Astronomy Picture of the Day——M13内恒星的颜色与星等图
The Colors and Magnitudes of M13
Image Credit & Copyright: TolgaGumusayak, Robert Vanderbei
Explanation: M13 is modestly recognized asthe Great Globular Star Cluster in Hercules. A ball of stars numbering in thehundreds of thousands crowded into a region 150 light years across, it liessome 25,000 light-years away. The sharp, color picture of M13 at upper left isfamiliar to many telescopic imagers. Still, M13's Color vs Magnitude Diagram inthe panel below and right, made from the same image data, can offer a moretelling view. Also known as a Hertzsprung Russell (HR) diagram it plots theapparent brightness of individual cluster stars against color index. The colorindex is determined for each star by subtracting its brightness (in magnitudes)measured through a red filter from its brightness measured with a blue filter(B-R). Blue stars are hot and red stars are cool so that astronomical colorindex ranging from bluer to redder follows the relative stellar temperature scalefrom left (hot) to right (cool). In M13's HR diagram, the stars clearly fallinto distinct groups. The broad swath extending diagonally from the bottomright is the cluster's main sequence. A sharp turn toward the upper right handcorner follows the red giant branch while the blue giants are found grouped inthe upper left. Formed at the same time, at first M13's stars were all locatedalong the main sequence by mass, lower mass stars at the lower right. Over timehigher mass stars have evolved off the main sequence into red, then blue giantsand beyond. In fact, the position of the turn-off from the main sequence to thered giant branch indicates the cluster's age at about 12 billion years.
M13内恒星的颜色与星等图
M13被公认为是武仙座内的大型球状星团。这个球体跨度150光年，是由成千上万颗恒星组成的，它距离地球约25,000光年。这幅影像左上方的M13是很多望远镜都拍摄过的。然而，在影像右下方，由相同的影像数据制作而成的M13颜色与星等图呈现了一幅更加生动的视图。它也被称为赫罗（HR）图，绘制了单个星团内恒星的视亮度与色指数的关系。我们通过从用蓝色滤光片（B-R）测量的亮度中减去通过用红色滤光片测量的亮度，就可以确定每颗恒星的色指数。蓝色恒星温度较高，红色恒星温度较低，所以从蓝色到红色的天文色指数范围遵循从左到右的相对恒星温标。在M13的HR图中，恒星明显地分为不同的星群。从影像右下方呈对角延伸的宽线条是星团的主序。当主序向右上方急转时，红巨星支紧跟在其后，而蓝巨星都聚集在视图左上方。M13的恒星在同一时期形成，起初它们都是按质量沿着主星序排列，右下方是质量较小的恒星。随着时间推移，质量更大的恒星已经脱离主星序，演化成红巨星，接着演化成蓝巨星和其它天体。实际上，从星团主星序折向红巨星支的位置可推测这个星团的年龄约为120亿年。
参考文献：
https://apod.nasa.gov/apod/image/1906/M13_and_HR_diag_updated6.jpg
近期文章：
Astronomy Picture of the Day——NGC 4676：双鼠星系