Sometimes the hardest things to understand are the things you are most familiar with. We may know our hometowns intimately, yet visitors or young children may still point out things we have never noticed before. They may not be as attuned to all the minutiae, but they often see the big picture better than longtime residents can. A similar situation faces astronomers who study the Milky Way: we are so deeply embedded in our home galaxy that we cannot see it fully. When we look at other galaxies, we can discern their overall layout but not their detailed workings. When we look at our own, we can readily study the details but perceive the overall structure only indirectly.

Consequently, we have been slow to grasp the big picture of the Milky Way's structure and history. Astronomers were not even sure that the galaxy was a distinct object, only one of many billions, until the 1920s. By the mid-1950s they had painstakingly assembled the picture that most people now have of the Milky Way: a majestic pinwheel of stars and gas. In the 1960s theorists proposed that our galaxy formed early in cosmic history--by the most recent estimate, 13 billion years ago--and has remained broadly unchanged ever since.

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