How was edwin hubble able to use his discovery of cepheids in andromeda to prove that the "spiral nebulae" were actually galaxies external to the milky way?

 The correct answer is :  From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.

Explanation:  Hubble's law says that the recession velocity of a galaxy is directly proportional to its distance from us. Hubble measured the distance to the Andromeda galaxy by applying the period-luminosity relationship to Cepheid.

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