On the other hand, all globular clusters in our galaxy have similar H-R diagrams, with short, stubby main sequences that lack hot, luminous stars.
A new study published in 2007, which took into account an improved understanding of the spacecraft's motions, led key Hipparcos team member Floor van Leeuwen (Cambridge University, UK) to an improved catalog and a more accurate H-R diagram for the 17,502 stars whose parallaxes are known to better than 7%.
Gaia will extend the H-R diagram to the lower right by measuring both cooler and fainter stars, and its more accurate measurements will provide much more detailed information on the main sequence.
So by the 100th anniversary of Russell's 1914 publications, we have plotted millions of stars on H-R diagrams and have an orbiting spacecraft that will provide measurements of millions more.
Pasachoff has been lecturing about the H-R diagram since he started teaching at Williams College in 1972 and writing about it in his textbooks, most recently the 2014 edition of The Cosmos: Astronomy in the New Millennium.
An H-R diagram I constructed for M13 from the professional catalogs in the publicly accessible VizieR database (http://vizier.u-strasbg.fr/viz-bin/VizieR) reveals a typical globular plot (facing page).
Recently, I constructed H-R diagrams for them, two of which appear in this article.
The first thing to notice is that my H-R diagram for M13 looks similar to the one produced from VizieR.
I can then compare pairs of H-R diagrams of the same cluster taken at different times.