# Distance of extra-galactic Classical Cepheids

There have been many questions and answers about finding the distance of a star from the earth. But as I did some research on the net, I found that we have specific approaches for finding the distances of different types of stars.

So, my question is more specific now.

How to determine how far a Classical Cepheid is?

What if the Cepheid is extra-galactic? Is there a good method to finding the solution? Can anybody suggest how to make a start at deriving a relation at finding the distance of a Cepheid from us?

There is this Period-Luminosity-Color relation. Can anyone at least enlist the journal articles where I can make a study to get closer to finding the answer to my question?

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Hi. I don't understand your question, sorry. If you are aware of the Period-Luminosity relation, what's the problem? Perhaps, is it that you need some explanations about how to use that relation to find the distance? –  Py-ser Jun 18 '14 at 2:38
Yup.I need to know how the distance from P-L relation was derived exactly. And 2nd-ly, I do not find any way to measure distance of an extra-galactic Cepheid. I am lost. –  MycrofD Jun 18 '14 at 5:17

Since it seems you don't know the equations, I will try to keep it simple. However, just keep in mind that, in principle, there is no difference between Galactic and extragalactic Cepheids, in this context.

Now, we know that a Period-Luminosity relation holds for the Cepheids:

$P\sim L$

Where $P$ is the pulsation period observed from the Cepheid, and $L$ is the observed luminosity.

We also know that:

$L\sim d$

where $d$ is the distance of the source.

Then it is like to write $P\sim d$.

This means that, if we observe the pulsation period of a Cepheid, we know its distance, and this allows to know the distance of the host galaxy as well.

Some basic sources:

Cepheid variable

Classical Cepheid variable with some numbers

And some diagrams too

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I had already come across those wiki articles and the third you mentioned. I meant to go deep into that. See, there are two equations. Which one to use? And extra galactic stars bring more extinction into the picture right. So it is not the same in that case with galactic stars. –  MycrofD Jun 18 '14 at 6:32
The principle is the same, then you can correct for extinction, but that is different process than from Cepheids modelling. What do you mean "which one to use"? You must use both –  Py-ser Jun 18 '14 at 6:40
Under P-L relation given in the 2nd link you posted in your answer, there are two equations. They are different. How can I use both? And which is more accurate? –  MycrofD Jun 18 '14 at 7:14
I thought you were referring to the equations I wrote in my answer. For the equations in the article, it depends on which type of Cepheids you are working with. As the article clearly states, one equation is suitable for Population I Cepheids, and the other for classical Cepheids. I don't actually know the difference. If you really need it I can look for on the web. –  Py-ser Jun 18 '14 at 7:28
Actually Pop I Cepheids and Classical Cepheids happen to be the same. I have been searching articles for satisfactory answers. Yet to learn about that. So I posted the question, just in case I might get some clue. –  MycrofD Jun 18 '14 at 8:36