Searching for 
Dim Companions 
Astronomers use a variety of techniques to find 
the less luminous partners in double star systems 
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by James W. Christy 
The major characteristic of the uni- 
verse is that it organizes matter into 
very small packages, thus producing 
its characteristic appearance in visible 
light of bright points on a black back- 
ground. The organizing force is grav- 
ity. Gravity has condensed matter 
from the vastness of space into gal- 
axies, stars, planets, comets, and even 
smaller bodies. Many of these small 
bodies are relatively dark objects. 
Consequently, in addition to the bright 
points on the black background, there 
are even more dark points about which 
we know little. Some are dead stars, 
notably the postulated black holes. 
Fortunately, it is possible that black 
holes, black dwarfs (stars too small 
to ignite and shine as a result of nu- 
clear reactions), and planets may oc- 
cur primarily in the presence of visible 
stars. This is demonstrated in the ex- 
istence of the planets around our sun 
and in the existence of double stars, 
or binary systems, where very small 
and dim stars sometimes orbit brighter 
ones. Many binary systems consist of 
stars of nearly equal brightness, but 
it is the dim companions that are es- 
pecially interesting. The frequency of 
systems with small companions relates 
directly to our understanding of the 
formation processes of stars and, fur- 
thermore, to the expected frequency 
of occurrence of systems with ex- 
tremely small companions, such as 
planets. 
That dim stars are often paired with 
brighter ones at least eliminates the 
frustration of searching the entire sky, 
535 billion square seconds of arc, when 
looking for nearly invisible objects. 
(An arc second is a measurement of 
how large an object appears to the 
viewer. It is a small unit that depends 
on the size and distance of the object. 
One arc second, for example, is about 
one eighteen-hundredth of the diam- 
eter of the full moon as seen from 
the earth.) However, this pairing 
means that many of the objects in 
the universe may lie very close to other 
stars, inside the atmospherically 
caused fuzziness that broadens images 
seen through earthbound telescopes. 
The size of such images is normally 
larger than one arc second. At the 
distances of most stars, such fuzziness 
would easily obscure an entire solar 
system. One might conclude that there 
is very little one can do to obtain in- 
formation about these less luminous 
objects from ground observations, but 
the fact is that during the last century 
astronomers have been amazingly suc- 
cessful, using a combination of tech- 
niques for pulling information out of 
fuzzy images. Because the success of 
these techniques depends primarily on 
the distance of the systems from the 
earth, let us divide them into three 
separate distance groups. First, there 
are those nearer than the bright star 
Vega, in the constellation Lyra, ap- 
proximately 27 light-years from the 
earth; second, those farther than Vega 
and out as far as the Pleiades star 
cluster, 410 light-years away; and 
third, those beyond the Pleiades and 
out as far as the center of our galaxy, 
30,000 light-years away. 
The sun is just eight light-minutes 
from the earth, and so Vega lies about 
the same distance from the sun as 
from the earth — twenty-seven light- 
years. Nearer than Vega there are 
about two hundred known stars, most 
of which are smaller and dimmer than 
the sun. Probably many smaller and 
even dimmer stars remain to be dis- 
70 
