Occasionally some well-meaning public radio program will try to guide listeners to a particular constellation. Look straight up, says a reassuring voice, and you’ll notice a triangle of stars.
The obvious problem with such simplistic directions is that they lead to a multitude of destinations. One can readily construct triangles-- or squares or octagons, for that matter--almost anywhere in the firmament. The sky is an infinite Rorschach playground for connect-the-dots fanatics, and September is a perfect time to join in the fun. Autumn features the heavens’ two most famous geometric shapes: the Great Square of Pegasus, in full view toward the east, and the Great Summer Triangle, overhead and to the west. Both are great as in great big chunks of celestial real estate; Orion could easily nestle within the summer triangle.
This particular triangle isn’t even a constellation. It’s an asterism--made up of stars borrowed from three separate groupings. Nor, despite its name, is it primarily a summertime phenomenon: it’s still on the rise in August and doesn’t reach its highest nightfall position until September and October. The summer triangle is indeed, however, a triangle, forged from the brightest overhead stars--all interesting white suns surrounded by cosmic delicacies.
The highest and brightest is Vega, in the constellation Lyra. Vega’s remoteness was revised earlier this year by the Hipparchos satellite from 26.5 light-years down to 25.3. A light-year or so notwithstanding, magnitude-zero Vega is the brightest star of autumn. Only Jupiter, lowish in the south, handily outshines it. Orange Arcturus, a spring star now sinking into the west, edges Vega out by a margin so small the eye has trouble deciding which is really the winner. Point a telescope just 7 degrees southeast of Vega to reveal more treasures: the ghostly glowing noose of the famous Ring nebula.
In the constellation Aquila lies Altair, the triangle’s second- brightest star. Altair, the fourth-nearest bright star to Earth, can be identified by two dim stars that hover like bodyguards on either side of it. Its distance, too, has been tweaked by Hipparchos, to just 16.8 light- years.
Finally there’s Deneb, the leftmost member of the triad. Deneb appears the dimmest but gains respect when we consider its awesome distance of 1,500 light-years. If Deneb were among the nearer stars, we would never have true darkness: our sky would be a deep, murky blue. Deneb is tied with Orion’s foot star, Rigel, as the most luminous bright star in the heavens-- a white supergiant, more dazzling than the famous red variety. Blazing with the light of 60,000 suns, it is the farthest first-magnitude star in the heavens, a lighthouse of the galaxy.
Cygnus (the Swan), its parent constellation, spills toward the right from Deneb. Two stars away from Deneb, in the middle of the swan’s long neck, sits a faint star (you can see it with binoculars) named hde 226868, which orbits one of the galaxy’s surest black holes. From this spot, a concentrated beam of X-rays streams down toward us. Then, one more star hop right, to the neck’s end, and you’ve reached Albireo, the night’s loveliest double star. This medium-bright star is easy to find since it sits about halfway between Vega and Altair in the rich, sugary overhead region of the Milky Way. Any magnification shows it as a pastel feast of golden yellow and blue--two multicolored suns separated by 100 times the radius of our solar system.
Now we know why the triangle deserves its name. The treasures within its boundaries are nothing short of great.