十万个为什么·科学启蒙30篇（3）
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What Is the Milky Way?
银河系是什么
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The Milky Way is our home galaxy—a vast, rotating disk of over 100 billion stars.
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It looks like a hazy band of light across the night sky because we see it edge-on from inside.
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Our Solar System sits about 27,000 light-years from the galaxy’s dense, star-packed center.
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Most stars, including the Sun, orbit the galactic center once every 225–250 million years.
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A supermassive black hole named Sagittarius A* lives at the very heart of the Milky Way.
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Dark matter makes up about 90% of the galaxy’s total mass, though we cannot see it directly.
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Spiral arms—like the Orion Arm where Earth resides—contain young stars and glowing gas clouds.
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The Milky Way is slowly merging with the smaller Andromeda Galaxy, a process that will take billions of years.
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Radio telescopes help map its structure by detecting invisible hydrogen gas emissions.
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From outside, astronomers believe it would look like a bright white spiral with soft blue arms.