docs: def visc_rad_kap_sc(T,rho,X):
      '''
      Radiative viscosity (Thomas, 1930) for e- scattering opacity
  
      Parameters
      ----------
      X : float
         H mass fraction
      T : float
         temperature in K
      rho : float
         density in cgs
  
      Returns
      -------
      nu : float
         radiative diffusivity in [cm**2/s]
  
      Examples
      --------
      >>> In [1]: import astronomy as ast
      >>> In [2]: l = 100*1.e5 # 100km
      >>> In [3]: v = 1.e5     # typical velocity
      >>> In [4]: T   = 90.e6  # temperature
      >>> In [5]: X   = 0.001  # H mass fraction
      >>> In [6]: rho = 100.   # density
      >>> In [7]: nu = ast.visc_rad_kap_sc(T,rho,X)
      >>> In [8]: Re=v*l/nu
      >>> In [9]: print "Re_rad = "+str('%g'%Re)
      >>> Re_rad = 4.43512e+08
  
      Notes
      -----
      Eqn. 14' in Schatzman, 1977, assume electron scattering opacity
      kappa_sc = 0.2*(1+X), Kippenhahn (2nd edn, Eqn 17.2)
  
      '''
      kappa = 0.2*(1.+X)
      nu_rad = 6.88e-26*(old_div(T**4,(kappa*rho**2)))
      return nu_rad