什么是 Limb darkening？

Limb darkening is the decrease of the surface brightness of a star from the centre of its disc to its edge, the limb. Near the limb the line of sight enters the star’s atmosphere obliquely and reaches unit optical depth in higher, cooler layers, which emit less. It is often written as a quadratic law in $\mu = \cos\theta = \sqrt{1 - r^2}$, with $r$ the distance from the centre of the disc in stellar radii: $I(\mu)/I(1) = 1 - u_1(1 - \mu) - u_2(1 - \mu)^2$.