星空故事：第谷新星

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　1572年發生的一件天文「異象」令教廷十分尷尬。那一年，年輕的第谷(1546-1601)看到仙后座附近出現一顆如木星般光亮的「新星」，原來由神主宰的宇宙並非永恒不變。歐洲之前並無類似的天文現象紀錄。

　透過第谷的觀測記錄(圖)，天文學家終在1952年找到這顆新星遺骸的電波訊息。第谷新星原來是一組雙星系統，這個系統由一顆高密度的白矮星(只有地球般大小)和一顆相當於太陽質量的恒星構成。白矮星以強大的引力，不斷吸取同伴的物質來增加自己的質量，當白矮星超越錢德拉塞卡極限時，便產生一次毀滅性的超新星爆炸。因類似的超新星爆炸具相同機制，釋出的亮度非常一致，成為天文學家測量其所在星系距離的線索。

Tycho's Nova

　In 1572 an astronomical event put the Vatican Church in a much-embarrassed situation. That year the young Tycho(1546-1601)saw a "new star", which rivaled Jupiter, in the vicinity of the constellation Cassiopeia. The heaven was not so perfect as God had intended it to be.

　With the help from Tycho's record, astronomers were able to identify his nova(new star)as a radio source in 1952. We now know that Tycho's nova was a common double-star system with an extremely dense white dwarf and a larger star like our Sun. The white dwarf was able to capture materials from its companion with its strong gravitational pull. When the mass of the dwarf reached the 1.4 solar-mass limit, it exploded as a supernova. All similar supernovae produce consistent peak luminosity. They allow astronomers to measure the distance to their host galaxies.　■甄枝強　資深天文愛好者　■圖片鳴謝：NASA/MPIA/Calar Alto Observatory, Oliver Krause et al.