The constellations shown on modern star atlases are all officially approved by the International Astronomical Union, but while constellations are official, asterisms are not. An asterism is often defined as a noteworthy or striking pattern of stars within a constellation, but that is not always the case. The larger asterisms — ones like the Big Dipper in Ursa Major and the Great Square of Pegasus — are often better known than their host constellations.
In a few cases, we have asterisms which are composed from more than one constellation. One such example is now evident low in our east-northeast sky as darkness falls this week. It is, in fact, the easiest pattern to visualize because it's composed of just three stars, but they are first-magnitude stars, and each is the brightest star in its own constellation.
The star pattern in question is known as the "Summer Triangle."
Looks can be deceiving
The brightest is the bluish-white star Vega, in the constellation Lyra, the lyre. Next in brightness is yellow-white Altair in Aquila, the eagle. Finally, there is white Deneb, in the constellation Cygnus, the swan. From our earthly viewpoint, Vega appears twice as bright as Altair and more than three times brighter than Deneb. But as the English idiom "don't judge a book by its cover" cautions: sometimes things are not always what they seem to be. We know, for instance, that Vega clearly is more luminous compared to Altair, because it's situated at a greater distance from us. Altair is 16.7 light-years away, while Vega is 25. Vega is 40 times more luminous than the sun, while Altair is only 11 times as lustrous.
But Vega actually pales in comparison with Deneb, one of the greatest supergiant stars known. Deneb's distance measures 1,550 light-years from Earth with a luminosity recently computed to be an incredible 196,000 times that of our sun.
If we could swap Vega's placement relative to us with Deneb, Vega would be visible only with a moderately large telescope. But seen from Vega's distance, Deneb would appear to shine about 100 times brighter than Venus; visible by day and casting distinct shadows at night. But because its light takes more than 15 centuries to reach us, Deneb merely appears as a fairly conspicuous but by no means particularly preeminent star.
Moniker and branding issues
Some believe that the Summer Triangle would be better served being associated with the autumn season as opposed to summer. On the day of the autumnal equinox, the Triangle can be found directly overhead at dusk compared to where we see it now low in the east-northeast. The difference is that the Triangle is visible all night long from now through mid-August. While admittedly in an excellent position right after sunset by late September, at that time of the year the Triangle is dropping out of sight by around 3 a.m. local time. Hence, we associate it with balmy summer nights when it always is in view from dusk until dawn.
There has always been some debate as to who first christened the bright stellar trio of Vega, Deneb and Altair as the Summer Triangle. Some say that the legendary British astronomer Sir Patrick Moore, was the first to coin the term. In his book "Fireside Astronomy" (John Wiley & Sons, 1992) Moore wrote: "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 1952, in his very popular sky guide, "The Stars — A New Way to See Them," Hans Augusto Rey (who was also the creator of the mischievous little monkey, Curious George), also used the Summer Triangle moniker. In addition, on page 54, Rey notes that: "Altair, Vega and Deneb form a huge right triangle, known to all navigators."
There could be some merit in Rey's comment, since 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.
Reappearance of the Milky Way
The appearance of the Summer Triangle also signals the return of the summer Milky Way. During the spring, the Milky Way runs all around the horizon, hidden in low-lying haze. But now as the Summer Triangle gradually ascends the eastern sky in the coming days and weeks, we'll begin to see one of the brightest sections of the Milky Way, cutting almost straight across the Triangle.
Perhaps the most striking visual feature of all here is the great dark rift, which appears to split the galaxy in two with the Triangle. Beginning near Deneb in Cygnus and stretching its way through Aquila above Altair, this obscuring lane creates two parallel branches or streams of stars angling southwestward down the sky.
It is not, however, a region devoid of stars but a tremendous band of dust and other obscuring matter that lies between Earth and the center of our Milky Way galaxy.
Our next "guest star?"
A final point concerning the Summer Triangle, of interest to visual and photographic observers: many naked-eye novas, or star explosions, have appeared in this part of the Milky Way in the past, and no doubt others will occur in the future.
The last one to appear was in August 1975 in Cygnus, not far from Deneb and appearing nearly as bright. Also, as has frequently been pointed out by professional astronomers, we are long overdue for another supernova — a massive star near the end of its life that literally blows itself apart. The last to appear in our galaxy was Kepler's Star in 1604; no doubt we are long overdue to see another. Such a cosmic spectacular might well burst in this part of the sky, bringing instant and certain fame to its discoverer.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.