4§§jF 
THE 
WOMAN 
THAT NEVER 
EVOLVED 
Sarah Blaffer Hrdy 
What does it mean to be female? 
Sociobiologist and feminist Sarah 
Blaffer Hrdy shows us that evolution- 
ary biology can provide surprising 
answers. She introduces us to our 
nearest female relatives- highly 
competitive, independent, sexually 
assertive female primates. 
Hrdy proves that the sexually pas- 
sive, non-competitive, all-nurturing 
woman of prevailing myth never 
could have evolved within the 
prima'te order. 
“A spectacular and pioneering 
work !” — Jane B . Lancaster 
“A stunning achievement.” 
— Jessie Bernard, author of 
The Female World 
$17.50 Illustrated 
At your local bookstore, or directly from 
Harvard 
University Press 1 
79 Garden Street, 
Cambridge, MA 02138 
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Sky Reporter 
Stellar Togetherness 
Most stars in the Milky Way are 
members of binary or multiple 
systems, but no one knows exactly why 
by Stephen P. Maran 
Most astronomical facts are gleaned 
from new observations, but sometimes 
clever analysis of existing data discloses 
a significant pattern that had been over- 
looked. A good example is a 1974 inves- 
tigation of binary stars by Virginia 
Trimble, an astrophysicist with the Uni- 
versity of California at Irvine and the 
University of Maryland at College Park. 
Her study of data on what are known as 
spectroscopic binary stars led to a better 
understanding of how binary systems 
are formed. 
A binary star, of course, consists of 
two stars in orbit around a mutual cen- 
ter of mass. A spectroscopic binary is a 
binary system that can be identified 
from the lines in its spectrum, whose 
wavelengths shift back and forth peri- 
odically owing to the Doppler effect as 
the two stars travel their orbits. Trimble 
analyzed the data on known spectro- 
scopic binaries by estimating the “mass 
ratio” of each such binary star. This 
quantity is simply the mass of the “sec- 
ondary,” or fainter, star in the binary 
divided by the mass of the primary star. 
Thus, if the primary has twice the mass 
of the secondary, the mass ratio is 0.5, 
whereas if the two stars have equal 
mass, the mass ratio is 1.0. 
Taking the ratio of two masses is a 
simple matter, but the results of Trim- 
ble’s analysis were still very significant. 
She found that the distribution of mass 
ratios has two peaks: there are a large 
number of binaries with mass ratios 
close to 0.7 (meaning that the secondary 
star in each case has about 70 percent 
the mass of its primary) and there are 
also many binaries with mass ratios near 
0.3 (the fainter star having just 30 per- 
cent the mass of the primary). By con- 
trast, there are relatively few binaries 
with intermediate mass ratios, such as 
0.5. The existence of two preferred val- 
ues, 0.7 and 0.3, immediately suggested 
to Trimble that two different processes 
must be involved in forming binary 
stars. One process is apparently effec- 
tive in making binaries with nearly 
equal masses (the 0.7 mass ratio sys- 
tems), while the other process forms 
binaries with very unequal masses (the 
0.3 mass ratio binaries). 
Most stars are members of binary or 
multiple star systems. (Although the 
sun, which is single, is considered to be 
an average star, in this respect, it is not 
average.) The rarity of being single, 
which has long been suspected, was con- 
firmed recently by several investigations 
carried out by Helmut A. Abt and Saul 
G. Levy, spectroscopists at Kitt Peak 
National Observatory in Tucson, Ari- 
zona. 
To determine the relative occurrence 
of binary, triple, and quadruple star 
systems, Abt and Levy made thousands 
of spectrograms of hundreds of stars. 
Typically, they observed a star on 
twenty different nights and then com- 
pared the twenty spectra to see if their 
spectral lines shifted as a result of or- 
bital motion. The results proved that 
single stars are indeed a minority. For 
example, in one 1976 study, Abt and 
Levy found that among stars resembling 
the sun (including those that are slightly 
hotter and brighter), 46 percent were 
observably binary, 9 percent were tri- 
ples, and 2 percent were quadruple 
stars. A minority of 42 percent seemed 
to be single. (These numbers don’t add 
up to 100 percent because they have 
been rounded off.) 
Since 42 percent is not an extreme 
minority, one might think that the sun, 
as a single star, is not so unusual after 
all. Forty-two percent is actually an 
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