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Hertzsprung-Russell Diagram The Hertzsprung-Russell Diagram (often abbreviated to 'HR diagram') is the most important state diagram1 in stellar astronomy. It bears the names of two astronomers, the Danish Ejnar Hertzsprung (1873 - 1967) and the American Henry Norris Russell (1877 - 1957). Hertzsprung had the basic idea in 1909. Around 1913, Russell developed it further and invented the HR diagram. Motivation Being extremely far away, stars are very reluctant to disclose their secrets. Even today it's difficult to see stars as objects with real expanse: in the biggest telescopes they are just dots! So the only thing we can examine is their light, namely its brightness and its spectrum. Knowing the visual brightness of the star in the sky and its distance2, we can then easily calculate its absolute (real) brightness. The spectrum (colour, spectral absorption lines) yields the spectral class of the star. Now we have two stellar properties that can be...

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