The Great Summer Triangle

By Dennis Mammana

July 18, 2019 4 min read

Week of July 21-27, 2019

Now that we in the Northern Hemisphere are firmly rooted in summertime, we can see high in the eastern sky after dark one of the most famous star groupings of the season. Stargazers know it as the "Summer Triangle" because ... well, its stars form a huge, prominent, bright triangle that appears in the summer.

The Summer Triangle is not actually a constellation but an asterism — a group of stars that we can trace to resemble something familiar. In fact, each of its three stars — Vega, Deneb and Altair — is the brightest of its own constellation. But that doesn't mean that the asterism has been passed over by early storytellers.

For example, in an ancient Chinese love tale, Deneb represents a magpie bridge over the Milky Way, which allows the separated lovers — represented by Altair and Vega — to be reunited on only one night each year.

The brightest of the stellar trio — and nearly overhead after dark — is Vega, the most prominent star in the tiny constellation of Lyra, the harp. This brilliant white star has a diameter and mass some three times greater than our sun and produces about 50 times more power than our star. As a result, astronomers believe that Vega will likely exhaust its fuel in only one-tenth the time, making its expected life span only about one billion years.

Movie buffs may remember this star as the one from which radio astronomers detected intelligent signals in the fictional 1997 motion picture "Contact." I guess that would make Vega — heh-heh — a movie star!

The southeasternmost of the three stars is Altair, the brightest in the constellation of Aquila, the eagle. Its name comes from the Arabic Al Nasr al Tair, meaning in English "the flying eagle." Altair lies about 98 trillion miles (16.7 light-years) from us. In other words, its light has been traveling through space for nearly 17 years, and its photons of light that strike our eyes tonight have been traveling through space since the summer of 2002.

Not only is Altair one of the nearest stars in our sky, it's also one of the most rapidly rotating. Astronomers have measured that this white star spins once every 6.5 hours, completing nearly four rotations for every one made by our own planet. As a result of this rapid spin, Altair is most likely so flattened by centrifugal force that it would appear more like an egg than a sphere.

Finally, farthest to the north lies Deneb, located in the tail of the great swan, Cygnus. Deneb lies some 8,200 trillion miles (about 1,400 light-years) from us — so far that we see it as it appeared in the seventh century. The light that leaves the star tonight won't arrive here until around the year 3419.

Deneb is not only the brightest star in Cygnus, it's one of the most luminous in our entire galaxy. From its great distance, Deneb shines with a luminosity equivalent to more than 261,000 suns. In fact, it generates more light in just one day than our sun has since the days of Marco Polo at the end of the 13th century!

 Summertime has brought the "Summer Triangle" to the eastern sky.
Summertime has brought the "Summer Triangle" to the eastern sky.

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