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Re: [Phys-l] the sun



I've been teaching astronomy for quite a few years, and it's been
interesting to watch the textbooks change over the years. A new edition
of the book I use comes out roughly every three years. You might
suspect this is because the publishers want to thwart the used-book
market. While there may be truth to that, I notice fairly numerous
changes (updates) from edition to edition.

Regarding the distribution of stellar masses, the information has not
changed in the last few years. So perhaps this is reasonably well
known.

The book I use is Astronomy Today by Eric Chaisson and Steve McMillan.
It just came out in the 6th Edition in time for me to use it in my class
that started in January 2008. So this edition is about 1-year old at
this time.

Here is what it says...

Blue Giants >20 solar masses 0.06%
Blue Giants 8-20 solar masses 0.3%
Blue Giants 4-8 solar masses 0.8%
White Stars 2-4 solar masses 3%
Yellow Stars 1-2 solar masses 8%
Orange Stars 0.5-1 solar mass 19%
Red Dwarfs 0.25-0.5 solar m 28%
Red Dwarf <0.25 solar mass 41%

This table is unchanged from the fourth edition dated 2002. If these
numbers are correct, our sun is not near the mean nor the median. If
you take the population-weighted average of these data (using the mass
from the center of the range, that is, saying 28% of the stars have mass
0.37, then the average star mass is about 0.64 solar masses. The
average star has about two-thirds of Sun's mass. If you start from the
low end and go up until you hit 50%, that will be somewhere around 0.3,
or about one-third of a solar mass.

So a rough calculation using data from the textbook I use would say that
the average star mass is about two-thirds of the Sun, and the median
solar mass is about one-third of the Sun.

Although the Sun isn't real close to either the average or the median, I
think you could certainly say it is closer to the average than to the
median.

If these data were to change in the next few years, I would expect the
mean and median would both go down.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu