Jimmy Westlake: Magnificent summer Milky Way
July 16, 2012
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.
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. In this case, the grains of sand are the stars that populate the Milky Way. Our sun is but one grain of sand on this cosmic beach.
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.
The Milky Way is 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 Milky Way 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 is caused by enormous clouds of opaque dust thousands of light years away that eclipse our view of the stars beyond.
South of the Great Rift and above the spout of the teapot asterism of 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 because of the intervening stars, gas and dust, but there is mounting evidence that a supermassive black hole lurks at the heart of our galaxy.
Experiencing the subtle beauty of the Milky Way has become a privilege that many Americans might never have. A large portion 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 go to a planetarium to see. 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 Northwest Colorado all we have to do is step out into 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. Check out his website at http://www.jwestlake.com.
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