ay be ever so much nearer to us than the other, the
result will give all the appearance of a related pair. A seeming
arrangement of two stars in this way is known as a "double," or double
star; or, indeed, to be very precise, an "optical double." Secondly, in
a pair of stars, both bodies may be about the same distance from us, and
actually connected as a system like, for instance, the moon and the
earth. A pairing of stars in this way, though often casually alluded to
as a double star, is properly termed a "binary," or binary system.

But collocations of stars are by no means limited to two. We find,
indeed, all over the sky such arrangements in which there are three or
more stars; and these are technically known as "triple" or "multiple"
stars respectively. Further, groups are found in which a great number of
stars are closely massed together, such a massing together of stars
being known as a "cluster."

The Pole Star (Polaris) is a double star, one of the components being of
a little below the second magnitude, and the other a little below the
ninth. They are so close together that they appear as one star to the
naked eye, but they may be seen separate with a moderately sized
telescope. The brighter star is yellowish, and the faint one white. This
brighter star is found _by means of the spectroscope_ to be actually
composed of three stars so very close together that they cannot be seen
separately even with a telescope. It is thus a triple star, and the
three bodies of which it is composed are in circulation about each
other. Two of them are darker than the third.

The method of detecting binary stars by means of the spectroscope is an
application of Doppler's principle. It will, no doubt, be remembered
that, according to the principle in question, we are enabled, from
certain shiftings of the lines in the spectrum of a luminous body, to
ascertain whether that body is approaching us or receding from us. Now
there are certain stars which always appear single even in the largest
telescopes, but when the spectroscope is directed to them a spectrum
_with two sets of lines_ is seen. Such stars must, therefore, be double.
Further, if the shiftings of the lines, in a spectrum like this, tell us
that the component stars are making small movements to and from us which
go on continuously, we are therefore justified in concluding that these
are the orbital revolutions of a binary system greatly compressed by
distance. Such conn