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the light from such a binary system, 
shifts in the lines in the spectra may 
be measurable. These Doppler shifts 
measure the radial velocity of the 
brighter of the two stars in the system 
(a star in a binary pair with, say, one- 
tenth the mass of its partner will con- 
tribute less than one percent of the 
light of a spectrum). The radial ve- 
locity is the component of the star’s 
motion that is directly toward or away 
from the earth. If we happen to be 
looking at the system so that the stars 
are moving at right angles to our line 
of vision, then their radial velocities 
will always be zero, and their orbital 
motions will be undetectable in the 
spectrum. For most binary systems, 
the stars are not moving at right angles 
to our line of vision, however, so that 
some radial velocity due to orbital mo- 
tion exists and can be measured if 
it is large enough and if the star is 
bright enough so that its spectrum 
can be obtained with an exposure time 
that is much less than the orbital pe- 
riod. 
In the case of Vega, if a one-tenth 
mass companion at a distance of 0.8 
astronomical units existed, the veloc- 
ity variation of Vega would be about 
3.5 miles per second in five months 
time. This velocity variation is just 
detectable, which tells us that in such 
a case the method is useful for sep- 
arations of less than 0.1 arc second, 
because the velocities become larger 
with decreasing orbital size for any 
pair of objects. In neither the posi- 
tional nor the radial velocity obser- 
vations for Vega itself has a pertur- 
bation been detected. Thus, the exist- 
ence of a companion star with a mass 
larger than one-tenth that of Vega is 
virtually excluded by present obser- 
vations. This does not hold true for 
all of the other bright stars nearer 
the earth than Vega, where the tech- 
niques of visual examination, inter- 
ferometry, perturbation measure- 
ments, and measurements of radial ve- 
locity perturbations have resulted in 
the detection of numerous stellar com- 
panions. 
However, most of the nearby stars 
are not visible to the unaided eye. 
In fact, the faintest known of these 
objects are one million times fainter 
than the brightest 10 percent of the 
nearby stars. Fortunately, the method 
of perturbation measurements still 
works well for these faint stars, al- 
though the other methods fail when 
applied to stars of fainter apparent 
magnitude. Photographic exposures of 
up to several hours are required for 
the faintest stars known, but the po- 
sitional measurements possible with 
such photographs are nearly as ac- 
curate as those of the very short ex- 
posures for stars one million times 
brighter. Furthermore, the faintest 
star known, Van Biesbroeck’s Star in 
the constellation Aquila, has a mass 
approximately one percent that of 
Vega. A hypothetical companion of 
Van Biesbroeck’s Star would thus pro- 
duce a wobble of Van Biesbroeck’s 
Star 100 times that produced by a 
similar companion of Vega. Using the 
above techniques during the rest of 
this century, astronomers should find 
all of the stellar companions of stars 
nearer than Vega. Most have been 
found already. 
Van Biesbroeck’s Star is the least 
massive star known, with a mass about 
thirty times that of Jupiter. The theory 
of stellar interiors indicates that a star 
with a mass smaller than this will not 
undergo nuclear burning in its interior, 
and thus stars of masses from roughly 
three to thirty times that of Jupiter 
are termed “black dwarfs.” Although 
they may glow from internal heat as 
do Jupiter and Saturn (in infrared ra- 
diation), they will be much fainter 
than true stars, such as the sun, which 
shine as a result of nuclear energy 
production. 
The problem of detecting black 
dwarfs and planets is not easily solved. 
When orbiting other stars, planets 
such as the four giant planets of our 
solar system — Jupiter, Saturn, Ura- 
nus, and Neptune — can be detected 
by the perturbation method only if 
the star they are orbiting is one of 
the nearest to our solar system and 
if the star has a small mass. However, 
with existing equipment even these 
conditions bring the planet only to the 
fringe of detectability. Fortunately, in- 
strumentation now in the development 
stage will be capable of greater pre- 
cision. Also, the launching into orbit 
of the Space Telescope in 1985 should 
provide an increase of more than ten- 
fold in our capacity to detect small 
masses. Thus the opportunity exists 
to detect some black dwarfs and plan- 
ets around nearby stars, and perhaps 
by the year 2000 to at least estimate 
the number of black dwarfs and plan- 
ets nearer than Vega. 
Let us now look at a more distant 
region of space, beyond Vega’s dis- 
tance of 27 light-years and extending 
as far as the Pleiades star cluster, 
410 light-years from the earth, or 
