Slide 12 of 48
Notes:
So we want to compare the amount of lost through a smooth, relatively dense medium with that lost through a clumpy medium, where cold clouds are suspended in a hot, low density gas, and we are seeing the star through a gap in the clouds.
Thus we need to calculate the flux actually observed in each case, F.
F = F_0 exp (-sigma_eff (E) x N_H)
we first calculate N_H in each case, from the distance to the star and the density of the interstellar medium. Then using this value for N_H and the effective cross-section, sigma_eff, we can calculate F for each scenario in terms of the intrinsic flux, F_0.
If we assume the case of a smooth medium, we calculate that we actually observe only 1/20th of the intrinsic flux, thus we would multiply the observed flux by 20 to find the intrinsic luminosity.
If in fact the medium is clumpy as described, and we see the star through a gap, then the star has only lost about 75% of its flux and we are vastly over-compensating for photoelectric absorption along the line of sight.
Observations support the clumpy medium model, with cold HI clouds (~80K) suspended in hot HII gas (~9000K). The clouds may contain 75% of the ISM mass but only 10% of the volume.
