Another point to be noticed is the arrangement of stars in classes,
according to their lustre. The brightest stars, of which there are
about twenty, are said to be of the first magnitude. Those just
inferior to the first magnitude are ranked as the second; and those
just lower than the second are estimated as the third; and so on. The
smallest points that your unaided eyes will show you are of about the
sixth magnitude. Then the telescope will reveal stars still fainter and
fainter, down to what we term the seventeenth or eighteenth magnitudes,
or even lower still. The number of stars of each magnitude increases
very much in the classes of small ones.
Most of the stars are white, but many are of a somewhat ruddy hue.
There are a few telescopic points which are intensely red, some exhibit
beautiful golden tints, while others are blue or green.
There are some curious stars which regularly change their brilliancy.
Let me try to illustrate the nature of these variables. Suppose that
you were looking at a street gas-lamp from a very long distance, so
that it seemed a little twinkling light; and suppose that some one was
preparing to turn the gas-cock up and down. Or, better still, imagine
a little machine which would act regularly so as to keep the light
first of all at its full brightness for two days and a half, and then
gradually turn it down until in three or four hours it declines to a
feeble glimmer. In this low state the light remains for twenty minutes;
then during three or four hours the gas is to be slowly turned on again
until it is full. In this condition the light will remain for two days
and a half, and then the same series of changes is to recommence. This
would be a very odd form of gas-lamp. There would be periods of two
days and a half during which it would remain at its full; these would
be separated by intervals of about seven hours, when the gradual
turning down and turning up again would be in progress.
[Illustration: FIG. 84.--Perseus and its Neighboring Stars, including
Algol.]
The imaginary gas-lamp is exactly paralleled by a star Algol, in the
constellation of Perseus (Fig. 84), which goes through the series of
changes I have indicated. Ordinarily speaking, it is a bright star of
the second magnitude, and, whatever be the cause, the star performs
its variations with marvellous uniformity. In fact, Algol has always
arrested the attention of those who observed the heavens, and in early
times was looked on as the eye of a Demon. There are many other stars
which also change their brilliancy. Most of them require much longer
periods than Algol, and sometimes a new star which nobody has ever
seen before will suddenly kindle into brilliancy. It is now known that
the bright star Algol is attended by a dark companion. This dark star
sometimes comes between Algol and the observer and cuts off the light.
Thus it is that the diminution of brightness is produced.