Model 2: the HR diagram

The HR diagram is a graph with star temperature (K) on the x axis and star luminosity (sun luminosity) on the y axis. The graph was created by plotting data collected by male scientists. However, the graph and trends noticed in the graph were discovered by female scientists. 

By plotting the stars data one learns that luminosity relates to temperature in a way that creates three major star classification categories: the Main Sequence stars (like our Sun), the Giant stars (middle aged), and the White Dwarfs (dying stars). Within these categories there are others (see supergiants and brown dwarfs).

Understanding the HR diagram led scientists to develop theories about the lifetimes of stars despite the fact that no one human can study a full lifetime of a star.