How did Edwin Hubble find distances using Cepheid variables as 'standard candles'?
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2$\begingroup$ Did you read the wiki article? $\endgroup$– pelaCommented Aug 5, 2016 at 16:40
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1$\begingroup$ yes. it was not very helpful as it was very vague $\endgroup$– nasastromasterCommented Aug 5, 2016 at 16:46
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$\begingroup$ Do you mean how did he calculate distances using Cepheid's, or else how can Cepheid's be calibrated such that distances can be determined by observing them? $\endgroup$– zephyrCommented Aug 5, 2016 at 17:06
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$\begingroup$ @pela Having read the entire article, I have to side with nasastromaster. The article talks a lot about the historical developments of cepheid variables in astronomy, including the issues and unknowns that limit the precision, but doesn't mention much of anything about exactly what you measure about them and then how you apply that to make them into "standard candles". I don't see a single mention of, or link to, the luminosity/distance/brightness relation, for example. The Classical cepheid variable page seems more instructive. $\endgroup$– zibadawa timmyCommented Aug 6, 2016 at 1:22
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$\begingroup$ Yes I agree, @zibadawatimmy and nasastromaster, it's actually quite bad. I'm sorry if I sounded condescending. But as zephyr says, you should probably specify what it is about the concept that you would like to hear about, e.g. the concept of standard candles or the luminosity-variability relationship. $\endgroup$– pelaCommented Aug 6, 2016 at 12:37
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Why not read his original paper? It's fairly short by today's standards, despite being such a monumental turning point in our understanding of the size and nature of the Universe.
In short though, Hubble measured the distance to Cepheid's the same way anyone does. There is a distinct relation between the luminosity oscillation period of the star and the star's luminosity. If he can measure the star's oscillation period with good accuracy, he can derive the luminosity. If he can also then measure the brightness (i.e., how bright it appears to be to him) then he can calculate the distance (since luminosity, distance, and brightness are all related).
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$\begingroup$ I'm still confused - since the luminosity is variable, it is not constant - so exactly what luminosity is referred to in the phrase "distinct relation between the luminosity oscillation period of the star and the star's luminosity". $\endgroup$– uhohCommented Sep 23, 2016 at 15:27
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$\begingroup$ @uhoh That's a good question. The luminosity in that instance is the average luminosity. If you know the period, and use the period-luminosity relations, what you get out is the average luminosity of the star. I suppose I should point out that technically the equations usually output the absolute magnitude, not the luminosity, but it's a pretty quick bit of math to switch between the two. Then just measure average flux (or equivalently apparent magnitude) and with those two you can calculate distance. $\endgroup$– zephyrCommented Sep 23, 2016 at 16:47
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$\begingroup$ Are you 100% it simply the arithmetic average of the luminosity, not something more complicated? Some stars have very sharp maxima, some have much smoother maxima. (side note, averaging flux and averaging magnitude are not equivalent) $\endgroup$– uhohCommented Sep 23, 2016 at 17:19