One of astronomy's most powerful tools is the Hertzsprung-Russell diagram, which plots a star's brightness against its temperature. The diagram reveals patterns that tell us how stars work. The curved line across the middle of the diagram forms the main sequence, which comprises stars that are in the prime of life, "burning" the hydrogen in their cores to make helium. Near the ends of their lives, the stars puff up to form giants or supergiants, which are plotted at the upper right. (The Sun will become a giant in a few billion years.) The final remnants of most stars, including the Sun, are white dwarfs, at lower left, which are hot but dead stellar cores. [ESO]
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
For more skywatching tips, astronomy news, and much more, read StarDate magazine.