Several years ago, my family and I drove from Tucson to Los Angeles on a clear and moonless summer night. There were few cars on the road that night, few headlights to ruin my night vision. Some hours into the ride, I noticed a diffuse band of light in the sky. Eager to get a better look at it, I stopped the car and turned off the headlights. Now I could see the luminous patchwork far more clearly, stretching across the sky from one point on the horizon to its opposite.

I immediately recognized what I was looking at, but it was no less wondrous just because I could name it. I had seen the Milky Way often, of course, but never under such ideal viewing conditions as on this dark night in the desert, far from the light pollution of any city. From this viewing site, the glow from the countless stars and nebulae of our home galaxy was not merely something that one could see, with a bit of effort. No, it was the first thing one noticed when looking up—the most prominent attraction in the star-studded sky.

Astronomers have learned a great deal about our Milky Way Galaxy, especially during the last century. Hundreds of billions of stars, sprinkled with thousands of star clusters and glowing gaseous clouds, are arrayed in pinwheel fashion throughout a more or less flat disk. Its dimensions are difficult for our minds to grasp. The diameter of the galactic disk is 100,000 light-years. That is, it would take light—travelling at its characteristic speed of 186,000 miles per second—100,000 years to travel from one edge of the disk to the opposite edge. Our solar system (the sun and its entourage of planets and lesser objects in orbit around it) is located about 30,000 light-years from the center, in the suburbs of the Milky Way. The band ...

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