托福閱讀材料之反射星云

　　Just weeks after NASA astronauts repaired the Hubble Space Telescope in December 1999, the Hubble Heritage Project snapped this picture of NGC 1999, a reflection nebula in the constellation Orion.

　　Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. The nebula is famous in astronomical history because the first Herbig-Haro object was discovered immediately adjacent to it (it lies just outside the new Hubble image). Herbig-Haro objects are now known to be jets of gas ejected from very young stars.

　　The nebula is illuminated by a bright, recently formed star, visible just to the left of center. This star is cataloged as V380 Orionis, and its white color is due to its high surface temperature of about 10,000 degrees Celsius, nearly twice that of our own sun. Its mass is estimated to be 3.5 times that of the sun. The star is so young that it is still surrounded by a cloud of material left over from its formation, here seen as the NGC 1999 reflection nebula.

　　NGC 1999 shows a remarkable jet-black cloud near its center, located just to the right and lower right of the bright star. This dark cloud is an example of a "Bok globule," named after the late University of Arizona astronomer Bart Bok. The globule is a cold cloud of gas, molecules and cosmic dust, which is so dense it blocks all of the light behind it. The globule is seen silhouetted against the reflection nebula illuminated by V380 Orionis. Astronomers believe that new stars may be forming inside Bok globules, through the contraction of the dust and molecular gas under their own gravity.

　　參考譯文：

　　1999年12月，就在NASA的宇航員修復哈勃太空望遠鏡后數周，哈勃遺產計劃小組捕捉到這張獵戶座反射星云NGC1999的照片(好記，都是1999)。

　　反射星云就象街燈周圍的霧，因為其內嵌的光源照亮了星云中的塵埃而發(fā)亮;星云本身不發(fā)出任何可見光。這個星云在天文學史上大名鼎鼎，因為第一個被發(fā)現的HH天體(赫比格-阿羅天體)就在它附近(該HH天體剛好不在圖中)。目前所知HH天體是由非常年輕的恒星噴出的高速氣流。

　　這個反射星云被一顆明亮的、新近形成的恒星(在圖像的中間偏左)照亮，該恒星編號為獵戶V380，因為表面溫度高達10000攝氏度(幾乎是太陽的2倍)而發(fā)出白色的光芒，其質量估計是太陽的3.5倍。這顆恒星如此年輕，以至仍被形成恒星的剩余原始材料環(huán)繞，這就是我們看到的NGC1999反射星云。

　　NGC1999的中心附近有一個不尋常的黑色噴流云，在圖中亮恒星的右下一點點。這個暗云是一個“博克球狀體”，以亞利桑那大學天文學家Bart Bok命名。該球狀體是一團非常稠密的冷氣體、分子、塵埃組成的云氣，擋住了它后面所有的光，在被V380照亮的反射星云襯托下顯示出它的輪廓。天文學家相信博克球狀體內的塵埃和分子云氣會在自身引力作用下收縮，然后形成新的恒星。