船尾座的三个星团
Three Clusters in Puppis
Image Credit & Copyright: Dave Doctor
Explanation: Galactic or open star clusters are young. The swarms of stars are born together near the plane of the Milky Way, but their numbers steadily dwindle as cluster members are ejected by galactic tides and gravitational interactions. Caught in this telescopic frame over three degrees across are three good examples of galactic star clusters, seen toward the southern sky’s nautical constellation Puppis. Below and left, M46 is some 5,500 light-years in the distance. Right of center M47 is only 1,600 light-years away and NGC 2423 (top) is about 2500 light-years distant. Around 300 million years young M46 contains a few hundred stars in a region about 30 light-years across. Sharp eyes can spot a planetary nebula, NGC 2438, at about 11 o’clock against the M46 cluster stars. But that nebula’s central star is billions of years old, and NGC 2438 is likely a foreground object only by chance along the line of sight to youthful M46. Even younger, aged around 80 million years, M47 is a smaller and looser star cluster spanning about 10 light-years. Star cluster NGC 2423 is pushing about 750 million years in age though. NGC 2423 is known to harbor an extrasolar planet, detected orbiting one of its red giant stars.
Tomorrow’s picture: peculiar galaxies
船尾座的三个星团
影像提供与版权: Dave Doctor
说明: 银河星团（又称疏散星团）很年轻。不过，这些同时诞生于银河系盘面附近的成团恒星，在银河潮汐和彼此重力的交互作用之下，偶而会被弹飞出去，也因此星团成员的数量会不断减少。这幅超过3度宽的望远镜影像，就记录了南天．航海星座─船尾座方向的3个很典型之银河星团。它们分别是影像左下角约5,500光年远的M46，中右方近在1,600光年之处的M47，以及中上方约2,500光年远的NGC 2423。在约3亿年稚龄的M46里，它的数百颗成员星挤在宽约30光年的空间之内。目光犀利的读者，可在M46内的11点钟方向，看到编录号为NGC 2438的行星状星云。不过这团星云的中心星之年龄高达数十亿年，也因此，NGC 2438可能只是一个恰好出现在年轻M46视线方向的前景天体。而远更年轻、稚龄约8千万年的M47，是一个宽约10光年，相对较小也较松散的星团。在年龄约有7亿5千万年的星团NGC 2423里，天文学家找到一颗正在绕行星团内诸多红巨星之一的系外行星。
明日的图片: peculiar galaxies
