A cepheid variable in a nearby galaxy looks "10^6" times fainter than an identical Cepheid in the Milky Way? The Cepheid in the Milky Way is 1000 parsecs away. How far away is the nearby galaxy?

1 Answer
George C.
Mar 27, 2016

10^6 parsecs

Explanation:

The absolute brightness (i.e. number of photons per unit time for a fixed size detector) decreases as the square of the distance.

To prove this for yourself, think of the photons passing through the surface of a sphere of radius r. The surface area is 4pi r^2, proportional to the square of the radius.

So if the absolute brightness decreases by a factor of 10^6 that means that the distance is a factor of sqrt(10^6) = 10^3 greater.

That makes the distant cepheid variable 10^3*10^3 = 10^6 parsecs away.

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