# Star Shining Bright

A cepheid variable star's brightness can be predicted by observing the period over which its luminosity varies.

A particular cepheid variable is found by this method to have a luminosity of $4\times 10 ^ {31} \text{ W}$. The intensity of its light on Earth is $0.00025 \text{ W/m}^2$. How far away is the star in metres?

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