about fifteen times as distant as Vega. 
The number of stars out there should 
increase approximately in proportion 
to the volume of the region, which 
is 15 X 15 X 15, or 3,375, times 
greater than the volume of the region 
between the earth and Vega. If we 
multiply 15 X 15 X 15 by 200, for the 
known stars out as far as Vega, we 
get 675,000 or more stars that may 
be nearer than the Pleiades. This re- 
gion of space, although only a tiny 
piece of our Milky Way galaxy, is 
critical to astronomy because in this 
region many types of stars exist that 
do not happen to be found nearer to 
earth than Vega. And in this region, 
relatively nearby within a 410-light- 
year radius, astronomers can calibrate 
the luminosity, or true brightness, of 
the stars because direct distance de- 
termination, which is impossible at 
greater distances, is feasible. At 
greater distances the brightness of the 
stars is generally used to make crude 
estimates of their distances. This is 
done by means of the very critical 
luminosity calibrations in the Pleiades 
and nearer. 
At distances between that of Vega 
and that of the Pleiades, we can do 
little to detect black dwarfs and plan- 
ets because the size of their wobbles 
decreases in proportion to their dis- 
tance. The discovery of stellar com- 
panions is still possible, however, al- 
though more difficult. But the number 
of stars to investigate for companions 
is very large, and most of the binary 
stars known (roughly 50,000) have 
been discovered in this region by di- 
rect visual examination of the image 
through the telescope. An experienced 
visual observer can detect stars sep- 
arated by only one-tenth of an arc 
second by waiting patiently for mo- 
ments of excellent viewing. Superb 
eyesight and telescope quality are re- 
quired for this demanding task, as well 
as unusual endurance on long, cold 
nights. 
Radial velocity perturbations do not 
decrease with distance, and therefore, 
for sufficiently luminous stars, the 
method of velocity perturbations re- 
tains its value out to great distances. 
However, the brightness of a star de- 
creases with the square of its distance. 
If Vega were moved fifteen times far- 
ther away, to the distance of the Ple- 
iades, it would be 15X15, or 225, 
times fainter, and the exposure re- 
quired to obtain its spectrum would 
be 225 times longer, or roughly an 
hour. In principle, we have the ca- 
pacity to discover in this region all 
companions that are of a luminosity 
brighter than that of our sun. In prac- 
tice, there are too many such objects, 
and we must be somewhat selective. 
Beyond the distance of the Pleiades 
lies 99.99 percent of our galaxy and 
roughly 100 billion other galaxies. All 
of the universe must be interpreted 
in terms of what is observed in the 
Pleiades and nearer because of the 
blurred images and faintness that limit 
most observations beyond the Pleia- 
des. The number of stars becomes 
huge, in the hundreds of billions within 
our galaxy alone. In such an enormous 
sample of stars, another detection 
technique becomes possible. Many bi- 
nary star systems will be found ori- 
ented in such a way that their orbital 
plane lies nearly exactly in the line 
of sight. The two stars of a binary 
system will pass in front of each other 
producing eclipses at intervals of one- 
half of their orbital period. The bright- 
ness variation makes such “eclipsing 
binaries” show up on repeated pho- 
tographs. Once the brightness changes 
of an eclipsing binary system are de- 
tected, detailed study can yield a 
rough idea of the shapes and surfaces 
of both stars. If the stars are bright 
and their radial velocities are obtain- 
able, astronomers can measure with 
precision the diameters of the stars 
in the pair by determining the lengths 
of the two eclipses in the orbital pe- 
riod. Thus even at distances of thou- 
sands of light-years, for a few select 
stars we can measure sizes down to 
one-millionth of the image diameter 
and, in a fashion, see two stars. 
In our solar system a similar tech- 
nique will yield a special surprise in 
the case of the planet Pluto and its 
moon Charon. A few years from now, 
Pluto’s slow orbital motion around the 
sun will line up its orbital plane and 
that of Charon in such a fashion that 
seen from the earth each object will 
alternately eclipse the other. For some 
five subsequent years this system will 
behave like an eclipsing binary, with 
an orbital period of about six days. 
Thus a brightness mapping of the sur- 
faces of both bodies will be obtainable, 
and we will at last be able to discern 
the details of Pluto and Charon despite 
their distance from the earth. 
James W. Christy is an astronomer 
at the U.S. Naval Observatory in 
Washington, D.C. He is credited with 
discovering Pluto’s satellite Charon 
in 1978. 
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