Establishing fundamental properties for Evolved Stars in Open Clusters

从明星的交往中可以学到很多关于他们的知识。没有什么地方比我们称之为疏散星团的美丽恒星群更明显的了，疏散星团是由数百到数千颗恒星组成的群体，拥有共同的年龄，距离和原始的化学组成。疏散星团之所以具有巨大的价值，是因为它们能够作为里程碑，指示在其中发现的稀有天体的距离，光度和进化状态。多年来，这包括造父变星，与太阳一样热但数量级更大的脉动恒星，它们是从比我们的太阳大3到30倍的恒星演化而来的，并且已经消耗了其热而致密的核心中所有的氢燃料。在1975年，造父变星星团的列表只包含了大约10个天体，但是这个数字现在已经超过了50个，许多新的增加都是由申请人的工作产生的。同样地，目前被确认属于疏散星团的行星状星云的数量是零，或者最多一个，但如果有一点集中的努力，这个数字可能是6个或更多。行星状星云代表了造父变星插曲之后的一个演化阶段，它们的距离尺度比造父变星的距离尺度更不可靠，甚至在造父变星被认为是疏散星团的成员之前。在疏散星团中发现的其他奇异物体包括大质量的脉动M超巨星，它们本身就像造父变星一样的标准烛光，但也是它们作为II型超新星爆炸之前的最后一个演化阶段。连接这些不同物体的共同线索是，它们代表了恒星演化的高级阶段，我们的知识基础不太安全。也许我们可以从疏散星团中的例子中了解更多关于恒星演化的这些阶段。

It is possible to learn a lot about stars from the company they keep. Nowhere are such connections more obvious than in those beautiful stellar ensembles we call open clusters, groups of hundreds to thousands of stars sharing a common age, distance, and original chemical makeup. What makes open clusters of immense value is their ability to serve as mileposts indicating distance, luminosity, and evolutionary status for rare classes of objects that are found within them. For many years that has included Cepheid variables, pulsating stars about as hot as the Sun but orders of magnitude larger, that have evolved from stars roughly 3 to 30 times more massive than our Sun and that have consumed all of the hydrogen fuel available in their hot, dense cores. In 1975, lists of cluster Cepheids contained only about 10 objects, but that number currently exceeds 50, many newer additions arising from work by the applicant. Likewise, the number of planetary nebulae currently recognized to belong to open clusters is zero, or one at most, but that number might be 6 or more with a bit of focused effort. Planetary nebulae represent a stage of evolution beyond the Cepheid interlude, and they have a distance scale that is known much less reliably than that of Cepheids, even before Cepheids were recognized to be members of open clusters. Other exotic objects found in open clusters include massive, pulsating M supergiants, standard candles in their own right much like Cepheids, but also the last stage of evolution prior to their explosion as Type II supernovae. The common thread linking these various objects is that they represent advanced stages in stellar evolution, where our knowledge base is less secure. Perhaps we can learn more about such stages of stellar evolution from the examples located in open clusters.