94 ASTRONOMY. 
The Double Stars ; Remarkable Collections of Stars; Nebulous Spots 
and Stars. 
15. By double stars is meant two stars, generally so near together that to 
the naked eye they appear but as a single star. There are nevertheless 
many double stars only visible through the telescope. These may consist 
of two kinds—optical, or apparent, and physical, or actual double stars. The 
‘first are such as, not related to each other, happen to fall nearly on the 
same line of vision ; the latter are those which, connected in one system, 
revolve the one about the other. One of these is frequently larger than the 
other, although sometimes their size is nearly equal. Their colors, how- 
ever, are always different. The single fixed stars and the optical double 
estars shine only with a whitish light, verging sometimes on yellow, some- 
times on red. The physical or actual double stars, of which alone we here 
treat, have only been studied within the last few decades. The peculiar 
motions of these remarkable stars appear to occur according to the 
Newtonian Jaws of gravitation, and Savary, Encke, John Herschel, and 
Madler, have already determined the orbits of many double stars, as 70 p, 
Ophiuchus, polar star, y Andromede, ¢ Ursze Majoris, ¢ Herculis, &c., as 
also their periods of revolution, which latter in some double stars amount to 
a few years, in others to many centuries. Since one of two double stars, 
as before stated, revolves around the other, it may readily happen that with 
respect to our earth one may pass before the other so as to cover it com- 
pletely. This has actually been confirmed by observation. Thus, for 
instance. stars which once were double are now single, and others which 
were once single are now seen as double. Consequently the apparent dis. 
tances of the double stars cannot be otherwise than variable. Struve has 
published a catalogue of 3112 double and multiple stars, arranged in order, 
which is thus far the most complete and accurate. 
Since the double stars generally present very slight points of light of 
various distance and distinctness, their observation may serve as the surest 
test of the excellence of a telescope. Achromatic telescopes, which, for 
instance, merely exhibit ¢, Ursa Major, or Mizar (pl. 12, fig. 3), and 4, 
Andromeda, Alamak (fig. 6), as double stars, are only of ordinary powei. 
Those, however, are much better which show as double stars Castor and 
Pollux, or « Gemini (pl. 12, fig. 1) and the pole star, or « Ursee Minoris 
(fig. 9), as also Mesarthim, or y Arietis (fig. 8), and Cor Caroli in Canes 
Venatici (fig. 10). An instrument that shall show y Virginis (fig. 2), 
o Arietis (fig. 5), & Orionis, or Rigel (fig. 7), and Ras Algithi, or « Her- 
culis (fig. 4), as double, is one of extraordinary excellence. The double 
star Vega, or « Lyre (fig. 11), is probably not really but only optically 
double, the smaller of the 12 magnitudes being distant about 43 seconds 
from the principal stars. 
16. Among the spots of the northern heavens richest in stars belong the 
groups figured on pl. 12; the Pleiades, or the seven stars, fig. 12, in the 
back of Taurus, the Hyades, or rain stars, fig. 138, in the forehead of 
Taurus, the little group of stars (fig. 14) between the tips, 6 and , of the 
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