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overestimate of the stars without com- 
panions, however, because some kinds 
of binary and multiple stars are hard to 
recognize as such (see “Searching for 
Dim Companions,” Natural History, 
June 1981). So-called selection ef- 
fects — that is, circumstances that may 
result in certain specimens being over- 
looked or underrepresented in a scien- 
tific survey — prevented Abt and Levy 
from identifying these difficult cases as 
binary or multiple stars. For example, 
binaries with very long orbital periods 
were missed because their orbital veloci- 
ties are correspondingly slow and hence 
the Doppler shifts of their spectral lines 
are too small to measure. Also, if a 
secondary star is much less massive than 
its primary, the primary will orbit at 
very low speed while the secondary star 
orbits at high speed. Because the mas- 
sive primary is much brighter, only its 
spectral lines will be detectable. The 
Doppler shifts of the massive primary 
star’s spectral lines will be too small to 
measure and thus the binary will pass 
for a single star. 
Still another selection effect occurs 
when the members of a pair orbit in a 
plane at nearly right angles to the line of 
sight from the earth. In that case, even 
though a pair may zip around their 
orbits at high speeds, there is no signifi- 
cant motion toward or away from the 
earth and thus no observable Doppler 
shift. Binaries of this type are also 
missed. 
When Abt and Levy extrapolated the 
observed percentages of binary and mul- 
tiple sunlike stars, increasing the per- 
centages to compensate for those missed 
due to selection effects, the “42 percent 
single” category shrank dramatically. 
After making these corrections, Abt and 
Levy concluded that nearly all stars are 
at least binary, with a typical star having 
an average of 1.4 companions. (Thus, 
among every ten stars seen in the sky, 
there will be a total of about fourteen 
companions.) 
Abt and Levy also made another im- 
portant finding that brings us back to 
Trimble’s discovery that two distinct 
processes are at work making binary 
stars. The Arizona team studied the 
masses of the secondary stars in binary 
systems and found that there are two 
types of secondaries that differ accord- 
ing to their orbital periods. Long-period 
binaries (those whose stars are widely 
separated from one another and whose 
large orbits take a century or more to 
complete) have secondaries that, as a 
class, are indistinguishable from the 
general population of stars in the Milky 
26 
