Here’s a powerpoint animation that explains the Hertzprung-Russel diagram, step by step.
The Hertzprung-Russel diagram is one of those great organizational schemes, like the Periodic Table, that manage to have great predictive and associative power. It’s not perfect (neither is the Periodic Table), but it CAN explain a great number of things about stars.
For instance: Colors. Astronomers have always been aware that stars varied in color. Notwithstanding the problems with Sirius being reported as red by the ancients, it was pretty clear that Betelgeuse is red, and Rigel is not. By the mid-1700s (see a previous post), comparisons with terrestrial fires had demonstrated that the blue ones were hotter, the red ones were cooler, and the white ones were somewhere in between.
The great innovation of Hertzprung (and Russel) was to add a second axis to the plot: luminosity (for which I’ve substituted absolute V magnitude), which can be calculated if you have a distance. Surprise! The blue ones are *much* brighter than the red ones, by factors of ten billion. They fall out in a roughly diagonal line called the “main sequence”, and this is where most stars fall. At the same time, the blue stars can get to be 100 times (maybe) more massive than the Sun, 10,000 times brighter, and 10 times wider, while the red stars can get to be 1/12 of the mass of the Sun, 1,000,000 times fainter, and 1/10 as wide. Fortunately for the little guys, there are FAR more tiny stars than there are big stars.
The rest of the stars are something else, of course. They are cool but very bright, which implies the emitting surfaces of the stars are gigantic: these are the giants, stars at the end of their lives that have run out of hydrogen in their cores to fuse and are now attempting to fuse helium. The H-R diagram can distinguish them from regular stars. There is another even rarer class of stars, the supergiants- these are exceedingly bright, and these are the stars that have moved on from helium fusion to even less efficient things. They are the ones on the verge of shedding their outer layers entirely and producing white dwarfs, or (for sufficiently large sizes) neutron stars and black holes.