2. Radiation Field Basics I Specific Intensity Ω = dddd cos d tA I E

The flux emitted by a star per unit area of its surface isν = ν + = πI ν∗ where I ν∗ is the intensity, averaged over the apparentstellar disk, received by an observer. This equality is why that fluxis often written as πF = , so that F = I ∗ , with F called theAstrophysical Flux.This explains the often confusing factors of π that are floating aboutin definitions of flux:= Monochromatic Flux or just the Flux; F = Astrophysical FluxThey are related by πF = .In terms of moments of the radiation field, the first moment isdefined as the Eddington Flux, H ν . For plane parallel geometry:Hν≡4π1 ν 1IνcosθdΩ = =1−1ν=4πF42Iνµ dµFlux = energy/second per areaLuminosity = energy/secondStellar LuminosityLν=νA*= 4π R2*π IνAssume I ν = B ν and integrate:L =L2νdν= 4πR* π Bνdν= 4π R2*σ T4