The Light of the Stars 55
tected by ordinary means are produced by further strip-
ping of the electrons from the atom. The amount of
work which is required to remove an electron from an
atom may also be measured by electrical means in a
suitably designed vacuum tube, as well as by the spectro-
scopic processes previously described. Combining the in-
formation that we get in the two ways, we know that
some elements, like sodium and potassium, are easily ion-
ized, others, like magnesium and zinc, with more difficulty,
while the permanent gases, hydrogen, oxygen and nitro-
gen, are much harder to ionize, and helium hardest of all.
We can now understand what happens in the atmo-
spheres of the stars. In the red stars, where the surface
temperature is lowest, the atoms are subjected to a rela-
tively mild stimulus, and the “ultimate,” or “furnace”
lines of the metals are prominent, with the arc lines of
the same substances present, but fainter. For hydrogen,
whose ultimate lines lie in the ultra-violet, the only ac-
cessible lines belong to a “subordinate series,” absorbed
only by atoms in which the electrons have already suffered
one shift; and at this temperature very few of the hydro-
gen atoms undergo this internal change, so that the hy-
drogen lines are weak, and those of the other permanent
gases, which are still harder to excite, are absent. As the
temperature rises to that of the Sun, the lines of oxygen
begin to appear. The arc lines of the metals grow
stronger. Some of the metallic atoms lose one electron
and are in a position to absorb the enhanced lines. For
other elements, such as potassium, which have no enhanced
lines in the accessible region, the lines of the neutral
atoms gradually fade out, with nothing to replace them.
By the time that we reach the F type, the arc lines of
the metals are getting fainter, though the enhanced lines