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# Cepheid Variables question (Doppler Effect) watch

1. I was wondering if anyone could give me a hand with this question:

A Cepheid variable with a period of about 1 day is 200 times as bright as the Sun. A cepheid variable with a period of 100 days is 10,000 times as bright as the Sun. If a 1 day cepheid variable in a nearby star cluster appears as bright as a 100 day Cepheid variable in a more distant cluster, how much futher away is the distant cluster than the nearer one.

Much appreciated!
2. I reckon about 23.

Use the fact that the brightness will decrease with the inverse square of the distance.
3. (Original post by morals_officer)
I reckon about 23.

Use the fact that the brightness will decrease with the inverse square of the distance.
thanks for the reply. I forgot to add that the book says the answer is ~7 times.
4. So it is! I missed a power of 10, oops!

Here's how it "is".

Firstly, you know that a 100 day cephid is 50 times brighter than a 1 day one. You also know that brightness decrases with inverse square of the distance. Hence, the second cephid is sqrt(50) times further away.

that okay?
5. Thanks that's brilliant. I kept on trying to find a way to use the doppler equation

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