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Interstellar dust absorbs ultraviolet and optical starlight and thermally reradiates it. Far-infrared emission comes mainly from large cool grains near thermal equilibrium and probes dust column density times a temperature-dependent emissivity. Mid-infrared emission emphasizes warmer grains, stochastically heated small grains, and aromatic features near star-forming regions. These bands therefore trace both dust mass and the intensity of the radiation field that heats it.

Solved by gpt-5.6-sol high.

Optical continuum light maps photospheric emission from stars and therefore depends on stellar density, luminosity, temperature, age, and composition. Recombination and forbidden lines additionally trace warm ionized gas. Strong wavelength-dependent absorption and scattering by interstellar dust obscure the inner disk, so an optical map is also a map of the foreground extinction.

Solved by gpt-5.6-sol high.

Near-infrared light is dominated over much of the Milky Way by old, cool, low-mass stars, with extra emission from young stars and hot dust in active regions. Because interstellar dust attenuates it much less strongly than optical light, near-infrared surface brightness and star counts are useful tracers of stellar mass and the obscured Galactic bulge and bar.

Solved by gpt-5.6-sol high.