= Tip of the red giant branch
The termination of the first-ascent <red giant> <luminosity function> near helium ignition provides a calibrated population-II distance indicator. In suitable passbands the tip <luminosity> varies relatively weakly over a useful old, metal-poor population range. Colour/composition corrections, foreground <extinction>, photometric completeness and contamination by other evolved <stars> must be controlled; an arbitrary brightest red <star> is not a reliable tip measurement.
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