Jimmy Westlake: The summer triangle

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There are few sights in nature more beautiful than the starry summer sky. When the sun goes down and the summer stars come out, three of the first ones you'll see, high in the northeastern sky, will be the trio of bright stars that forms the corners of an unmistakable asterism called the Summer Triangle.

Asterisms are dot-to-dot drawings in the sky that are widely known but are not counted among the 88 official constellations. In many cases, asterisms are easier to recognize than the official star patterns.

The Summer Triangle is a case in point.

The brightest star in the Summer Triangle and the first to rise is Vega, named for "the plunging vulture." At a distance of only 25 light years, Vega is among the closest stars to our solar system. Vega became a real "movie star" in 1997 when astronomer Carl Sagan chose it as the source of the first extraterrestrial signal detected by radio astronomers on Earth in his fictional book and movie "Contact." In real life, Vega was one of the first stars discovered to have a ring of planetary material surrounding it, possibly a family of planets in the process of formation.

The second star to rise in the Summer Triangle is its faintest member, the blue supergiant star named Deneb, meaning "the tail of the swan." Although Deneb appears nearly as bright as Vega, it does so from a distance of 1,500 light years away. If Deneb were moved in to the same distance from Earth as Vega, it would shine over 1,700 times brighter than Vega and cast distinct shadows at night. Deneb is one of the most luminous stars known to astronomers.

Finally, the third member of the Summer Triangle, marking its southernmost corner, is the star named Altair, which means "the flying eagle." Altair is the closest of the three stars in the Summer Triangle, lying at a distance of only 17 light years.

Each of the stars in the Summer Triangle falls in a different constellation: Vega is the brightest star in Lyra the Harp, Deneb belongs to Cygnus the Swan, and Altair is part of Aquila the Eagle. The fact is, this region of the sky is filled with constellations representing flying creatures of one kind or another.

You can use the Summer Triangle as a guide to locate many of the official constellations of summer. We'll learn to do that in another Cosmic Moment.

Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA's "Astronomy Picture of the Day," Spaceweather.com, Space.com, Discover.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines.

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