Cam Bragg, age 10, of Oak Hill, West Virginia, for her question:
How many miles is the Milky Way from end to end?
Things up there in the sky can fool you and what you think you see is not always so. For example, take our view of the Milky Way. It looks for all the world like a foggy white arch over the sky. Surely astronomers should be able to measure the long, looping line from end to end. Or so you would think. But actually, the Milky Way is not shaped like a line or a loop at all.
For thousands of years, our star gazing ancestors were mystified by the Milky Way. They believed their eyes and thought that it really is a hazy white line arching over the sky. Then in 1609, the great astronomer Galileo trained his small telescope on its ghostly glow. For the first time, an earthling saw that the glimmering arch is really a host of stars. They are so crowded and far away that their brilliant lights blend together in a hazy white fog.
Later astronomers learned more about it and gradually pieced the whole thing together. And that long line of shaggy white fog certainly fools the human eye. We now know for sure that the Milky Way is an enormous wheel of at least 100 billion starry suns. From the earth we get an edgewise view across the crowded hub of the Big Wheel and we see only half of it at a time.
A circle, of course, has no ends so there is no way to measure the wheeling Milky Way from end to end. But astronomers have measured its diameter, which is a straight line from side to side through the center. They estimate this enormous distance to be about 100,000 light years. One light year, as we know, is the distance a beam of light travels in one earth year and the speed of light is 186,000 miles per second. A light year equals about six million million earth miles, or a figure six with a tail of twelve zeros.
From these figures we can estimate the stupendous distance across the Milky Way in mere earth miles. We simply multiply the six million million miles of a light year by the 100,000 light years across the Big Wheel. No space ship could make the trip in 100,000 years, because we cannot imagine one capable of reaching the speed of light. But suppose we invented one to travel at a million miles per hour. It would be seven times slower than light. But in 700,000 years it might be able to cross the Milky Way, traveling non stop at full speed.
Our earth is out toward the rim of the Big Wheel and we get a tilted view across it. The crowded hub is beyond our summer constellation Sagittarius, hidden by dusky cosmic clouds. The distance from here to the center is estimated to be about 30,000 light years. In earth miles, this is roughly 36 plus 16 zeros.