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The first 200 words of this essay...
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.
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 measured rather simply and accurately: absolute brightness and spectral class. This is very important, because other things cannot be seen directly; for example the age of the star, its mass, radius, core temperature and many more. But we do have these two
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