For a Java demo of the effects of temperature on EM radiation, see this Davidson Webphysics applet. . The mass of a star turns out to be the key to determining its life cycle. Newton showed that the time taken to complete one orbit by an object in a gravitational orbit around another is related to the combined mass of the two objects. During the 19th and early 20th centuries, small telescopes were often used for this kind of study. Norton's Star Atlas contains good lists of the brighter binary star systems with information on their motions.
Binaries often present nice color constrasts in small telescopes as well.
The masses of normal stars have a smaller range than do their intrinsic brightnesses. Their work depended on the painstaking accumulation of measures of accurate distances to stars. The important point is that stars are not randomly scattered in the diagram but confined to well-defined sequences. Brighter MS stars are also hotter. The HRD is a basic clue to stellar physics.