蜗牛壳

　　美丽的大螺旋星系M81（也叫波德星系）是地球天空中最亮的星系之一，它位于北天大熊座内，距离我们1180万光年，大小和我们银河系相近。在下面这幅深空影像中，除了M81明亮的黄色核心区域外，还展示了它壮观的蓝色旋臂和尘埃带及分布其上的暗淡结构，称之为“亚伯圈”的巨大拱形结构看起来象是从星系盘的右上方盘旋升起。在1960年代的研究中认为，亚伯圈是一个引潮尾，是M81中的物质被邻近大星系M82的重力作用拉出所形成的；但最近的研究表明，亚伯圈的大部分可能位于我们银河系之中。在可见光和红外波段，拱形结构的色彩与目前尚未探索的距离银河盘面之上仅几百光年远的卷须尘埃云颜色相近，这些尘埃云与银河系中的恒星一起，分布在该图的前景中。M81的矮伴星系霍姆伯格九在大螺旋的左上方。在天空中，该图展示的区域约占0.5度，差不多是一个满月大小。

Explanation: One of the brightest galaxies in planet Earth's sky and similar in size to the Milky Way, big, beautiful spiral M81 lies 11.8 million light-years away in the northern constellation Ursa Major. This deep image of the region reveals details in the bright yellow core, but at the same time follows fainter features along the galaxy's gorgeous blue spiral arms and sweeping dust lanes. It also follows the expansive, arcing feature, known as Arp's loop, that seems to rise from the galaxy's disk at the right. Studied in the 1960s, Arp's loop has been thought to be a tidal tail, material pulled out of M81 by gravitational interaction with its large neighboring galaxy M82. But a recent investigation demonstrates that much of Arp's loop likely lies within our own galaxy. The loop's colors in visible and infrared light match the colors of pervasive clouds of dust, relatively unexplored galactic cirrus only a few hundred light-years above the plane of the Milky Way. Along with the Milky Way's stars, the dust clouds lie in the foreground of this remarkable view. M81's dwarf companion galaxy, Holmberg IX, can be seen just above and left of the large spiral. On the sky, this image spans about 0.5 degrees, about the size of the Full Moon.

　　每日天文一图（APOD）是美国宇航局天体物理学部每日定期发布的天文图片，并配有专业的简短解释，是小子时常光顾的地方，地址为：http://apod.nasa.gov/apod/。现将部分酷图转来并对原文解释做了翻译，英文及天文知识均有限，译错之处敬请各位指正。
　　秋天的麦兜 2011年2月14日 于济南。