2 Observing through the interstellar medium
In all the analysis of stellar properties discussed so far we have made an implicit assumption - that light emitted by a star is not changed between its emission and its arrival outside the Earth's atmosphere, except by the inverse square law (i.e. it is reduced by a factor of d2, where d is the distance to the star) and by the Doppler effect. However, this may not be the case.
What will happen to the position of a star on the H-R diagram if interstellar material causes a reduction in its brightness?
If the interstellar material causes light to be absorbed then the star will appear fainter and hence be placed lower than its true position on the H-R diagram.
We will now investigate some of the properties of the interstellar material and how it affects the radiation we observe from stars.