The first graph shows the variation of apparent brightness of a Cepheid star with time. The second graph shows the average luminosity with period for Cepheid stars.

from first graph period $=5.7$ «days» $\pm 0.3$ «days» ✔ from second graph $\frac{L}{L_{sun}}=2300$ « $\pm 200$ » ✔ $d=$ « $\frac{2500 \times 3.8 \times 10^{26}}{4\pi \times 1.1 \times 10^{-9}}=8.3 \times 10^{18} \text{m}$ »$=250$ «pc» ✔

Cepheids have a definite/known «average» luminosity ✔ which is determined from «measurement of» period OR determined from period-luminosity graph ✔ Cepheids can be used to estimate the distance of galaxies ✔