Jimmy Westlake: Spot the Summer Triangle
July 9, 2013
When the summer sun goes down, three of the first stars to peak through the lingering twilight are the trio of bright stars that form the unmistakable asterism called the Summer Triangle, high in the northeastern sky. Asterisms are dot-to-dot drawings made of stars that widely are recognized but are not counted among the 88 official constellations. The Big Dipper is another example of an asterism. In many cases, asterisms like the Summer Triangle and Big Dipper 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 proto-planetary debris 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 in Earthly skies, 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 1,700 times brighter than Vega in our sky and cast distinct shadows at night. Deneb is one of the highest-wattage 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. Altair has a very rapid 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 is the alpha star 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. His "Celestial News" column appears weekly in the Steamboat Today newspaper, and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Check out Westlake's astrophotography website at http://www.jwestlake.com.