GV-42 PHOTOGRAMMETRIC ENGINEERING 
The annual parallax is slightly over ".03 at a distance of 100 light years, the 
approximate limit of distance penetration by the photographic method. 
10. NEARBY STARS 
The parallaxes of the nearest stars have yielded both individual and statis- 
tical results of interest. Consider, for example, the known 56 stars within the 
arbitrary distance limit of 16 light years. Of this cosmic sample fewer than a 
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5 10 15 20 microns 
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Fic. 4. The annual parallactic motion of the nearby star Ross 248 (distance 10.2 light years), 
as caused by the earth’s orbital motion around the Sun. The photographic observations have been 
grouped into 16 points, whose accuracy (probable error of +”.006) is represented by the radii of the 
small circles. These normal points clearly determine the part of the star's parallactic orbit accessible 
to observation. Since the observations are always made close to the meridian, the other half of the 
parallactic orbit falls in daylight, and is therefore not observable. 
dozen stars are visible to the unaided eye. The majority are faint telescopic 
objects which were first recognized as being close to us because of their large 
proper motion. Not all 56 stars are isolated like the sun. Of these 31 appear 
single, 20 are grouped,—two by two—in 10 binary systems, while 6 are grouped 
—three by three—in two triple systems. 
Knowing the distance to a star permits the calculation of the intrinsic 
brightness, or luminosity, which is conveniently expressed in terms of the sun's 
luminosity. In the present sample only three stars are brighter than the sun, 
while the faintest star is only 1/63,000 as luminous as the sun. 
Stars differ in size and color, or what amounts to the same thing—in tem- 
perature. With few exceptions, there appears to be a remarkable relation be- 
tween luminosities and colors. The fainter a star, the redder its color, so that 
an arrangement of the nearer stars according to decreasing luminosity also 
proves to be a sequence of increasing redness, and of decreasing diameter. The 
astronomer refers to this relation as the main sequence. However, the situation 
is rather more complex, particularly if there is included more distant stars— 
and the main sequence must be considered only as a first, basic stepping stone