Astronomy & Celestial Mechanics
the sky, computed
Concept

Scattering — where it appears

The redirection of light or particles by matter without their being absorbed, such as photons bouncing off free electrons or atoms. It randomises direction, so it blurs and traps radiation while leaving its energy almost unchanged.

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

A peak at 0.55 µm, and therefore a grain size. Interstellar polarisation against wavelength — the Serkowski law, p(λ) = p_max exp[−K ln²(λ_max/λ)] with K = 1.66 λ_max. The heavy curve peaks at 0.550 µm, measured off the drawing rather than read back from the parameter, and falls to half its peak at 1.314 µm on the red side, against the closed form λ_max exp√(ln2/K) = 1.315. That peak wavelength is the measurement. It is set by the size of the grains doing the aligning — bigger grains, longer λ_max — and it is tied to the shape of the extinction curve along the same sight line by R_V ≈ 5.5 λ_max, which gives 3.03 here against the diffuse-medium value of 3.1. The two faint curves are populations peaking at 0.35 µm and 0.75 µm: the same amount of polarisation, distributed differently, and a different dust. Nothing in a photometric measurement of the same star distinguishes them.

The direction a photon count throws away

A photometer records how many photons arrived. It discards a two-component quantity that survives every attenuation on the way, and that quantity carries a magnetic field direction, a grain size, and the shape of an exploding star nobody can resolve.

starlight · Polarimetry 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

Named alongside it

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

Absorption lineAsphericityCollisional excitationEddington barbier relationGrain alignmentInterstellar dustLocal thermodynamic equilibriumMagnetic fieldNon-LTEPolarimetric biasPolarisationPosition angle

All concepts