While plotting a star on an H-R diagram, which of the following measurements is not required?

In the context of plotting a star on the Hertzsprung-Russell (H-R) diagram, the primary measurements required are the star's surface temperature and its luminosity (which can be derived from apparent brightness and distance). The H-R diagram is a graphical representation that relates a star's luminosity to its temperature, allowing astronomers to categorize stars and understand their evolutionary stages.

The measurement of mass, while significant in understanding a star's life cycle and in astrophysical modeling, is not directly needed to plot a star on the H-R diagram. The surface temperature, which can often be determined through spectral analysis, directly corresponds to the position on the horizontal axis, while luminosity (calculated from apparent brightness and distance) places the star on the vertical axis.

Thus, while mass can influence a star's properties and behavior, it is not a necessary parameter for the basic plotting of a star on an H-R diagram.