User Guide

A NEW SUN
APPENDIX 5
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The second clue is that the more quickly they revolve around each other, the
closer they are likely to be to each other. (Mercury, very close to the Sun,
revolves around the Sun in 88 days — Earth days, that is. Jupiter takes almost
12 Earth years for the same revolution.)
Alpha Centauri A and B have one of the widest apparent separations and one
of the quickest revolutions (80 years) of all binary stars. The evidence mount-
ed that Alpha Centauri is very close to us.
PARALLAX
As telescopes got better and better, it eventually became possible to measure
the parallax of closer stars. Parallax is a measure of how much an object
appears to shift, when the observer shifts position. In the case of stars, the only
way to measure perceivable parallax is to observe a star at two times in the year,
six months apart from each other, so that the Earth (and the observer) have
shifted from one side of the solar system to the other. In the 1830s Thomas
Henderson was able to measure Alpha Centauri’s parallax. He determined that
Alpha Centauri is 4.4 light-years away from us. (Remember that Alpha Centauri
is a binary, so sometimes Alpha Centauri A is closer to us, and sometimes Alpha
Centauri B is closer.) No star has since been found to be closer.
PROXIMA CENTAURI
Okay, there is one star closer. Sometimes. In 1913, Robert Innes discovered an
eleventh-magnitude star very close to the Alpha Centauri system, with high par-
allax. It is 0.15 light-years (a trillion miles, or more than 13,000 AUs*) from Alpha
Centauri A and B, but turns out to be part of the Alpha Centauri system,
nonetheless. Its orbit around the other two stars takes hundreds of thousands of
years; for now, it is closer to us than either Alpha Centauri A or B, by the same
0.15 light-years. Officially, it is Alpha Centauri C. Unofficially, it is Proxima
Centauri. We’ll keep calling Alpha Centauri a binary system, because Alpha
Centauri C is too far away, and too small, to materially affect the two larger stars.
* An AU is an astronomical unit, the distance from the Earth to the Sun.