def visc_rad_kap_sc(T,rho,X):
kappa = <NUM_LIT>*(<NUM_LIT:1.>+X)<EOL>nu_rad = <NUM_LIT>*(old_div(T**<NUM_LIT:4>,(kappa*rho**<NUM_LIT:2>)))<EOL>return nu_rad<EOL>
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)