Hertzsprung–Russell diagram

The diagram was created independently in 1911 by Ejnar Hertzsprung and by Henry Norris Russell in 1913, and represented a major step towards an understanding of stellar evolution.

[1] Hertzsprung noted that stars described with narrow lines tended to have smaller proper motions than the others of the same spectral classification.

He took this as an indication of greater luminosity for the narrow-line stars, and computed secular parallaxes for several groups of these, allowing him to estimate their absolute magnitude.

Early studies of nearby open clusters (like the Hyades and Pleiades) by Hertzsprung and Rosenberg produced the first CMDs, a few years before Russell's influential synthesis of the diagram collecting data for all stars for which absolute magnitudes could be determined.

Color distortion (including reddening) and extinction (obscuration) are also apparent in stars having significant circumstellar dust.

Another prominent feature is the Hertzsprung gap located in the region between A5 and G0 spectral type and between +1 and −3 absolute magnitudes (i.e., between the top of the main sequence and the giants in the horizontal branch).

RR Lyrae variable stars can be found in the left of this gap on a section of the diagram called the instability strip.

Following Russell's presentation of the diagram to a meeting of the Royal Astronomical Society in 1912, Arthur Eddington was inspired to use it as a basis for developing ideas on stellar physics.

[15] The paper anticipated the later discovery of nuclear fusion and correctly proposed that the star's source of power was the combination of hydrogen into helium, liberating enormous energy.

Eddington managed to sidestep this problem by concentrating on the thermodynamics of radiative transport of energy in stellar interiors.

In the 1930s and 1940s, with an understanding of hydrogen fusion, came an evidence-backed theory of evolution to red giants following which were speculated cases of explosion and implosion of the remnants to white dwarfs.

The term supernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a pre-supernova star, a concept put forth by Fred Hoyle in 1954.

An observational Hertzsprung–Russell diagram with 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence . In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants , giants and supergiants . The Sun is found on the main sequence at luminosity 1 ( absolute magnitude 4.8) and B−V color index 0.66 (temperature 5780 K, spectral type G2V).
An HR diagram with the instability strip and its components highlighted
HR diagrams for two open clusters , M67 and NGC 188 , showing the main-sequence turn-off at different ages