Yet the significance of the three
predominating colours--blue-white, yellow, and red--has been
sustained by the spectroscope. It is the series of colours
through which a white-hot bar of iron passes as it cools. And the
spectroscope gives 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.
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