When the summer sun goes down, three of the first stars to peep through the lingering twilight are the bright stars that form the unmistakable asterism called the Summer Triangle.
By 9:3 0p.m., you should be able to pick out the three stars of the Summer Triangle about halfway up to the zenith in the eastern sky before the myriad other summer stars reveal themselves.
Asterisms are dot-to-dot drawings made of stars that are widely recognized but not counted among the 88 official constellations. The Big Dipper is another example of an asterism. In many cases, asterisms such as the Summer Triangle and the Big Dipper are easier to recognize than the official star patterns to which they belong.
The brightest star in the Summer Triangle, and the closest to the zenith, 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 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 rocky debris surrounding it — possibly a family of planets in the process of formation.
The second star in the Summer Triangle is its faintest member, the blue supergiant star named Deneb, meaning “the tail of the swan.” Though Deneb shines nearly as brightly as Vega in Earthly skies, it does so from a distance of 1,500 light years away. If Deneb were moved to the same distance from Earth as Vega, it would shine 1,700 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 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 rapid rotation, spinning once on its axis in only 8.9 hours. By comparison, our sun takes 28 days to spin once. This rapid motion causes Altair’s equator to bulge outward, making it 20 percent fatter around its middle.
Each of the stars in the Summer Triangle is the alpha star of its own 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 Steamboat Today. Check out Weslake’s astrophotography website at jwestlake.com.