Joanne Williams, age 13, of Salt Lake City, Utah, for her question:

How big is the Milky Way Galaxy?

On a clear starry night, we get a view of the great Galaxy from the planet Earth. It looks like ragged scarves of pale, pale light arching over the sky. But this view does not give us a fair picture of the real thing. Our Milky Way Galaxy is an enormous cartwheel of about a hundred billion stars    one of which is our starry sun. We are far from the crowded center and our view looks out across the big, flat wheel.

Obviously a galaxy of 100 billion to 200 billion widely separated stars must be enormous and we need something bigger than miles to measure it. We can use the light year, which equals about six million million earth miles. Astronomers estimate that the diameter of the Big Wheel is about 100,000 light years. Its thickness through the center is about 20,000 light years. We cannot travel at the speed of light, which is 186,282 miles per second. But even if we could, it would take us 100,000 years to cross the Galaxy from side to side.