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圖為「八裂星雲」是一個位於南天的行星狀星雲，星雲中央較暗的恒星就是噴出雲氣物質的白矮星。行星狀星雲實際上與行星無關，只因當初這類星體被發現時，看起來像遙遠行星般呈細小的面積，因而得名。當一顆紅巨星把它的外層大氣向外噴射，便形成行星狀星雲，而核心則只剩下如地球般大小的星核—一顆白矮星。噴射出去的雲氣物質不斷向外膨脹，與星際物質融合，所以與恒星上億年的壽命相比，行星狀星雲只是短短數萬年的過眼雲煙。百分之九十五的恒星臨至生命終結時，均會經歷行星狀星雲階段。50億年後，我們的太陽也不會例外，到時如果地球仍存在的話（有部分理論指出地球早於太陽的紅巨星階段滅亡），人類或可目睹小如金星的太陽從東方升起，並放射出相當於100個現今太陽總和的光芒。
A dying sun
The above image is a planetary nebula commonly called the "Eight-Burst Nebula". The fainter star near the centre is actually the white dwarf that has ejected the gaseous material. A planetary nebula is formed when a red giant star ejects its outer layers as clouds of luminescent gas, revealing the dense, hot, and tiny white dwarf star at its core. Planetary nebulae come in many shapes, from rings to elegant goblets. Their complex structures yield clues about the final days of a star's life. 95% of all stars that we see in our own galaxy, the Milky Way, will ultimately become planetary nebulae. This includes our Sun. If the earth still survives 5 billion years from now, the human race might be able to witness the remnant Sun no larger than Venus but more brilliant than 100 present Suns put together. ■甄枝強 資深天文愛好者 圖片來源：Hubble Heritage
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