84 Recent Advances in Stellar Astronomy
We are now in a position to evaluate another very im-
portant physical characteristic of the stars—their mean
densities. Among the dwarfs, we have seen that the aver-
age mass of an individual star diminishes slowly from
twice that of the Sun for Class F to half the Sun’s for
Class M, and that the radii ran from sixty per cent,
greater than the Sun to forty per cent, less than the Sun’s.
This shows that the densities of all these stars are of the
same order of magnitude. If we set the average density
of a dwarf F-star as half of the Sun’s, of a G-star as equal
to the Sun’s and of an М-star as twice as great, we shall
be in good agreement with our data. For a very faint star
like that mentioned above, we may estimate a diameter
one-third and a mass one-seventh of the Sun’s, giving a
density four times that of the Sun, or about equal to the
density of the Earth. Our knowledge of the surface bright-
ness of these dwarf stars is not accurate enough to justify
much reliance on the differences between these figures, but
the general conclusion that the dwarf stars are comparable
in density with the Sun, and most of them denser than
water, appears certain. For the stars of Class A, we are on
firmer ground, for most of the eclipsing variables belong
here. In every such system, where we know the period
and the relative sizes of the stars and orbit, a fortunate
mathematical relation permits us to compute the density
without having to know the dimensions or distance. For
69 stars, of spectrum between BS and AS, the average
density is 1/5 that of the Sun. They are decidedly similar
to one another—⅞ of them having densities between four
times the mean and ¼ of it. The few eclipsing variables
of early B type indicate a much smaller mean density—
about 1/40 that of the Sun.
Passing to the giant stars, we may take as an example of