User Guide

The motion that Halley discovered is called proper motion, because it is
motion of the star itself, rather than movement of the Earth. Halley couldn’t
see Alpha Centauri, but other astronomers soon discovered that it, too, has
proper motion. In fact, its motion is greater than that of any other bright star.
On the other hand, it’s not surprising this motion wasn’t detected by earlier
astronomers — it takes more than 500 years for Alpha Centauri to transit an
arc of space as wide as the moon.
However, the proper motions of several very dim stars — including 61 Cygni
(fifth magnitude) and Barnard’s Star (ninth magnitude) — are greater even than
that of Alpha Centauri. Barnard’s Star is almost three times as fast as Alpha
Centauri. So which is the better predictor of closeness, brightness or proper
motion? (Note that a star coming directly at us, or moving directly away from
us, would have no detectable proper motion, no matter how close it is.)
DOUBLE STARS
Double stars are pairs of stars that are apparently so close that it’s hard to dis-
tinguish them individually. Alpha Centauri turns out to be a double star. It
wasn’t originally thought that double stars were actually close to each other,
just that they were on the same line as seen from the Earth. However, when
astronomers started listing double stars, there turned out to be far too many
for random distribution of this sort. It seemed likely that many of the double
stars were actually part of the same system. Double stars that are bound
together in one system are called binary stars. Alpha Centauri is a binary, con-
taining Alpha Centauri A and Alpha Centauri B.
Observation of a binary system gives us two more indicators to its distance
from our own solar system. The farther apart the two stars appear to be from
each other, the closer the system is likely to be to us. This likelihood is drawn
from the assumption that all binary stars are actually the same distance apart
from each other, which is obviously not true. However, it can serve as an indi-
cator, especially when combined with other data.
A NEW SUN
APPENDIX 5
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