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?
A)He measured the parallaxes of the Cepheids in Andromeda to determine their distances. He showed that Andromeda was far outside the Milky Way Galaxy.
B)As a Cepheid is a type of luminous galaxy, Hubble's discovery of Cepheids in Andromeda proved that it was a separate galaxy from the Milky Way.
C)From the period- luminosity relation for Cepheids, he was able to determine the distance to Andromeda. He showed that it was far outside the Milky Way Galaxy.
D)There are no Cepheids in the Milky Way, so his discovery proved that Andromeda had to be in another galaxy.
E)He used main- sequence fitting to determine the distances to the Cepheids. He showed that Andromeda was far outside the Milky Way Galaxy.

Question

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Edwin Hubble used the period-luminosity relation for Cepheids to determine the distance to Andromeda. By showing that it was far outside the Milky Way, he proved that the spiral nebulae were actually galaxies external to our own.

How Was Edwin Hubble Able to Use His Discovery of Cepheids