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今天讓我們認清組成「大三角」3顆亮星的真正身份。如果我們把大三角看作一個直角三角形，天琴座的主星織女星，便是構成直角的亮星，也是全天第5顆最亮的恒星，它距地球約25.3光年。此外，織女星的質量為太陽的2.5倍，每秒所釋出的能量卻比太陽強50倍。基於歲差現象，織女星將於公元14,000年左右取代現今北極星的地位。
大三角的尖端是牛郎星，距離地球僅16.3光年，表面溫度比太陽的6,000度高出約1,000度，在眾亮星中排行第12。
天津四不及前二者亮，但實際情況卻剛好相反，根據最新測定，天津四離地球約1,550光年，是最遠的一等亮星。不止如此，天津四的直徑更相當於太陽直徑的200至300倍，是最巨大星之一。
The Summer Triangle
The Summer Triangle resembles a right-angle triangle. Vega is the right-angle star. Being 2.5 times more massive than the Sun, it gives off 50 times as much energy. At a distance of only 25.3 light years, no wonder why Vega ranks 7th in the bright star catologue. In 14,000 A.D., Vega will replace Polaris as our Pole Star.
At the tip of the triangle is Altair. While not as massive as Vega, it is still hotter than the Sun. At 16.3 light years, it comes 12th in the bright star ranking.
Deneb is the least bright of the three but you would be surprised to know the truth. At an immense distance of 1,550 light years, it is actually the most distant first magnitude star. Deneb is also huge. It has a diameter almost 200 to 300 times that of the Sun, making it one of the biggest stars known.
■甄枝強 資深天文愛好者
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