twenty stars in the sky are popularly considered as
being of the first magnitude, yet several of them are much brighter than
an average first magnitude star would be. For instance, Sirius--the
brightest star in the whole sky--is equal to about eleven first
magnitude stars, like, say, Aldebaran. In consequence of such
differences, astronomers are agreed in classifying the brightest of them
as _brighter_ than the standard first magnitude star. On this principle
Sirius would be about two and a half magnitudes _above_ the first. This
notation is usefully employed in making comparisons between the amount
of light which we receive from the sun, and that which we get from an
individual star. Thus the sun will be about twenty-seven and a half
magnitudes _above_ the first magnitude. The range, therefore, between
the light which we receive from the sun (considered merely as a very
bright star) and the first magnitude stars is very much greater than
that between the latter and the faintest star which can be seen with the
telescope, or even registered upon the photographic plate.

To classify stars merely by their magnitudes, without some definite note
of their relative position in the sky, would be indeed of little avail.
We must have some simple method of locating them in the memory, and the
constellations of the ancients here happily come to our aid. A system
combining magnitudes with constellations was introduced by Bayer in
1603, and is still adhered to. According to this the stars in each
constellation, beginning with the brightest star, are designated by the
letters of the Greek alphabet taken in their usual order. For example,
in the constellation of Canis Major, or the Greater Dog, the brightest
star is the well-known Sirius, called by the ancients the "Dog Star";
and this star, in accordance with Bayer's method, has received the Greek
letter [a] (alpha), and is consequently known as Alpha Canis
Majoris.[29] As soon as the Greek letters are used up in this way the
Roman alphabet is brought into requisition, after which recourse is had
to ordinary numbers.

Notwithstanding this convenient arrangement, some of the brightest
stars are nearly always referred to by certain proper names given to
them in old times. For instance, it is more usual to speak of Sirius,
Arcturus, Vega, Capella, Procyon, Aldebaran, Regulus, and so on, than of
[a] Canis Majoris, [a] Booetis, [a] Lyrae, [a] Aurigae, [a] Canis Minoris,
[a] Tauri, [a] Leo