What is POP?

WHat is the Distance to a variable Cepheid Pop I and Pop II?

• Im having trouble determing the relationship between the distances to POP I and POP II Cepheids. My Data is: Period : 10 days Apparent Visual Mag: 18 What is the Distance if it is POP I and POP II? This is what I know so far: Mv(pop I) = -2.76*(log(P) - 1.0) - 4.16 and mv - Mv = 5log(d) - 5 Thus I can find d for POP I. But what is it for POP II? Thanks

• Answer:

  Type II Cepheids (also known as W Virginis variables) belong to population II (low-metallicity stars). According to the Wikipedia article mentioned below, they are about 1.5 magnitudes fainter than Type I Cepheids. As you can see from the second website below, both types of Cepheids have similar period-luminosity relationships except for a constant ratio of luminosity (which is a constant offset in magnitudes). Hence, the period-luminosity relationship for Type II Cepheids is the same as the one in your question, but with an extra 1.5 magnitudes: Mv(pop II) = Mv(pop I) + 1.5 = -2.76*(log(P) - 1.0) - 4.16 + 1.5 = -2.76*(log(P) - 1.0) - 2.66 (Or, equivalently, you can do it this way: Solve the problem first for a Type I Cepheid. If a Type II Cepheid of the same period were at the same distance, it would be 1.5 magnitudes fainter. For the Type II Cepheid to have the same apparent brightness as the Type I Cepheid, it must be closer. How much closer? Enough to increase its brightness by 1.5 magnitudes. I'll let you figure out the distance ratio.)