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  Reflection Nebula

  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.

  参考译文:

  1999年12月,就在NASA的宇航员修复哈勃太空望远镜后数周,哈勃遗产计划小组捕捉到这张猎户座反射星云NGC1999的照片(好记,都是1999)。

  反射星云就象街灯周围的雾,因为其内嵌的光源照亮了星云中的尘埃而发亮;星云本身不发出任何可见光。这个星云在天文学史上大名鼎鼎,因为第一个被发现的HH天体(赫比格-阿罗天体)就在它附近(该HH天体刚好不在图中)。目前所知HH天体是由非常年轻的恒星喷出的高速气流。

  这个反射星云被一颗明亮的、新近形成的恒星(在图像的中间偏左)照亮,该恒星编号为猎户V380,因为表面温度高达10000摄氏度(几乎是太阳的2倍)而发出白色的光芒,其质量估计是太阳的3.5倍。这颗恒星如此年轻,以至仍被形成恒星的剩余原始材料环绕,这就是我们看到的NGC1999反射星云。

  NGC1999的中心附近有一个不寻常的黑色喷流云,在图中亮恒星的右下一点点。这个暗云是一个“博克球状体”,以亚利桑那大学天文学家Bart Bok命名。该球状体是一团非常稠密的冷气体、分子、尘埃组成的云气,挡住了它后面所有的光,在被V380照亮的反射星云衬托下显示出它的轮廓。天文学家相信博克球状体内的尘埃和分子云气会在自身引力作用下收缩,然后形成新的恒星。


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