io8 
THE GUIDE TO NATURE 
of these stars would receive any atten- 
tion if it were separated from the 
others. But as a group they at once 
attract the attention. In ancient lit- 
erature the group was always regarded 
as consisting of seven stars. What has 
happened to the seventh star, if one 
ever appeared, is uncertain. Fourteen 
stars have been seen with the naked 
eye by at least three persons under 
very favorable conditions. Eleven were 
charted by a student of Kepler. This 
was before the days of telescopes, so 
that there is no doubt that he saw 
them. A few of the other stars are 
shown on Figure 2. The star most 
easily seen after the six is the one to 
the right of those joined with the lines. 
Its name is Calaeno. Those joined by 
the lines are named from left to right 
Atlas, Alcyone, Merope, Electra, Maia 
and Taygeta. A two-inch telescope 
shows seventy-five stars in the group. 
The whole region is filled with stars 
just a little too faint to see with the 
naked eye. On a photograph covering 
three square degrees about the Pleiades 
and showing stars down to the six- 
teenth magnitude 2,326 stars were 
counted. 
The group is sometimes called the 
Uittle Dipper, as the stars are arranged 
in that form. Ursa Minor, however, is 
better known as the Uittle Dipper. The 
Pleiades are often called the Seven 
Sisters. In Europe they are known as 
the Hen and Chickens. 
Southeast of the Pleiades near the 
bright star Aldebaran in Taurus is an- 
other group of faint stars usually asso- 
ciated with the Pleiades. They are 
called the Hyades. This group has 
been found to be a globular cluster of 
stars about one hundred and thirty 
light years away. The stars all give 
out more light than the sun, some as 
much as a hundred times as much. 
They move together through space. 
jjc ;jc 
Algol. 
At C, Figure 1. in the constellation 
Perseus is the best known variable star. 
Beta Persei or Algol. It is usually of 
2.3 magnitude, but regularly, at inter- 
vals of two days and twenty-one hours, 
its brightness decreases. In four and 
a half hours its brightness decreases to 
3.5 magnitude. It remains at this mag- 
nitude for twenty minutes and then in- 
creases in brightness to 2.3 magnitude 
again in three and a half hours. The 
star will be at its minimum brightness 
at the following times in the early even- 
ing in December: December 6, 11:30 
P. M. ; December 9, 8:15; December 
12, 5:05; December 29, 10:00. It is 
best to locate the star and compare its 
brightness with that of the surround- 
ing stars some evening before that of 
the minimum brightness. 
The variations of the star must have 
been known for a long time, for the 
name Algol, applied to it by the Arabs, 
Figure 3. The system of Algol. 
means “demon star. - ’ As early as 1783 
it was suggested that the variations 
were caused by some dark body re- 
volving about the one we see and thus 
cutting off a part of its light, when the 
other body passed between us and the 
star. The very accurate measures of 
brightness now possible show that 
there is also a very small decrease in 
brightness half way between the prin- 
cipal minima. By noting the changes 
in the star’s brightness with the time 
we can, after a mathematical discus- 
sion of the problem, learn not only that 
an eclipse causes the variation but the 
relative dimensions of the two bodies, 
the distance between them, the relative 
brightness, the position of the orbit, 
and we can set an upper limit to the 
densities of the stars. This shows that 
the dark body is somewhat larger than 
the bright one and only a twentieth as 
bright. 
