Old-aged stellar distance indicators are present in all Galactic structures (halo, bulge, disk) and in galaxies of all Hubble types and, thus, are immensely powerful tools for understanding our Universe. Here we present a comprehensive review for three primary standard candles from Population II: (i) RR Lyrae type variables (RRL), (ii) type II Cepheid variables (T2C), and (iii) the tip of the red giant branch (TRGB). The discovery and use of these distance indicators is placed in historical context before describing their theoretical foundations and demonstrating their observational applications across multiple wavelengths. The methods used to establish the absolute scale for each standard candle is described with a discussion of the observational systematics. We conclude by looking forward to the suite of new observational facilities anticipated over the next decade; these have both a broader wavelength coverage and larger apertures than current facilities. We anticipate future advancements in our theoretical understanding and observational application of these stellar populations as they apply to the Galactic and extragalactic distance scale.

中文翻译：

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老年人主要距离指标

老年恒星距离指示器存在于所有星系结构（光晕、凸起、圆盘）和所有哈勃类型的星系中，因此是了解我们宇宙的极其强大的工具。在这里，我们对来自种群 II 的三个主要标准蜡烛进行了全面审查：(i) RR Lyrae 型变量 (RRL)，(ii) II 型造父变星 (T2C)，以及 (iii) 红巨星分支的尖端 (TRGB ）。这些距离指标的发现和使用在描述它们的理论基础和展示它们在多个波长的观测应用之前被置于历史背景中。用于为每个标准蜡烛建立绝对比例的方法在观察系统学的讨论中进行了描述。最后，我们期待未来十年预期的新观测设施套件；与现有设施相比，它们具有更广的波长覆盖范围和更大的孔径。我们预计未来在我们对这些恒星族群的理论理解和观测应用方面会取得进展，因为它们适用于银河系和河外距离尺度。