View Full Version : Cosmological Distance Equations!

2012-Mar-02, 03:28 AM
I need help with a rather specific question, and I was hoping I could find some guidance here... Basically, my question is, how does one find the distance to a variable star (lets say for this example a type II cepheid variable) with only the light curve of the star (Luminosity vs. Time). I understand that one can determine the period by looking at the graph, but after that I'm stuck. Any help would be appreciated.

2012-Mar-02, 01:12 PM
There is a strong relationship between a Cepheid variable's luminosity and its pulsation period. They don't all 'pulse' at the same rate, but whatever that pulse is tells you the star's intrinsic luminosity. Then you compare how much less luminous the star appears to its intrinsic luminosity to give you the distance. If you know a candle has an intrinsic brightness of "x", but when you measure the brightness it's only one-fourth of "x", then you know it's twice as far away.

2012-Mar-02, 01:27 PM



2012-Mar-02, 03:09 PM
SMan's links should help. We know the narrow range of brightness of a Cepheid of a particular period and type (type by looking at metal content of its spectrum) by looking at places that have many of them (like the Large Mag. Cloud) and finding the distance to that cloud using other means. You need to take the total luminosity of such a star as known from a formula calculated by lots of previous work on the distance ladder.

BTW, I put a link to a paper about this in today's "Fun Papers"

2012-Mar-06, 03:44 AM
Thanks guys, the links were especially helpful.