On a clear, dark night, far from the glow of city lights, we can see a hazy, luminous band stretching across the sky: a thin river of light called the Milky Way.

Not until Galileo's day did we begin to understand the nature of the Milky Way. What looked like a diaphanous band of light to the naked eye became myriad individual stars when seen through a telescope.

Today, we recognize the Milky Way as our home galaxy, a vast continent of perhaps 400 billion stars—some in star clusters—with nebulae and interstellar dust and gas, all bound together by gravity. Nearly everything visible to the naked eye is within the Milky Way.