The Hertzsprung-Russell diagram shows the relationship between different properties of stars and illustrates trends among stars. The diagram was created in 1910 by Ejnar Hertzsprung and Henry Norris Russell, and represented a huge leap forward in understanding stellar evolution, or the 'lives of stars'.
In this HR diagram, 22,000 stars are plotted from the Hipparcos catalogue and 1000 from the Gliese catalogue of nearby stars. An examination of the diagram shows that stars tend to fall only into certain regions on the diagram. The most predominant is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence.
Hot stars give off more blue light than red; cool stars give off more red light than blue. Coloured filters are used to measure different wavelengths of light from stars. The magnitude of the star is measured first through a standardized B-band ("blue") filter. Then the star's magnitude is measured through a V-band ("visible", peaking in green) filter. The value of V is subtracted from B to get the B-V colour index. This leads to the spectral classification of a star.
As a star gets cooler and therefore more red, the B-V colour index increases, since smaller magnitudes correspond to brighter light. Hot stars have a small B-V and cool stars have a large B-V. Hotter stars therefore appear to the left on the HR diagram and cooler stars appear on the right.
Most of the stars occupy the region in the diagram along the line called main sequence. During that stage stars are fusing hydrogen in their cores. The next concentration of stars is on the horizontal branch (helium fusion in the core and hydrogen burning in a shell surrounding the core).
The H-R diagram can also be used by scientists to roughly measure how far away a star cluster is from Earth. This can be done by comparing the apparent magnitudes of the stars in the cluster to the absolute magnitudes of stars with known distances (or of model stars). The observed group is then shifted in the vertical direction, until the two main sequences overlap. The difference in magnitude that was bridged in order to match the two groups is called the distance modulus and is a direct measure for the distance. This technique is known as main-sequence fitting, or, confusingly, as the spectroscopic parallax.
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