The summer triangle consists of Deneb in Cygnus the Swan, Altair in Aquila the Eagle, and bluish Vega in Lyra the Harp.
1. Look for Cygnus the Swan, the Northern Cross. Deneb is the tail of the swan, or the top of the cross (as the long end of the cross represents the swan's outstretched neck).
2. Look for Vega, the second brightest star in the northern hemisphere, closely rivaling Arcturus. Vega soars almost directly overhead in summer, while the bright stars of winter nights are hidden almost directly beneath our feet. Look for a small parallelogram of stars near Vega which forms the frame of the harp.
3. Altair in Aquila.
These three stars make an almost perfect right triangle. Use the summer triangle to find your way around the summer sky!
As explained in Starstruck Tonight:
Going back to the Big Dipper, trace away from it above the open bowl. This line runs to Deneb, the tail of the constellation Cygnus the Swan. With wings abreast, and long neck outstretched, Cygnus flies along the milky river. One legend relates that the swan was the hero Orpheus, who enchanted all who heard him with his magic harp. Cygnus is also known as the Northern Cross, and around Christmas Eve at sunset it stands upright on the northwest horizon.
Deneb (DEN-ebb; the tail of the swan) is a bluish-white supergiant, one of the most luminous stars known. Because it is so far away, Deneb is only the 20th brightest star in the sky. But if Deneb were as near to us as Sirius, which we see as the brightest star, then it would shine as brightly as the Moon. If Deneb were as close to us as Alpha-Centauri, we could read by its light.
Albireo (al-BEER-ee-oh), the beak of the swan, is one of the most beautiful of all double stars. In close proximity, one of the star-pair shines with a brilliant gold, and the other with a sapphire blue.
A small, massive object orbits a giant blue star near the center of Cygnus. Discovered in 1965 and known as Cygnus X-1, it cannot be seen with optical telescopes, but it emits intense, flickering x-rays. Most astronomers believe that Cygnus X-1 is a black hole. Matter from the bluish companion star spirals down toward the black hole, emitting x-rays as it reaches the boundary and disappears inside the black hole forever.
Deneb and two other bright stars form the "summer triangle," an asterism found high overhead all summer long amid the splendid sweep of the Milky Way.
The bright star Altair (ALL-tare) lies in the constellation Aquila the Eagle, which was a servant of Zeus.
Altair means "the flying one," and Altair flies around its axis once every 6 and a half hours. Astronomers calculate that because of this rapid rotation, it must be twice as wide at its equator as at its poles.
The summer triangle consists of Deneb... Altair... and bluish Vega. Vega is the second brightest star in the northern hemisphere, closely rivaling Arcturus. Vega means Swooping Eagle in Arabic. It soars almost directly overhead in summer, while the bright stars of winter nights are hidden almost directly beneath our feet.
Vega is in the constellation Lyra the Harp. This is the lyre which belonged to Orpheus according to the Greeks, or to King Arthur according to English legend. Look for a small parallelogram of stars near Vega which forms the frame of the harp.
Shakespeare tells us that when Orpheus would play his lyre:
everything that heard him play,
even the billows of the sea,
hung their heads, and then lay by.