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Locating the Summer Triangle

December 28, 2010
By Aileen O'Donoghue

Summer fades to memory as smoke curls from chimneys and forests of sticks emerge from brilliant blankets of red and gold. The robins and red-winged blackbirds are long gone from our yards and meadows. Shrinking flocks of geese rise from lakes where ice now forms, circle, honk their farewells and head south. But there are three summer birds that linger well into winter in the increasingly early darkness. The constellations of Cygnus the swan, Aquila the eagle and the bright white star Vega, "swooping eagle" hover over the western horizon as night falls.

As shown in the diagram, the brightest star of Cygnus, Deneb ("tail), and that of Aquila, Altair ("flying eagle"), join with Vega (VEE'-GUH) in Lyra to form the Summer Triangle, a large asterism, about one-and-a-half times the size of your hand held at arm's length. This asterism, straddling star fields and nebulae of the Milky Way, rises in June to dominate the sky through summer and fall.

All three stars are bright in our sky with Vega as the brightest and Deneb the dimmest, but their distances from us are very different. Altair is closest at 17 light-years (Ly) away. That means the light we see from Altair has been traveling for 17 years at 186,000 miles per second. Thus traversing about 6 trillion miles per year, the light from Altair has covered about 100 trillion miles since it left the star's surface in 1993, when Bill Clinton was inaugurated as president.

Vega is 25 Ly, 150 trillion miles, away, so we're actually seeing it as it was in 1985, at the start of Ronald Regan's second term. But Deneb, the dimmest of the three in our sky, is at the startling distance of 1,425 Ly, almost 60 times farther than Vega and 84 times farther than Altair. Its light has traversed 8,400 trillion miles of interstellar space since it left Deneb in the late sixth century, when the Prophet Mohammed was a teenager. To be as bright as it is at this distance, Deneb has to be an extremely bright giant, shining 55,000 times brighter than the sun and about 115 times its diameter. If it were at the distance of Vega, it would be 20 times brighter than Venus, easily visible during the day.

If we replaced our sun with Deneb, it would span 60 degrees of the sky and make Earth far too hot for liquid water or life. We'd have to move 235 times farther out, eight times farther from the sun than Neptune for Earth, to keep her oceans.

In the sky, Deneb is the top of its own smaller asterism, the Northern Cross. Below it is the horizontal bar of the cross made up of Gienah (GEE'-NAH, "wing"), Sadr (SAD'-ER, "breast"), and Delta, creating an easily recognizable triangle. Sadr is even more impressive than Deneb, shining 65,000 times brighter than the sun from a distance of 1,500 Ly. Dust between us and this star, though, makes it appear about 2.5 times dimmer than it actually is. The staff of the cross stretches down through Eta to Albireo (AL-BEER'-EE-OH). In a telescope, Albireo will separate into blue and gold stars, mimicking the official colors of the University of California at Berkeley that has taken Alberio as its campus star. This is a favorite target of amateur astronomers due to the beauty of the contrasting colors.

The wings of the swan, gliding south along the Milky Way, are completed by adding Zeta to the left and Iota and Kappa to the right beyond the ends of the crossbar. In Lyra, Vega forms the top of a small triangle attached to a similarly small parallelogram with Sheliak (SHELL'-YAK, tortoise) and Sulafat (SUE'-LUH-FAT, also meaning tortoise) marking the far edge. Closer to the horizon, Altair and its neighbor Tarazed (TAR'-UH-ZED, "balance") are a distinctive pair flanked by dimmer Alshain. Though it is known as the eagle, the mnemonic stick figure I use to pick out the constellation is more of a manta ray with wings stretched out to Theta and Deneb el Okab ("tail of the eagle") and the end of the tail marked by Althalimain (AL-THAL'-UH-MAIN, "the Ostrich").

With the time of sunset getting earlier until Dec. 9 and then creeping later quite slowly, these constellations will remain visible in the western sky into January. By the beginning of February, the Northern Cross will stand on the western horizon at sunset then the swan will quickly plunge, beak first, into the brightening dusk and re-emerge in the evening skies of early summer.


For help finding your way around the sky or more information, please visit the Adirondack Public Observatory web site at



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