H-R Diagram 
Stars come in a bewildering array of sizes, colors, and temperatures. To sort out this diversity, astronomers use a powerful tool called the Hertzsprung-Russell diagram, which made its debut a century ago.
The diagram takes its name from Danish scientist Ejnar Hertzsprung, who first published the diagram, in 1911; and American astronomer Henry Norris Russell, who, unaware of Hertzsprung’s work, produced a similar diagram two years later.
The H-R diagram, as it’s known for short, plots a star’s luminosity — its true brightness — against its surface temperature. On this diagram, 95 percent of stars — including the Sun — fall into a band that slopes diagonally from hot, bright, blue stars like Regulus, to cool, dim, red stars like Proxima Centauri. This band is called the main sequence. Although main-sequence stars span a huge range, each generates energy the same way — by converting hydrogen to helium in its core.
A second group of stars — the giants and supergiants — were once main-sequence stars, but they exhausted the fuel at their centers and puffed up to many times their original size.
The third and final group is the dim white dwarfs. These are dead stars — former giants that shed their outer layers, leaving only their hot, dense cores. They form a group that’s below and parallel to the main sequence on the H-R diagram — a tool that depicts the lives of the stars. And we’ll talk about some examples of main-sequence and giant stars tomorrow.
Script by Ken Croswell, Copyright 2012