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The description of the word you requested from the astronomical dictionary is given below.
The Hertzsprung-Russell Diagram is a diagram showing a measure for the brightness of stars (usually along the vertical axis) against a measure for the temperature of stars (usually along the horizontal axis). This kind of diagram was invented around 1910 by Ejnar Hertzsprung and Henry Norris Russell. It turns out that stars concentrate in certain parts of such a diagram, depending on the phase of their life.