How does one work with a Hertzsprung-Russell diagram?!?
It depends on what you are doing. If you have observations of a cluster, you can assume that all the stars have approximately the same distance, and then the difference between absoltue and apparent magnitude is the same for all your stars.Same applies if the stars are all in the same galaxy. In fact we can use this to determine the distance to a cluster. Do you have the luminosity class? If you you can estimate the absolute magnitude. But you are correct, if you don’t have the distance and you just have a random collection of stars, you cannot plot them on the HR diagram. Do you have parallax measurements? That would give you the distance.
