|MadSci Network: Astronomy|
Yes, you can use the H-R diagram to infer some very general compositional properties of a whole cluster plotted, but precise determinations of elemental abundances relative to hydrogen require a lot more work.
There are two ways you can think about the question of composition and the H-R Diagram.
Globular clusters are old spherically shaped clusters of stars
that occupy orbits all around the Galactic center (not usually in
the disk of the Galaxy). These clusters formed early in the
Galaxy's history, before many generations of stars could form and
distribute the heavy elements, so globular clusters tend to be what
Metal-poor just means there
much of anything other than hydrogen and helium there.) Because so
many of the stars in them are very old even at low masses (Heavier
stars burn their fuel faster than less-massive stars), an H-R
diagram of a globular cluster has more stars in an intricate
pattern far above the main sequence. Those old stars will trace out
the evolutionary path in the diagram up and to the right from the
top of the main sequence for that cluster, on the Asymptotic Giant
Brach, and then back over to the left on the Horizontal
Branch. I won't go into the details, but these sections of the
diagram are places that represent states of nuclear burning after
the core hydrogen has run out. So, if you see this highly evolved
pattern, it's likely that you're looking at a globular cluster, and
you can say that it's
Open clusters are young clusters of stars that have all kinds of
geometrical arrangements of the stars, but the clusters exist only
in orbits in the disk of Galaxy with the rest of us. The reason
they're young is that orbiting in the plane of the Galaxy makes a
cluster formed here disperse pretty fast, before all but the most
massive stars have had a chance to get off the main sequence after
hydrogen burning. That means that if you see an open cluster, it's
relatively young. So, if you see an H-R diagram with most of the
stars on the Main Sequence (or little to the right of the bottom of
the main sequence for the cluster), it's probably an open cluster.
Since open clusters form in the warm dusty parts of the disk of our
galaxy where lots of generations of stars have formed and
distributed heavy elements, they tend to be (relative to globular
From a single star's position on the H-R diagram, you can't tell very much about its composition. That's because even though the trends above exist for globular clusters and open clusters, you can find stars of all ages in the Galaxy. You can tell its spectral type (surface temperature), which tells you which spectral lines of what elements ought to be excited if the element is present. But then you need go out and get spectra to see if how bright those lines are, and do a lot of physics, like I mentioned in the beginning.
So, I think the cluster populations are what your teacher is driving at. A really well-written introductory book on general astronomy is Astronomy: A Physical Perspective, by Marc Kutner. It goes describes more detail than most intro astronomy books, but in very easy-to-understand language.
I used the following references in putting together my answer:
Astronomy: A Physical Perspective, Marc Kutner
Astrophysical Concepts, 2nd ed., Martin Harwit
Principles of Stellar Evolution and Nucleosynthesis, Donald D. Clayton
Try the links in the MadSci Library for more information on Astronomy.