While not the brightest star in the sky, Polaris, the North Star, is among those with
the broadest of stellar roles. Ranking 50th, it's the luminary of
Ursa Minor, the Smaller Bear, and sits at
the end of a beloved pattern, the Little Dipper. The star is easy
to find, as the two front bowl stars of the Big Dipper point toward it. It's close to
the North Celestial Pole, the point
about which the stars seem to rotate over the course of the night.
Face Polaris and you look north. As you walk toward the star, it and
the celestial pole rise higher above the horizon. The first rule of
celestial navigation is that the elevation of the pole above the
horizon in degrees is your latitude. We are at a latitude of 40 degrees
north. Look 40 degrees up from the horizon and there it is.
'Twas not always so. The Earth's spin axis wobbles about the orbital axis (to
which it is inclined by 23.4 degrees, the cause of the seasons) over
a period of 26,000 years. The "North Star" thus keeps changing. The
current alignment of Polaris near the pole (actually 3/4 of a degree
away) is thus an accident. In Homeric times, the Little Dipper's
brighter front bowl star (Kochab) was near
the pole, while the ancient Egyptians followed Thuban in Draco
(the celestial dragon). In 12,000 years we get bright Vega in Lyra, and in
26,000 we're right back to Polaris.
About 150 BC, the Greek astronomer Hipparchus divided stars into six
groups, from first "magnitude," the brightest
(22 of them), to sixth, the faintest he could see. We use his scheme
yet today. Polaris, which is visible any time from almost all the
northern hemisphere, initially set the modern system's mathematical
scale at magnitude 2.0. But Polaris's brightness is slightly
variable, which makes it a poor standard. It's an ageing type of
variable called a "Cepheid"
(after the constellation Cepheus, in which the first one was found)
with a mass some six times that of the Sun.
Cepheids were the initial candlesticks that allowed Edwin Hubble to
discover the expansion of
the Universe in the 1920s. Polaris is unique in that over the
twentieth century its variability nearly ceased, allowing us an
unusual opportunity to watch stellar evolution in action.
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Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the Winter-Spring 2016 Clark-Lindsey Village Voice.