HUMANS have always been captivated by the Milky Way. To the ancient Greeks, it was a squirt of breast milk, while the Mayan farmers of Mesoamerica saw it as a growing maize stalk. Today, astronomers understand that what we see when we gaze up at the hazy streak of light stretching across the night sky is actually just one part of our host galaxy-one of four spiral arms emanating from an enormous disc of dust and gas that together contain at least 100 billion stars.
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