the brightness at each wavelength) representing the spectrum of some object's light may generally form a curve, i.e., nearby wavelengths have nearly the same brightness. A spectral line can be a "spike" in such a SED (a short stretch of wavelengths within the SED where light has a noticeably higher magnitude than adjacent wavelengths) or a dip (where a short stretch of wavelengths has a noticeably lower magnitude than adjacent wavelengths). If the spectrum is displayed as a rectangular area with the source's wavelengths spread across a dimension (per a plate produced a spectrograph), each of these shows as a line, brighter or darker than its surroundings.

In a source's spectrum, bright lines are caused by emission at specific wavelengths (emission lines), and dark lines are caused by absorption at specific wavelengths (absorption lines), generally by material that the electromagnetic radiation (EMR) is passing through. Lines are related to chemical makeup of the light source or absorber, indicating changes of energy level of electrons (electron excitation), which have characteristic values based upon quantum mechanics. In addition to its wavelength, the line has a shape (spectral line shape): it is not infinitely thin, but in detail is a rise and fall of brightness over the course of a small wavelength-interval, which in a SED graph, shows like a little hill or valley. This shape can reveal characteristics of the light source or intervening absorber such as velocity, turbulence, etc. The term line broadening refers to the mechanisms that create this shape, e.g., in EMR from a star.

Among the spectral lines widely used in astrophysics are the hydrogen series of lines, e.g.,

• Lyman series
• Balmer series
• Paschen series
• Brackett series
• Humphreys series
• Pfund series

Atomic spectral lines are sometimes identified by element and ionization, specified by the chemical symbol followed by a Roman numeral, I meaning neutral, II meaning singly ionized positive, III meaning doubly ionized, etc. For example, a line due to neutral hydrogen can be termed an HI line.

Spectral-line mapping refers to imaging or surveying a portion of the sky for the presence of particular spectral lines, often noting their redshift, to determine the source material's radial velocity. Its uses include investigations of accretion disks, jets, molecular clouds, the entire Milky Way, and other galaxies.