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# Cepheid Variables Watch

1. How are cepheid variables used to determine distances to nearby star clusters??

I understand that you have to find some cepheids whose distances are already known from a nearby galaxy. Then work out the luminosity (using observed brightness) and periods and plot them in a graph (linear relationship: longer period means greater luminosity).

Then find a cepheid in another galaxy of interest. Measure the period of the cepheid and its observed brightness. Then you read off the luminosity from the previous established graph that showed the linear relationship.

What do you do after this?

add point: How is observed brightness worked out?
2. I think you're asking about the relation between apparent magnitude (m) and absolute magnitude (M)...

The equation of magnitudes given in the formulae sheet is
m-M=5*log (d/10) with d being distance in parsecs.

Tbh this seems like more of a definitional query than a conceptual one. I.e. One you could quite easily sort out for your self with a library book or astronomy website.
3. (Original post by park1996)
How are cepheid variables used to determine distances to nearby star clusters??

The key piece of information that is missing from your OP is that you need the distance to at least one Cepheid variable as a point of reference for the other calculations. Fortunately, Delta Cephei is close enough for us to get a parallax view (ie, two observations on either side of the earth's orbit) which allows us to know its distance accurately.
4. (Original post by Pastaferian)

The key piece of information that is missing from your OP is that you need the distance to at least one Cepheid variable as a point of reference for the other calculations. Fortunately, Delta Cephei is close enough for us to get a parallax view (ie, two observations on either side of the earth's orbit) which allows us to know its distance accurately.

Is it possible if you could tell me why Henrietta Leavitt had to assume that the Cepheids she was analysing were roughly the same distance away?
5. (Original post by park1996)
Is it possible if you could tell me why Henrietta Leavitt had to assume that the Cepheids she was analysing were roughly the same distance away?
The Cepheids that she studied were all in the Small Magellanic Cloud, so they were all roughly the same distance away (i.e., she wasn't making an assumption about that). She was then able to find the relationship between apparent brightness and period for those stars. By studying Delta Cephei, whose period was known and whose distance was calculated using parallax, it was possible to calculate the distance to all Cepheid variables which have the same period (the relationship between luminosity, apparent brightness and distance is simple physics). This fixed the distance to the SMC, and all the stars within it. After that, the distance to any other Cepheid variable can be calculated by comparing its brightness with that of a star in the SMC with the same period. See http://en.wikipedia.org/wiki/Henriet...Leavitt#Career for a brief explanation of Leavitt's work (she was considered for a Nobel Prize, but she had died several years before her work became widley appreciated).

HTH
6. (Original post by Pastaferian)
The Cepheids that she studied were all in the Small Magellanic Cloud, so they were all roughly the same distance away (i.e., she wasn't making an assumption about that). She was then able to find the relationship between apparent brightness and period for those stars. By studying Delta Cephei, whose period was known and whose distance was calculated using parallax, it was possible to calculate the distance to all Cepheid variables which have the same period (the relationship between luminosity, apparent brightness and distance is simple physics). This fixed the distance to the SMC, and all the stars within it. After that, the distance to any other Cepheid variable can be calculated by comparing its brightness with that of a star in the SMC with the same period. See http://en.wikipedia.org/wiki/Henriet...Leavitt#Career for a brief explanation of Leavitt's work (she was considered for a Nobel Prize, but she had died several years before her work became widley appreciated).

HTH

Thank you!

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