M13内恒星的颜色与星等图
影像来源与版权:Tolga Gumusayak, Robert Vanderbei

说明:M13被公认为是武仙座内的大型球状星团。这个球体是由聚集在一片跨度为150光年区域中的成千上万颗恒星构成,它距离我们约25,000光年远。这幅影像左上方的M13清晰彩色照片被许多望远镜成像仪所熟悉。然而,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红色滤光片测量的亮度(以星等为单位),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(高温)到右(低温)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弯时,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。随着时间的推移,质量更大的恒星已经离开主星序,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。
翻译:北京天文馆

The Colors and Magnitudes of M13
Image Credit & Copyright: Tolga Gumusayak, Robert Vanderbei

Explanation: M13 is modestly recognized as the Great Globular Star Cluster in Hercules. A ball of stars numbering in the hundreds of thousands crowded into a region 150 light years across, it lies some 25,000 light-years away. The sharp, color picture of M13 at upper left is familiar to many telescopic imagers. Still, M13’s Color vs Magnitude Diagram in the panel below and right, made from the same image data, can offer a more telling view. Also known as a Hertzsprung Russell (HR) diagram it plots the apparent brightness of individual cluster stars against color index. The color index 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 color index ranging from bluer to redder follows the relative stellar temperature scale from left (hot) to right (cool). In M13’s HR diagram, the stars clearly fall into distinct groups. The broad swath extending diagonally from the bottom right is the cluster’s main sequence. A sharp turn toward the upper right hand corner follows the red giant branch while the blue giants are found grouped in the upper left. Formed at the same time, at first M13’s stars were all located along the main sequence by mass, lower mass stars at the lower right. Over time higher mass stars have evolved off the main sequence into red, then blue giants and beyond. In fact, the position of the turn-off from the main sequence to the red giant branch indicates the cluster’s age at about 12 billion years.
Astronomy Picture of the Day 06/13/2019