Star patterns that act as street maps to summer sky
Step out into the woods near a pond or lake on a summer night, and critters large and small sound from all directions. Discord reigns.
But listen harder, and distinct sounds prevail from the high-pitched calling of a tree frog, to the near-silent flutter of a bat, or the distant "neighing" of an eastern screech owl.
The countless stars that canopy the sky on a clear night likewise present a visible confusion. How to bring order to the myriad twinkling?
For stellar neophytes, just look east-southeast after 10 p.m. You'll see two configurations as distinct to the eye as a bullfrog is to the ear. Each is a street map to the summer sky in northern latitudes.
First, and most famous, is the great triangle of summer, three stars so bright that each is easily visible from Manhattan's Central Park on a clear night. Connect the three with imaginary lines and they become an isosceles triangle on its side pointing southward. Vega passes almost directly overhead. Deneb (pronounced DEN-ab) is below and slightly to the left of Vega. Altair is to the right (south) of both Vega and Deneb.
When we are away from city lights among the tens of thousands of stars visible, these three are the brightest in their constellations: Vega in Lyra (the Lyre), Deneb in Cygnus (the Swan), and Altair in Aquila (the Eagle). Even though Vega appears brighter, it can't hold a candle to Deneb, one of the brightest, yet most distant, stars visible to the naked eye in the Milky Way. Deneb is 3,200 light-years from Earth (one light-year is 6 trillion miles). A blue super giant, it has mass in excess of 30 suns. Its circumference is almost 200 times that of the sun, and the nuclear explosions occurring at its core give it a luminosity 300,000 times as bright as our sun. Its incredible distance diminishes its true brightness.
Vega and Altair, on the other hand, are "only" 25.3 and 16.8 light-years, respectively, from Earth. Astronomically speaking, they are next-door neighbors, two of the closer stars to our solar system. ("Close" is a relative term. The American space probe Pioneer 11, which left Earth in 1973 and continues in the direction of Altair, will pass its sparkling white radiation in about 1 million years.) And since Vega is 50 times as luminous as our sun, one has to look closely to see the four other stars that give a diamond shape to the Lyre.
What is truly wonderful about this triangle in the sky is how it gives a mental leg up on the mathematics of very large numbers. Even though Deneb has a diameter of 280 million kilometers (174 million miles) it appears smaller than Vega, with a diameter of 4 million kilometers (2.5 million miles), and Altair, with a diameter of 2 million kilometers (1.2 million miles). Distance, size, luminosity simple yet profound wonders of the night sky. Just draw a triangle and begin the cosmic geometry lesson.
The second great marker of the summer sky is the constellation Cygnus. Each night, Cygnus flies south with the main current of the Milky Way. The star to fix on to see the swan is Deneb of the great summer triangle.
The ancients were predisposed to view Deneb as the feathered tail of a swan (Deneb is from the Arabic word for tail). The swan's long neck stretches (southward) to the star Albireo, which forms its beak. Between Deneb and Albireo and perpendicular to them, three more stars, Delta Cygni, Sadr, and Gienah, form the swan's wingspan.
On a clear night, with good binoculars, you can see that Albireo, some 400 light-years away, with a luminosity of 700 suns, is a double star. But come early winter, the swan dives into the northwest horizon appearing perpendicular from land or sea. Since the age of sail, Deneb was transformed from a feathery tail to the top point of the cross, the Northern Cross. The latter image has always been more compelling to mariners on the treacherous North Atlantic.