Astronomy & Celestial Mechanics
the sky, computed
Concept

Absorption line — where it appears

A narrow dark feature in a spectrum where atoms or ions in cooler gas absorb light at one of their transition wavelengths. Its depth, width and position give the gas's composition, temperature, pressure and velocity.

Named by 3 essays across 2 fields — each of them below, with the objects they name alongside it.

An absorption spectrum at 5772 K. A blackbody continuum at 5772 K with absorption lines cut out of it, each line's depth computed from how much of the gas is in a state that can absorb it. 8 of the 8 lines are strong enough to see at this temperature, which is the whole reason the spectral sequence is a temperature sequence.

Composition, read from what is missing

The dark lines in a stellar spectrum are wavelengths that never arrived. Which ones are absent names the elements present — and the strength of a line says more about temperature than about abundance.

starlight · Spectra An absorption line made by scattering alone. Emergent line profiles, as a fraction of the continuum, against distance from line centre in Doppler widths, for a strong line — 10⁵ times the continuum opacity at its centre — in an atmosphere at one temperature throughout, for destruction probabilities ε = 0.01, 10⁻⁴, 10⁻⁶. In local thermodynamic equilibrium this atmosphere makes no line at all: every depth radiates at the same temperature, and the line's extra opacity merely moves the depth from which the light comes. With scattering the line is deep, its central intensity 0.198, 0.021, 0.002 of the continuum, about twice √ε. The darkness is not a low temperature anywhere; it is photons scattered out of the line and back into the gas until they are destroyed or escape through the continuum, never reaching the observer at the line's frequency.

A line darker than any temperature in the gas

An absorption line is usually read as a temperature: the line is dark because its light comes from higher, cooler gas. A strong resonance line breaks that reading. Its photons are scattered thousands of times before a collision can destroy them, so near the surface the line's source function falls to the square root of the destruction probability times the Planck function — and even a gas at one temperature throughout shows a line almost black at its centre.

starlight · Spectra One sight line, two spectra: emission peaks at 46 K while the absorption reaches τ = 1.10. The 21-centimetre line along one sight line through a cold cloud (5·10²⁰ atoms cm⁻² at 60 K) embedded in warm gas (3·10²⁰ cm⁻² at 6000 K, half in front and half behind), computed by radiative transfer. Above: the emission spectrum, brightness temperature against velocity — a broad pedestal from the warm gas (dashed, alone) with the cold cloud's narrow peak on top, capped at 46 K because the cloud is nearly opaque at line centre. Below: the same gas seen in absorption against a bright radio source behind it, the fraction of the source's light transmitted. The absorption shows the cold cloud and almost nothing of the warm gas, whose optical depth is a hundred times smaller per atom. Read in the optically thin approximation, the emission gives a column of 6.26·10²⁰ cm⁻² against a true 8·10²⁰.

A temperature that belongs to none of the gas

The hydrogen between the stars announces itself at 21 centimetres twice — in emission, which counts atoms whatever their temperature, and in absorption against a radio source behind, which counts atoms divided by their temperature. The ratio of the two is a temperature. Along any real sight line it is the harmonic mean of a cold phase and a warm one, a number no parcel of the gas actually has — and where the cold gas is thick enough, the emission stops counting it at all.

galaxies · Interstellar medium

Named alongside it

The objects these essays reach for when they reach for this one.

Brightness temperatureCollisional excitationCurve of growthEddington barbier relationInterstellar mediumIonisationLocal thermodynamic equilibriumMetallicityNon-LTEOptical depthRadiative transferScattering

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