Slide 11 of 48
Notes:
One of the problems, as I mentioned earlier, with measuring the amount of absorbing gas along our line of sight to a star, is that measurements of the 21cm line emission are made using a very wide beam, hence low spatial resolution. So it is not known whether there is any fine structure in the ISM or not. When we measure the 21cm emission at a certain position, we assume that this is for a smooth distribution of cold hydrogen gas. But in reality, we could be seeing the star through a gap in cold clouds which are suspended in a hot, low density medium. This makes a dramatic difference to amount of flux that is actually transmitted. And if the amount of absorption is overestimated because we assume a smooth distribution of hydrogen, then we calculate a much higher intrinsic luminosity for the source than is actually the case.
