Print out the HR diagram chart found in Moodle or at

http://www.csun.edu/~boregan/astrolab/manual/hrdiagram.pdf

The vertical axis of the diagram is the Absolute Magnitude, M. The scale will start at the top with a -10 and increase downward to a +20. Each horizontal line is separated by 1. For example, the lines above the -5 line are -4, -3, -2, and -1.