Jimmy Westlake: The Summer Triangle
June 11, 2012
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 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. Although it's called the Summer Triangle, it first becomes visible in the early evening during late spring and hangs on in our evening sky until early winter.
Asterisms are dot-to-dot drawings in the sky that are widely known but are not counted among the 88 official constellations. The Big Dipper is another well-known example of an asterism. In many cases, asterisms like the Summer Triangle are easier to recognize than the official star patterns.
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 the late astronomer Carl Sagan chose it as the source of the first extraterrestrial radio signal detected by 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 shines nearly as bright as Vega, it does so from a distance of 1,500 light years. If Deneb were the same distance from Earth as Vega is, it would shine more than 1,700 times brighter than Vega in our sky 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 southern-most corner, is 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. Altair has a very fast rotation, spinning once on its axis in only 8.9 hours. For comparison, our sun takes 28 days to spin once. This rapid motion causes Altair's equator to bulge outward by 20 percent of its diameter.
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 the alpha star of Aquila the Eagle. You can use the Summer Triangle asterism as a jumping-off point to locate many other stars and constellations in the summer sky.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out his astrophotography website at http://www.jwestlake.com.
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