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?

Question

Luke Allen · Accepted Answer

#10^6# parsecs

The square of the distance causes the absolute brightness, or the number of photons per unit of time for a fixed size detector, to decrease. 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.

Ariana Acosta · Answer

undefined The nearby galaxy is 1000 times farther away than the Milky Way Cepheid, or 1000,000 parsecs away. We can calculate the distance to the nearby galaxy using the inverse square law of brightness. Since the nearby Cepheid appears 10^6 times fainter than the one in the Milky Way, the distance ratio will be the square root of 10^6, which is 1000.