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A so-called "planetary nebula" is a confusing misnomer because these clouds have nothing to do with planets. As a medium-size star nears the end, it puffs out into a red giant — about 100 to 1,000 times its original size — eventually engulfing the space around it, including any nearby worlds.
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人 民 网 版 权 所 有 ，未 经 书 面 授 权 禁 止 使 用
Tony Jolliffe BBC
Each puzzle features 16 words and each grouping of words is split into four categories. These sets could comprise of anything from book titles, software, country names, etc. Even though multiple words will seem like they fit together, there's only one correct answer.