H-R Diagram of media stars

Graphing variables is a critical skill in science. If something depends on something else -- like the speed of sounds depends on air density, or the surface gravity of an object depends on its size -- then if you plot the two things on a graph, you should see a pattern. The result is a line, or a curve. If the two things don't depend on each other, you get a random collection of dots: a scatter plot. About a hundred years ago, two astronomers plotted the brightness of stars against their color (from blue to red) and what they found was amazing: a clear connection between the two! In fact, stars fell into several groups, and over the years we've learned about why that happens. Most stars are stable, like the Sun, and fall into the Main Sequence of the plot. Some are old, some young, some dying, some ...