greybody

A greybody is an object whose thermal radiation is close to that of a blackbody but with finite, often wavelength-dependent emissivity less than one. The emitted spectral energy distribution is proportional to the emissivity times the Planck function, Lν ∝ ε(ν) Bν(T). If the emissivity is constant with frequency, the spectrum resembles a blackbody scaled in intensity; if ε varies with frequency, the spectrum deviates in shape. In many astrophysical contexts, emissivity is modeled as a power law ε ∝ νβ or ε ∝ λ−β, with β > 0.

Physically, a greybody represents a medium that absorbs and emits imperfectly. Its radiative output depends both on its temperature and on how efficiently it radiates at different wavelengths. This framework is a convenient approximation when the optical depth or material properties prevent a perfect blackbody behavior and when a simple, frequency-dependent emissivity captures the essential physics.

Applications of the concept are widespread in astronomy. The thermal emission from interstellar dust and circumstellar disks is typically modeled as a modified blackbody, sometimes called a greybody, with Sν ∝ εν Bν(T) and εν ∝ νβ. Typical β values are in the range 1–2, and inferred dust temperatures span roughly 10–60 K depending on the environment. The greybody/modified blackbody approach helps interpret observations in the far-infrared and submillimeter bands and provides a practical alternative to assuming ε = 1 for all wavelengths.