(1) Yes, the comment from Tim Folkerts to John Denker that the HR diagram
doesn't tell us anything about the mass is incorrect.
but...
(2) Yes, Tim is still correct that the HR diagram does not answer the
original question, because you can't tell by looking at the HR diagram what
the numerical data is for the actual mass distribution of stars. First of
all, most HR diagrams don't contain a point for every known star. Second,
even if the plot contained all known stars, what are you going to do, count
the dots? You need tabulated data, or a summary of tabulated data like I
used, to figure out what the median stellar mass is and what the mean
stellar mass is. Astronomers have that data, and the HR diagram is not it.
I can't tell from looking at the HR diagram that the mean stellar mass is in
the ballpark of 0.6 solar mass and the median stellar mass is in the
ballpark of 0.3 solar mass.
(3) As Brian Whatcott points out, you don't have to be "lucky" as John
Denker suggests to know more about stellar mass than high mass is hot and
bright and low mass is cool and faint. Binary stars are very prevalent. In
fact, almost all stars are in multiple star systems containing two or more
stars, and among these, binary systems predominate. Rather, you have to be
unlucky if the star for which you want to know the mass is not in a binary
system. Chaisson and McMillan in the "Astronomy Today" textbook state that
perhaps the most uncommon thing about our sun is that it is not part of a
binary system. Everything else about the sun is pretty normal.
(4) The website Brian led us to is totally plagiarized from Chaisson and
McMillan. I am sitting here looking at that web page and also looking at my
textbook, and every single picture on that web page is an exact copy of the
figures in Astronomy Today, and no reference to this source is given. A
quick trip to Professor Kuhlman's web page for the astronomy course he is
teaching this semester shows that he is using the text, "Astronomy: A
Beginner's Guide to the Universe" by Chaisson & McMillan. This is a
simplified version of the "Astronomy Today" text that I use. It may be
"fair use" for him to put stuff from the textbook on web pages for his
class, but it should be footnoted, especially if the pages are going to be
accessible to the general public.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu