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Question
Why does the Cepheid distance method fail us beyond about 20 Mpc?A) Distant galaxies are seen in the past, and Cepheids evolve with time.
B) Distant galaxies are younger than our Milky Way; too young to have Cepheids yet.
C) The intergalactic dust reddens the Cepheid's light and apparent brightness.
D) Even with the HST, the most luminous Cepheids are too faint to be seen beyond 20 Mpc.
E) The periods of Cepheids grows longer as the distances increase.
Answer
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