In the 1920s Edwin Hubble began answering the question about the nature of spiral "nebulae" when he discovered Cepheid variables in
A) the Milky Way Galaxy.
B) the Whirlpool Galaxy (M51).
C) the Andromeda Galaxy (M31).
D) the Small Magellanic Cloud.

Question

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Edwin Hubble discovered Cepheid variables in the Andromeda Galaxy (M31), which allowed him to calculate its distance. This discovery proved that Andromeda was far outside the Milky Way, confirming that spiral nebulae are indeed separate galaxies.

In the 1920s Edwin Hubble Began Answering the Question About