In order to understand clusters better, we will use a tool that you are already familiar with, the H-R diagram. Remember this diagram is a plot of the temperature (or spectral type) of a star against the luminosity (or absolute magnitude) of the star. But what we measure with our telescope is not the absolute magnitude, M, but the apparent brightness, or apparent magnitude, m. However, remember that the difference between these depends on the distance to the star.  Since all stars in a cluster are at about the same distance from us, the difference between their apparent and absolute magnitudes will be the same for all of them. So we can make a diagram plotting the apparent magnitudes that we directly measure instead of the absolute magnitudes, and it will look the same except all the points will be shifted in the vertical direction. Let's look at an H-R Diagram of the globular cluster M 5. Each point represents a single star.

The Y-axis is the apparent magnitude. In previous labs we have learned about two different magnitudes an object has: the absolute magnitude, M, and the apparent magnitude, m. (Refresh your memory about magnitudes with this link).  The X-axis is the temperature of the star.