Observational characterization of intermediate mass stars

被称为疏散星团的丰富恒星群是我们银河系中的美丽景象，其成员恒星的共同年龄、距离和原始化学成分使它们成为相关天体内在属性(光度、温度和演化状态)的理想校准器。特别重要的是造父变星，也就是脉动的超大恒星，用作到附近星系的距离指示器，因为它们的特征亮度周期的持续时间直接与固有亮度有关。用周期变化率和脉动幅度描述的另一个特征与它们的进化状态更精确地相关，也可能解决关于脉动模式的问题。年轻星团的另一个常见组成部分是脉动的M超巨星，它们本身就是标准的蜡烛，很像造父变星，但也是恒星演化的最后阶段，这些恒星的质量是太阳的20-25倍，然后作为II型超新星爆炸。与造父变星一样，将这些天体联系在一起的共同线索是，它们代表了恒星演化的高级阶段，在这些阶段，现有的观测数据可以用来检验日益复杂的恒星演化模型的预测与现实。在M超巨星的情况下，可能会探测到一个观测信号，为它们即将灭亡提供提前警告。

Rich ensembles of stars known as open clusters are beautiful sights in our Galaxy, and the common age, distance, and original chemical composition of their member stars make them ideal calibrators of intrinsic properties (luminosity, temperature, and evolutionary status) for associated objects. Especially important are Cepheid variables, pulsating supergiant stars used as distance indicators to nearby galaxies because the duration of their characteristic brightness cycles relates directly to intrinsic brightness. An additional characteristic described by rate of period change and pulsation amplitude correlates more precisely with their evolutionary status, and may resolve questions about pulsation mode as well. Another common constituent of young clusters are pulsating M supergiants, standard candles in their own right much like Cepheids, but also the last stage of evolution for stars 20-25 times more massive than the Sun prior to their explosion as Type II supernovae. Like Cepheids, the common thread linking such objects is that they represent advanced stages of stellar evolution, where existing observational data can be used to test predictions from increasingly-sophisticated stellar evolutionary models with reality. In the case of M supergiants, it may be possible to detect an observational signature that provides advanced warning of their forthcoming demise.