us good ground to
conclude that the stars are cooling.

When a glowing gas (not under great pressure) is examined by the
spectroscope, it yields a few vertical lines or bars of light on a
dark background; when a glowing liquid or solid is examined, it gives a
continuous rainbow-like stretch of colour. Some of the nebulae give the
former type of spectrum, and are thus known to be masses of luminous
gas; many of the nebulae and the stars have the latter type of
spectrum. But the stretch of light in the spectrum of a star is crossed,
vertically, by a number of dark lines, and experiment in the laboratory
has taught us how to interpret these. They mean that there is some
light-absorbing vapour between the source of light and the instrument.
In the case of the stars they indicate the presence of an atmosphere
of relatively cool vapours, and an increase in the density of that
atmosphere--which is shown by a multiplication and broadening of
the dark lines on the spectrum--means an increase of age, a loss of
vitality, and ultimately death. So we get the descending scale of
spectra. The dark lines are thinnest and least numerous in the blue
stars, more numerous in the yellow, heavy and thick in the red. As the
body of the star sinks in temperature dense masses of cool vapour gather
about it. Its light, as we perceive it, turns yellow, then red. The next
step, which the spectroscope cannot follow, will be the formation of
a scum on the cooling surface, ending, after ages of struggle, in the
imprisonment of the molten interior under a solid, dark crust. Let us
see how our sun illustrates this theory.

It is in the yellow, or what we may call the autumnal, stage. Miss
Clerke and a few others have questioned this, but the evidence is too
strong to-day. The vast globe, 867,000 miles in diameter, seems to be a
mass of much the same material as the earth--about forty elements have
been identified in it--but at a terrific temperature. The light-giving
surface is found, on the most recent calculations, to have a temperature
of about 6700 degrees C. This surface is an ocean of liquid or vaporised
metals, several thousand miles in depth; some think that the brilliant
light comes chiefly from clouds of incandescent carbon. Overlying it is
a deep layer of the vapours of the molten metals, with a temperature
of about 5500 degrees C.; and to this comparatively cool and
light-absorbing layer we owe the black lines of the solar spectrum.
Abov