Jimmy Westlake: The summer Milky Way
July 1, 2007
When the bright moon is not in the sky, the dark summer night reveals one of its most spectacular treasures – the soft, misty glow of the “Via Lactea,” or the Milky Way. Contrary to what the ancient Romans believed, the Milky Way is not made of milk, but is the combined light of billions of distant suns, unresolved by the unaided eye.
In China, the Milky Way has been known as the “silver river,” and many Aboriginal peoples know it as “the backbone of the night.” One of my favorites is the Cherokee name that means “the way the dog ran away.” Their legend tells of a mischievous dog that grabbed a bag of cornmeal and ran off to the north, spilling a trail of cornmeal along the way.
If you look through a pair of binoculars and sweep slowly across the Milky Way, you will rediscover what Galileo did in 1610: a multitude of faint stars. The situation is similar to flying high over a sandy beach and seeing the white sand stretch for miles in both directions, and yet you cannot see the individual sand grains that make up the beach.
The Milky Way is really our home galaxy, a spiraling collection of hundreds of billions of stars more or less like our sun. The word “galaxy” literally means “milk,” from the Greek word “galax.” Because it is flattened like a pancake and we peer outward from within that pancake, we see the spiral arms wrapped gently around us in a narrow band that runs almost north to south on summer evenings.
Look closely and you’ll notice that the band of the Milky Way seems to split into two parallel branches just south of the familiar Summer Triangle of stars, Vega, Deneb and Altair. This Great Rift in the Milky Way, as it is known, is caused by an enormous cloud of opaque dust and gas thousands of light years away that blocks our view of the stars beyond.
South of the Great Rift and above the Teapot asterism in Sagittarius is one of the brightest regions of the Milky Way. When we peer off into this direction, we are looking directly toward the center of the Milky Way galaxy, some 28,000 light years away. We cannot view the center directly, though, because of the intervening stars, gas and dust.
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Experiencing the subtle beauty of the Milky Way has become a privilege many Americans will never have. A large percentage of our nation’s population lives in or near a large city where human light pollution drowns out this natural wonder. To them, the Milky Way is something that you read about in a book or see pictures of in a planetarium. Streetlights and spotlights aimed skyward scatter so much unnecessary light into the night sky that it overwhelms all but the brightest stars. Many cities are making efforts to curb this urban light pollution to preserve the beauty of the nighttime sky.
For the time being, at least, here in northwestern Colorado, all we have to do is step out in our backyard to see the galaxy of which we are a part – the majestic Milky Way.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA’s “Astronomy Picture of the Day,” Spaceweather.com, Space.com, Discover.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines.