86 Recent Advances in Stellar Astronomy
very molecules themselves. The atmosphere above our
heads—equivalent to five miles of air of standard density—
scatters about one-eighth of the incident light, even when
perfectly free from dust. After passing through fifty
miles of air, only thirty per cent, of the original beam
would remain; at a hundred miles, nine per cent.; after
two hundred, less than one per cent.; and at the end of
six hundred miles, not one part in a million. Such a thick-
ness of perfectly clear air would be substantially opaque—
the Sun itself would be barely visible through it. Now an
easy calculation shows that a ray of light which passed
centrally through Antares (taking the star as of uniform
density, simply for the purpose of this calculation) would
in our lowest estimate have passed through a quantity of
gas equivalent to 800,000 miles of air—or more than a
thousand times enough to secure substantial opacity. We
need, therefore, trouble ourselves no more upon this score.
The very low densities of the giant stars permit an ex-
planation of the spectroscopic differences upon which
Adams has based his determination of the distances of the
stars. The temperature of the atmosphere of a star of a
given spectral class is probably much the same, whatever
its absolute magnitude, but the density of this atmosphere,
like that of the whole mass, must be very much greater in
a dwarf star than in a giant. What spectroscopic differ-
ences might arise from this can be seen upon Saha’s theory.
The relative strength of the ordinary lines and en-
hanced lines of any element depends upon the relative
number of the atoms of this element, in the star’s atmos-
phere, which are in the neutral and ionized states. Now
an atom becomes ionized whèn its internal energy becomes
so great as to lead to the ejection of an electron, and the
rate at which this happens will be practically independent