摘要或動機

By comparing eight different epoch images of the crab nebula taken through 1942 to 2004, we have calculated the expansion velocity of 27 optical bubble features and 60 filaments. The mean expansion velocity of bubble features and filaments is 0.173 arcsec/yr and 0.15 arcsec/yr, respectively. We also estimated the maximum radial velocity of the expansion by analyzing the emission spectrum of the nebula. The maximum radial velocity is 1385.5 km/s. Combining these measurements indicates that the crab nebular is approximately 5870 light year away. In addition, if we assume that the nebula has been expanding at a constant rate, our expansion velocity projected backward indicates the mean date of the supernova event as A.D 1124, more than 70 yrs later than the accepted date of 1054. The result confirms the well-known acceleration in the crab's expansion. Although we have analyzed eight images with a 62 yr baseline, the acceleration still can't be derived from this study. 透過量測由1942年到2004年之間八張不同年代的蟹狀星雲中爆炸後殘骸的位置變化，可以計算出蟹狀星雲爆發的擴張速度。本研究選定了27個包狀物和60個纖狀物，計算出的擴張速度分別為0.173 arcsec/yr.和0.150 arcsec/yr。再透過分析蟹狀星雲的光譜所計算出的徑向速度（radial velocity）為1385.5 km/yr，進而推得蟹狀星雲的距離分別為5430光年和6370光年，平均值為5870光年。 另外，如果假設擴張速度是等速運動，那麼把求得的擴張速度倒推出的爆發日期是在西元1124年，這比中國紀錄中超新星爆發的1054年晚了70年。這顯示出蟹狀星雲的確非等速擴張而是有加速度的狀態，才會造成以等速倒推發生日期時，晚了70年。雖然本研究中分析了相差62年之久的八張影像，仍然無法分析出星雲的擴張的加速度情形。