oximately measurable. The first star to which this
test was applied with success was that known as 61 Cygni, which is thus
shown to be no less than 40 billions of miles away from us--many
thousand times as far as we are from the Sun. The nearest of the Stars,
so far as we yet know, is [Greek: alpha] Centauri, the distance of which
is about 25 billions of miles.

The Pleiades are considered to be at a distance of nearly 1500 billions
of miles.

As regards the chemical composition of the Stars, it is, moreover,
obvious that the powerful engine of investigation afforded us by the
spectroscope is by no means confined to the substances which form part
of our system. The incandescent body can thus be examined, no matter how
great its distance, so long only as the light is strong enough. That
this method was theoretically applicable to the light of the Stars is
indeed obvious, but the practical difficulties are very great. Sirius,
the brightest of all, is, in round numbers, a hundred millions of
millions of miles from us; and, though as bright as fifty of our suns,
his light when it reaches us, after a journey of sixteen years, is at
most one two-thousand-millionth part as bright. Nevertheless, as long
ago as 1815 Fraunhofer recognised the fixed lines in the light of four
of the Stars; in 1863 Miller and Huggins in our own country, and
Rutherford in America, succeeded in determining the dark lines in the
spectrum of some of the brighter Stars, thus showing that these
beautiful and mysterious lights contain many of the material substances
with which we are familiar. In Aldebaran, for instance, we may infer the
presence of hydrogen, sodium, magnesium, iron, calcium, tellurium,
antimony, bismuth, and mercury. As might have been expected, the
composition of the Stars is not uniform, and it would appear that they
may be arranged in a few well-marked classes, indicating differences of
temperature, or perhaps of age.

Thus we can make the Stars teach us their own composition with light,
which started from its source years ago, in many cases long before we
were born.

Spectrum analysis has also thrown an unexpected light on the movements
of the Stars. Ordinary observation, of course, is powerless to inform
us whether they are moving towards or away from us. Spectrum analysis,
however, enables us to solve the problem, and we know that some are
approaching, some receding.

[Illustration: Fig. 55.--Displacement of the hydrogen line