22 
The Astronomical Survey of the Universe [ch. i 
available to us of the binaries in space. We accordingly conclude that only a 
small fraction of binaries are spectroscopic, and that the mean period of visual 
binaries is of the order of 100 years. 
Table III. Binary Systems within 10 parsecs of the Sun {Hertzsprung). 
Star 
Period in years 
(where known) 
log P 
X Draconis 
0-770 
-o-i 
£ Ursae Maj. 
1-80 
0-3 
/3 395 
25-0 
1-4 
£ Herculis 
34-46 
1-5 
Procyon 
40-23 
1-6 
/3 416 
41-47 
1-6 
/x Herculis 
42-23 
1-6 
Lacertae 353 
— 
1-7 
Sirius 
49-32 
1-7 
Kruger 60 
54-9 
1-7 
a Centauri 
78-83 
1-9 
70 Ophiuchi 
87-86 
1-9 
£ Bootis 
152-8 
2-2 
o 2 Eridani 
180-03 
2-3 
61 Cygni 
— 
2-6 
Sh. 190 
— 
2-8 
Struve 2398 
— 
2-9 
Struve 1321 
— 
3-0 
k Tucanae 
— 
30 
Groombridge 34 
— 
3-4 
y Leporis 
4-1 
22. The number of Binary Stars. We have just seen that 21 of the 87 
stars which are known to be within 10 parsecs of the sun are binary, while of 
the 26 stars within 5 parsecs of the sun (cf. Table I) 8 are binary. These 
figures suggest that something like a quarter, or possibly a third, of the 
stars in the sky are binary. We must be on our guard against putting the 
proportion too high. It may be true that some of the 87 and 26 stars just 
mentioned may be binary without their binary nature having yet been dis 
covered; on the other hand there may be a number of stars still undiscovered 
which must be added to our figures of 87 and 26, and these are far more 
likely to be simple stars than binary systems. But it seems probable that 
fully a quarter of the whole number of stars in the sky are binary, while in 
special classes of stars the proportion is higher. For instance Hertzsprung has 
found that 9 out of 15 stars in the Ursa Major cluster are binary, while 
Frost found that half the stars in the Taurus cluster are binary. Next to 
the simple featureless star, the binary star is the commonest object in the 
sky. 
23. Eclipsing Variables. The majority of binary stars shine with a per 
fectly steady light, the total light emission of the system being the sum of 
the emissions from the two components separately. In a certain proportion of