Description acceptable. A 2024 paper by Adam Riess et al. used JWST observations of NGC 4258 to validate the linearity of this relation and show that crowding does not significantly bias HST’s determination of it. For 10 points each:
[10m] Walter Baade classified stars into “populations” after finding that this relation depended on metallicity. This relation calibrates the lowest rung of the extragalactic distance ladder and was discovered by Henrietta Swan Leavitt.
ANSWER: the Cepheid period-luminosity relation [accept any answer that mentions Cepheid variables, their period of oscillation, and their luminosity; prompt on answers that only specify Cepheid variables by asking, “between what two quantities?”; prompt on answers that only specify the period-luminosity relation by asking, “of what objects?”]
[10e] Improper calibration of the Cepheid period-luminosity relation could be the source of a seven-sigma tension between measurements of this quantity. This quantity measures the rate of the universe’s expansion.
ANSWER: Hubble’s constant [or Hubble parameter; or H-naught; or H; accept Hubble tension]
[10h] The Carnegie-Chicago Hubble Program used this point on the HR diagram as a standard candle to observe an intermediate value of H-naught. Stars at this point are about to undergo the helium flash.
ANSWER: the tip of the red giant branch [or TRGB; or the tip of the RGB; prompt on tip; prompt on red giant branch or RGB]
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