What specific stellar feature did Hubble use to determine the distance to the Andromeda Nebula?

Answer

Cepheid variable stars whose period-luminosity relationship was known

Hubble observed Cepheid variable stars within the Andromeda Nebula. Knowing the period-luminosity relationship of these stars allowed him to use them as a crucial cosmic yardstick to calculate the immense distance.

What specific stellar feature did Hubble use to determine the distance to the Andromeda Nebula?

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