Nebula星云
Historically, the term "nebula" was used to describe any object, light or dark, that appeared as a definite object in the telescopes but lacked the sharp definition of a star or planet. Some were found to be galaxies, but the term is now generally used to describe clouds of interstellar dust or gas. Many are catalogued in the Messier catalog, such as M8, M16, M17 and M20 - these are now known as emission nebulae, glowing clouds of hot interstellar matter. Emission nebulae are some of the most interesting and beautiful objects in the sky. 历史上，'星云'一词用于描述在望远镜中出现为明确物体，但缺乏恒星或行星清晰定义的任何物体，无论是明亮的还是暗淡的。一些被发现是星系，但如今该词一般用于描述星际尘埃或气体云。许多星云被收录在梅西耶星表中，例如M8、M16、M17和M20——这些现在被称为发射星云，即发光的高温星际物质云。发射星云是天空中最有趣和美丽的天体之一。
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Index Galaxy concepts Reference Chaisson & McMillan Ch 18 索引 宇宙概念 参考 Chaisson & McMillan 第18章 | ||||||||||
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