Way. For example, among every 1 00 of 
these long-period secondaries, there are 
as many stars having 1 solar mass as 
there are among 100 stars chosen at 
random in the Milky Way, and there are 
as many stars of Vi solar mass as are 
found among 100 stars of the galaxy’s 
general population. Stars with 'A solar 
mass seem to be the most numerous of 
all in the Milky Way, and they are also 
the most numerous among the long- 
period secondaries. 
But among short-period binary sys- 
tems (in which the stars are fairly close 
together and take less than 100 years to 
complete their orbits), the secondaries 
do not have the same occurrence rates 
according to their masses as stars of the 
general Milky Way population. Instead 
of being most numerous at about 1 A solar 
mass, these secondaries tend to have 
masses approaching those of their pri- 
mary stars. If we were to study 100 
short-period primaries, each having 1 
solar mass, for instance, we would find 
that ‘A-solar-mass stars are not the most 
numerous among their secondaries. In- 
stead, secondaries with closer to 1 solar 
mass would be the most numerous. 
This distinction in the properties of 
mass between the secondary stars of 
long- and short-period binary systems 
was the second striking clue that two 
different processes form binary stars. 
From this second clue, we deduce that 
one process creates long-period binaries, 
in which the secondaries resemble the 
general population of stars. The other 
process, which creates the short-period 
binaries, somehow produces secondaries 
that are slightly smaller copies of their 
primaries. 
The leading theories of binary star 
formation invoke the phenomena of fis- 
sion, capture, fragmentation, disruption 
of clusters, and condensation of adja- 
cent protostars, respectively. A star be- 
gins as a protostar, condensing from a 
cool nebula of gas and dust. As the 
protostar contracts and warms on its 
way to igniting nuclear reactions and 
becoming a real star, it spins ever faster. 
According to the fission theory, if the 
protostar spins fast enough, it will throw 
off matter, which then forms a compan- 
ion protostar. The pair will eventually 
become true stars, constituting a binary 
system. Some astronomers believe that 
fission is inevitable and that it explains 
why almost all stars are binary or multi- 
ple. Nevertheless, there are objections 
to this theory. One is that tidal forces 
might prevent the ejected matter from 
condensing, just as tidal forces around 
Saturn seem to have prevented the par- 
ticles in its rings from collecting into one 
or more moons. In addition, computa- 
tions by Leon B. Lucy of Columbia 
University, an expert on binary stars, 
indicate that the matter ejected from a 
rotating protostar is sufficient only to 
form a secondary star with x k the mass 
of the primary. Thus, the many close 
binaries whose primary and secondary 
stars have comparable masses could not 
form by fission. Finally, fission could not 
hurl the secondary star far enough from 
the primary to form a long-period bi- 
nary. 
According to the capture theory, a 
binary is created when two unrelated 
stars happen to pass close enough to 
each other to enable their gravitational 
attraction to form a permanent bond. 
This theory is more applicable to the 
long-period binaries with their large sep- 
arations than to short-period binaries 
that are fairly close together, because 
stars passing randomly in space are 
more likely to pass at a large distance 
than at a small one (j ust as a badly 
thrown dart more often hits an outer 
target zone than the bull’s-eye). The 
objection to this theory is that simple 
analysis shows that two passing stars will 
almost always keep on going; capture 
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