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The Boston Globe

Metro

star watch

Three star-birds circle on clear September nights

As summer nears its end, the starry Summer Triangle completes its months-long climb to the summit of the sky and floats sprawling across the zenith after dark.

The Summer Triangle consists of Vega, Deneb, and Altair, three of the brightest distant suns shining over the Earth. Face south after nightfall and look up. The first you will spot is Altair. Now crane your neck. Brighter Vega and slightly less-bright Deneb are nearly straight up, with Deneb left of Vega. The triangle they make is three or four fists tall at arm’s length.

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All three stars represent bird constellations, or parts of birds, circling in late summer. With a little imagination you can see for yourself.

Deneb (tail in Arabic) is at the tail of big Cygnus the Swan, flying with wings outstretched and his long neck extending far into the Triangle. His brightest stars, the inner ones of his pattern, are also known as the Northern Cross.

Altair is the brightest star of Aquila the Eagle. In one version it is his bright eye.

Vega shines in Lyra, the Lyre, a constellation the ancient Greeks invented relatively late. Before Vega and its surrounding stars became a musical instrument, they were a bird of prey.

The three birds date to the cultures of Mesopotamia, about as far back as any Western legends can be traced. The names Vega and Altair come from there by way of Bedouin cultures in Arabia and North Africa. The names are medieval European corruptions of the Arabic names “Diving Eagle” and “Soaring Eagle.”

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The Diving Eagle had Vega as its body and the two faint stars next to it as its wings in a V. It is diving to the west. Whether you can see the wing stars depends how much light pollution you have to look through.

The Soaring Eagle, a bit larger, consists of Altair and two star-wings outstretched to its upper right and lower left. It is banking in a turn.

By comparison, the Swan is the size of a pterodactyl. Or maybe it is just not as high.

And the stars? Altair is a fast-spinning oblong globe bigger and hotter than our sun and 17 light-years distant. Vega is another big, hot, fast spinner, 25 light-years away. Deneb is something else: a vastly larger white supergiant lighting up our part of the Milky Way from a distance of about 1,500 light-years. Picture Altair and Vega as fireflies 17 and 25 yards from you in the night, and Deneb would be a porch light bulb nearly a mile off.

There is another creature here, incongruous among birds: Delphinus, the little leaping Dolphin. Look for it in a dark sky upper left of Altair by a little more than a fist at arm’s length.

Revealing illusions

The stars are scattered as randomly as dust in the wind. But humans are pattern makers and storytellers, seeing meaning everywhere. We find animals and faces in clouds, the green man in forest foliage, and monsters in ink-blot tests. So every culture worldwide has grouped the stars into constellations and made up stories to explain them.

The interesting thing about star-stories is that the raw material — unlike clouds or wall stains or tea leaves — has remained changeless in plain view to everyone for tens of thousands of years. So sky stories can be extremely old. There is evidence, admittedly weak, that at least a few of our modern constellations are prehistoric: predating the invention of writing and civilization by unknown ages.

Anyone can make up their own constellations with their own stories. Amateur astronomers do this all the time with favorite bits of the sky and especially with the small, faint patterns richly available in binoculars and telescopes. “Asterisms,” these unofficial constellations are called.

There are websites devoted to people sharing asterisms — the Golf Putter, the Swoosh, the Flying Minnow, Smiley Face, the Klingon Battlecruiser, the Broken Engagement Ring, Napoleon’s Hat, Kemble’s Cascade.

Most of them live and die with their inventors. A few become popular and stick. The Summer Triangle took hold when the BBC’s “The Sky at Night” radio program popularized it in the late 1950s, and it shows no sign of going away.

If people like them, asterisms can last for thousands of years.

Whole-sky maps

Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/gettingstarted.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.

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