It may be October and we’re well into the autumn season, but during the early evening hours this month, the famous Summer Triangle will appear almost directly overhead. Although its name suggests that it’s a star pattern most favorably situated during the warm summer months, that is not entirely true. In fact, as late as mid-November, stargazers can still see the Triangle nearly overhead and high in the western night sky.
There is some debate as to who first christened the bright stellar trio of Vega, Deneb, and Altair as the Summer Triangle. Some say that the late legendary British astronomer, Sir Patrick Moore (1923-2012), was the first to coin the term. Noted Moore, “During a 1958 television broadcast on the BBC, I introduced the nickname of the Summer Triangle and everyone now seems to use the term, even though it is completely unofficial.” However, several years earlier, in 1954, in his popular sky guide Find the Constellations, H.A. Rey (1898-1977) also used the Summer Triangle moniker.
Still others believe that the term was popularized through U.S. navigator training manuals during World War II. Interestingly, Romanian astronomer, Oswald Thomas (1882-1963), described Vega, Altair, and Deneb as Grosses Dreieck (Great Triangle) in the late 1920s and Sommerliches Dreieck (Summerly Triangle) in 1934.
Brightest of the three stars of the Triangle is the bluish-white Vega, in the constellation of Lyra, the Lyre. Next in brightness is the yellowish-white Altair in Aquila, the Eagle. Finally, there is white Deneb in Cygnus, the Swan. To our eyes, Vega appears twice as bright as Altair and more than three times brighter than Deneb. Apparent brightness can be deceiving, however, and in terms of actual luminosity a different tale is told: brilliant Vega is 48 times brighter than our Sun and 25 light years away. Altair is 12 times brighter than the Sun and is one of our nearer neighbors at just 17 light years away.
But Deneb is one of the greatest supergiant stars known. Its luminosity is nearly 80,000 times that of the Sun, yet at 1,467 light years away it merely appears to us as a fairly conspicuous– but by no means particularly notable — star. But if it were somehow possible to move Deneb to Vega’s distance from us, it would then appear to shine 16 times brighter than Venus, capable of casting distinct shadows and even be visible in the daytime.
So bundle up, and head outside, approximately 90 minutes after sunset, and look overhead for the constellation with the summer name!
Photo: “Summer Triangle” by Jim Thomas – Licensed under GPL via Wikimedia Commons
Joe Rao is an esteemed astronomer who writes for Space.com, Sky & Telescope, and Natural History Magazine. Mr. Rao is a regular contributor to the Farmers' Almanac and serves as an associate lecturer for the Hayden Planetarium in New York City.