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The Hertzsprung-Russell (H-R) diagram

As a star evolves, different mechanisms of element production, i.e. nucleosynthetic processes, are brought into play. The evolution of a star may be shown as a path on a Hertzsprung-Russell diagram (i.e. a graph of luminosity versus effective temperature). The basic route for an approximately solar mass star is shown schematically in Fig. 2.1. As a star grows older it will run through the H-R diagram on an evolutionary track fixed from the beginning by its basic characteristics, mass and composition.


Figure 2.1: A basic schematic representation of the evolution of a ~ 1M tex2html_wrap_inline518 star on a Hertzsprung-Russell diagram. The x-axis is temperature in kelvin, the y-axis is luminosity in units of solar luminosity (i.e. 1=1L tex2html_wrap_inline518 )

Since, for the purposes of this thesis, we are interested only in the post-main sequence evolution, Fig. 2.2 shows the evolutionary track of stars of various initial masses from the main sequence to the top of the asymptotic giant branch*. It is easily seen that the initial mass has an effect on the precise evolutionary track.


Figure 2.2: The Hertzsprung-Russell diagram of the post-main-sequence evolution of stars with different initial masses (from Zeilek & Smith 1987).


* the 9Mtex2html_wrap_inline518 and 15Mtex2html_wrap_inline518 initial masses are shown for completeness. These stars do not have AGB phases since they do not develop degenerate cores.


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