Red Giant Evolution
For stars of a solar mass up to 5-10 solar masses, post main sequence evolution will take the star to the
red giant region of the HR diagram. This evolution is marked by large increases in luminosity, decreases
in surface temperature, and the establishment of shell burning sources in the interior.
This
animation illustrates.
Giant Branches
The figure on the right shows the three branches of giant stars expected for evolved clusters;
the branches are labeled more schematically in the diagram shown below. These
branches correspond to regions of the HR diagram where stars evolving through the giant region spend
relatively large amounts of time (see the right panel).
Thus, at any one time we are more likely to find giant stars on these
branches than anywhere else.
Appearance of the HR Diagram
The absence of stars below about absolute magnitude 7 and the width of the part of the
main sequence shown for M5 are observational effects.
The cluster M5 is about 7 kiloparsecs
away. At that distance it is hard to observe faint stars in it. This has two effects.
For stars fainter than about absolute magnitude 7, the observations are too uncertain even
to plot the points. For the rest of the main sequence (down to absolute magnitude of
about 4) the stars are bright enough to see, but the assignment of their color indices and
magnitudes is uncertain and this causes a scatter in the plotted points because of
unavoidable observational errors.
