MadSci Network: Astronomy

Re: Can I use the Hertzsprung-Russell diagram to find the compisition of stars?

Date: Thu May 11 17:38:23 2000
Posted By: Kristin Nelson-Patel, Grad student, Infrared Astrophysics, Smithsonian Astrophysical Observatory
Area of science: Astronomy
ID: 957642805.As

Dear Lynsey,
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.

    1. 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 astronomers call metal-poor. (Metal-poor just means there isn't 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 metal-poor.

    2. 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 clusters) metal-rich.

  1. 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.

Good Luck!

I used the following references in putting together my answer:

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