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Loading contentOld, pulsating horizontal-branch stars with a well-defined luminosity, especially tight when measured in the infrared. Abundant in globular clusters and the haloes of galaxies, they are a standard candle for old stellar populations and, like the Cepheids, are calibrated directly by parallax.
Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
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