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A Cepheid variable in a nearby galaxy looks 20x107 times fainter than an identical Cepheid in...

A Cepheid variable in a nearby galaxy looks 20x107 times fainter than an identical Cepheid in the Milky Way. The Cepheid in the Milky Way is 132 parsecs away. How far away is the nearby galaxy, in lightyears?

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Answer #1

The flux of a star at a distance r from the star is

where P is the power emitted by the star.

For identical Cepheid variables the right hand side is a constant. For two identical Cepheid variables

It is given that the Cepheid variable in a nearby galaxy looks 20x107 times fainter than an identical Cepheid in the Milky Way. The Cepheid in the Milky Way is 132 parsecs away.

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