Jimmy Westlake: Cassiopeia ushers in autumn
September 7, 2011
Steamboat Springs — Summer is slipping away from us, and the changing constellations are a sure sign of the approach of autumn. The Big Dipper that rode high in the sky during spring and summer evenings now is sinking into the northwest. The Summer Triangle also is migrating westward. A whole stage full of new constellation characters is rising in the east to take their places.
One of the first star patterns to catch your eye in the late summer and early fall is a distinctive group of five bright stars forming the shape of a W in the northeastern sky. This familiar pattern represents Cassiopeia, the queen. The five stars of the W pattern actually form the outline of the queen's chair, hanging upside down in the sky. Why upside down?
Greek mythology explains that Cassiopeia is being punished for her boastful ways. She had a habit of doting on her beautiful daughter, Andromeda, and once went so far as to claim that Andromeda was more beautiful than the sea nymphs, who were the pride and joy of Poseidon, the mythological god of the sea. Poseidon punished Cassiopeia by placing her in the sky close to the pole star so that, as she rotates around the pole, she would spend half the year upside down, clinging to her throne for dear life. Let this be a lesson to all the vain and boastful people out there.
The star Caph, at the top of the W, is the nearest of Cassiopeia's five main stars at a distance of 54 light years from Earth. The star marking the middle of the W is the most distant, at 613 light years. This star, simply referred to as Gamma, is the brightest unnamed star in the northern hemisphere. Perhaps it was much fainter centuries ago when the Greeks and Arabs were naming the stars. Gamma today is known to be an unpredictable variable star that occasionally increases dramatically in brightness.
From Northwest Colorado, Cassiopeia is a circumpolar constellation, which means that it never dips below the northern horizon. It just barely skims over the mountaintops to the north before rising again. Cassiopeia's chair is diametrically opposite the North Star from the Big Dipper, so one or the other star patterns is visible at all times. While the Big Dipper dominates the spring sky, Cassiopeia rules the autumn nights. Just follow the hazy band of the Milky Way northward to find Cassiopeia.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus.