The Light of the Stars 53
most electrons of all is shifted, the attraction of the
nucleus upon it is relatively small, and is modified, too, by
that of the other electrons—less work is done, and the
corresponding waves are long enough to be called light.
There are many of these possible states, and still more
ways in which a shift from one to another may take place.
This explains why the spectrum of a given element may
contain a great number of lines. Each individual atom
at a given instant probably absorbs or emits but one kind
of light—a single line—but while one change is going on
in one atom, different changes are happening in others,
giving different lines, and so the mass of gas as a whole
absorbs or emits them all.
In a gas at low temperature—or at least at the lowest
temperature at which the element in question can be
vaporized—almost all the atoms will have their outer
electrons in the state corresponding to the smallest con-
tent of energy. When light is passed through this gas
the spectral lines corresponding to a shift of the electron
from this position to those others which (so to speak) can
be directly reached from it, will be absorbed. In a hotter
gas, where the atoms collide with each other more vio-
lently and increase their stores of internal energy, the
light will find some of them with their electrons already
in the “higher” positions of greater energy content, and
the shifting of these electrons to still higher energy levels
will cause the absorption of additional lines. At first
there will be relatively few atoms in these other states,
and the new lines will be faint. But as the temperature
is raised still more, the proportion of atoms which are
thus stimulated by collision will rise, and the new lines
will increase in intensity, compared with the “ultimate”
lines which at first appeared alone.