Consider the Great Square of Pegasus. Three of its stars belong to the Winged Horse, but the fourth is the alpha star of Andromeda. All three of these sky shapes are currently visible after sunset.
“Great” in size only (the magnitudes of this quartet of stars range from 2.0 to 2.8 — second-class citizens as stars go), the slightly lopsided Square is nevertheless an imposing sight in the barren-looking autumn sky. The Great Square is a useful guidepost for locating nearby constellations and sky objects. A year ago (“Beyond the Milky Way,” October 2003), we showed you how to use Alpha (α) Andromedae (the top left star in the Square) to find M31, the Andromeda Galaxy.
The Great Square is useful for pointing out other constellations and also can serve as an indicator of the transparency of the night sky. “Transparency” is the term astronomers use to describe night-sky clarity. Determined by a number of factors, such as atmospheric dust, haze, invasive moonlight, or light pollution, transparency changes from night to night — even hour to hour. If transparency is good, you’ll see faint naked-eye stars. Binoculars and telescopes will reveal much greater detail in deep-sky objects than can be visible on an average night.
Live in a remote rural area? As long as you’re observing on a clear night when the Moon isn’t lighting up the sky, you have a shot at picking out more than a dozen stars inside the Great Square.
You’ll need reasonably transparent skies to see our next shape — the Circlet of Pisces. As the finder chart shows, it’s located a stone’s — er, meteoroid’s — throw below the Great Square. The most recognizable part of Pisces, the Circlet, is made up of about a half dozen 4th and 5th magnitude stars arranged in a 5° by 7° oval. (Hey, the Square wasn’t perfect. Neither is the Circlet.) If you survey the Circlet with binoculars, you might stumble on a reddish star on the left (eastern) end. This is the star 19 (TX) Piscium. To see its ruddy color well, you’ll need to hold those binoculars steady.
We’ve seen a square and a circle. Wanna go for the Triangle? No problem. From Beta (β) Pegasi at the top right corner of the Great Square, trace an imaginary line to Alpha Andromedae (top left), then extend it about one-and-a-half times beyond. This should bring you close to the alpha star of Triangulum. Not to be confused with the Summer Triangle, which we explored in August, Triangulum is a rather small and obscure stellar trio. The illustration shows the location of the galaxy M33 is not far from Alpha (α) Trianguli. If you’re an astronomical newcomer, M33 will prove to be a difficult naked-eye target. It’s bright (magnitude 6.4) as galaxies go, but the light is spread over an area twice the Moon’s apparent diameter. This makes M33 an elusive target, something you won’t want to tackle unless skies are extremely transparent. In a future edition of this column, we’ll show you how to notch this galaxy.
Don’t forget about the Orionid meteors after midnight on the 21st and the lunar eclipse one week later. Try a Great Square star count at the beginning of the eclipse, while the Full Moon is still bright. Then count the stars again during totality, when the Moon’s light is substantially dimmed. You won’t believe the difference!
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Next month: We’ll take a close-up look at the constellation Cassiopeia.