Life cycle of a star

The Hertzsprung-Russell diagram (or HR diagram) is constructed in logarithmic coordinates. In this graphic construction, each star is a point marked on the ordinate by its brightness (luminosity or absolute magnitude) and on the abscissa by its spectral type (or surface temperature). By making such a graph for a cluster of stars, H.N. Russell publishes an article in Nature in 1914 where he notes that stars do not place themselves randomly in his graph.

He thus identifies three settlement areas:

1. The main sequence that includes a large majority of the population of stars. The Sun is in the middle of this area.
2. The area of red giants (very bright but rather "cold").
3. The area of white dwarfs (very hot but rather dark).

Such a diagram is a powerful analytical tool because it allows one to draw conclusions about the mass, size, chemical composition, age and evolutionary stage of a star. 

Luminosity is expressed in solar light (L⊙). The radius of the stars is expressed in comparison with the radius of the Sun (R⊙).

Bibliography and credits:

• Stellarium constellation art by Johan Meuris : http://johanmeuris.eu/en/portfolio_page/stellarium-constellation-art/ distribuées sous licence Free Art License (http://artlibre.org/licence/lal/en/)
• HR-Diagram by Prof. James Schombert, University of Oregon: http://abyss.uoregon.edu/~js/ast122/lectures/lec11.html
• http://www.astrosurf.com/luxorion/vie-etoiles2.htm (in french)
• Stellar Structure and Evolution Simulator : http://rainman.astro.illinois.edu/ddr/stellar/
• Astrophysics lectures - University of Montreal : http://craq-astro.ca/phy1971/index2.html (in french)