User Guide

A NEW SUN
APPENDIX 5
211
Brief Mythological Sidetrip. Other lands also have myths of half-horse, half-
human creatures and races, but Centaurs are best known for their role in Greek
mythology. They have the body and legs of a horse, with the torso, arms and
head of a human. In some myths they are lustful and aggressive; in other tales
they are wise and reserved.
BRIGHT STARS
It’s obvious to anyone who sees it that Alpha Centauri is bright, but how bright
is it? About 130 B.C., Hipparchus divided the visible stars into “magnitudes.” He
recorded six levels of magnitude, later more precise measurements have deter-
mined that each magnitude is about 2.5 times brighter than the next magni-
tude. He listed 18 first-magnitude stars; four more were added to his list by later
astronomers, stars that were too far south for him to see. These four stars are
Alpha and Beta Crucis (in the Southern Cross) and Alpha and Beta Centauri.
In fact, Alpha Centauri is the third-brightest star in the sky, behind Sirius
(magnitude -1.42) and Canopus (-0.72), with a magnitude of -0.27. It’s over
three times brighter than the average first-magnitude star. (For the record, it’s
the tenth-brightest object in the sky, behind the Sun, the Moon, Venus, Mars,
Jupiter, Sirius, Mercury, Canopus and Saturn.)
But why is it so bright? The answer seems obvious today, but let’s briefly fol-
low the trail of discovery that astronomers followed.
PROPER MOTION
In the early 1700s, Edmund Halley discovered that a few stars (Sirius, Procyon
and Arcturus) actually shifted position in the sky. In particular, bright stars
seemed to be more likely to move. That made it likelier that bright stars are
bright because they are closer to us. (It had already been decided that stars,
unlike planets, were too far away to reflect the light of the Sun, because they
didn’t seem any larger when viewed under magnification. The resulting theo-
ry was that stars generated their own light.)