Radiative diffusion inside an opaque star
In an optically thick stellar region close to local thermodynamic equilibrium, radiative diffusion relates luminosity to the temperature gradient through density and an appropriate mean opacity.
Key knowledge
In an optically thick stellar region close to local thermodynamic equilibrium, radiative diffusion relates luminosity to the temperature gradient through density and an appropriate mean opacity. Larger opacity makes a steeper gradient necessary to carry a given radiative flux.
Values and equations
dT/dr = −3κρL / (64πσSB r²T³)
- T
- local temperature K
- κ
- Rosseland mean opacity m² kg⁻¹
- ρ
- density kg m⁻³
- L
- luminosity through radius r W
- r
- radius m
- σSB
- Stefan–Boltzmann constant W m⁻² K⁻⁴
Conditions and scope
- Use Rosseland mean opacity in this diffusion regime.
- Diffusion does not describe the optically thin photosphere or a region whose energy transport is substantially convective.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- Stellar Structure, pp. 3–4: radiative temperature gradientIan J. M. Crossfield — MIT, Astrophysics I course notes (2019)Textbook chapter · PDF
- Stellar Structure and Evolution, §5.2.3 printed pp.58–59: equations5.19–5.24 and Rosseland meanO. R. Pols — Utrecht University (September 2011)Textbook chapter · PDF
Evidence and review
Read Crossfield radiative-gradient equation and radiation-transport context, and Pols actual §5.2.3 frequency-integrated diffusion equations5.19–5.24 defining the Rosseland harmonic mean weighted by ∂Uν/∂T. Stated thick/LTE diffusion and opacity scope explicitly; no full-opacity-table or whole-text validation is claimed.
2026-10-10 · Independent AI editorial review: All26 Life-authored Cosmos compact statements, conditions, displayed equations and all56 origin semantic links read. Entire26-entry author statement/source matrix and explicit exclusions assessed. Targeted independent primary sections are listed below; this does not claim every referenced source body or proof was independently re-read. This is scoped AI reference-content review, not human disciplinary certification or whole-field completion. Receipt: cosmos-life-independent-content-review.1.
Entry details
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- uni-cosmos-stellar-diffusion
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- University core expansion