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1764年，法國天文學家查理士．梅西爾在狐狸座發現了一個細小而暗淡的天體，這個星體異於他以前所觀測過的星雲或星團。1779年，天文學家又在天琴座找到另一個更細小的天體。早期的觀測者對這兩個天體的屬性茫無頭緒，只能以「暗淡的行星」作出形容。直至1784 年，英國天文學家威廉．赫歇爾把這種天體歸類為「行星狀星雲」，這名稱一直沿用至今。行星狀星雲其實與行星無任何關係，相反，它是細小質量恆星在生命結束前的謝幕「秀」，整個星雲原本是恆星的外層大氣，當一顆如太陽大小的恆星演化至紅巨星階段時（太陽還須等50億年），會產生強大的脈動，把外層氣殼從兩個相反的方向拋射出去，成為行星狀星雲，留下的恆星核則演化成白矮星。
Planetary nebulae
In 1764 French astronomer, Charles Messier, found a faint, tiny object in the constellation Vulpecula. This object was unlike any he had observed before. In 1779, another tiny object was discovered. Astronomers in those days could not understand the nature of these objects. British astronomer, William Herschel, finally catalogued such objects as ‘planetary nebulae’ in 1784. Planetary nebulae have nothing to do with planets. In fact they were the firework displays of dying dwarf stars. When a low mass star evolves to become a red giant, it blows off its outer atmosphere in opposite directions. This shell of gases expands and becomes visible to us as a planetary nebula. The stellar core that is left behind evolves to become a small white dwarf. ■甄枝強 資深天文愛好者
Image credit: Ring Nebula: NASA/Hubble; Dumbell Nebula: ESA
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