Dan,
So what you are saying is that there have been enough measurements to keep Cephied distance values as valid? It would seem that any changes in Cephied luminosity values could easily be applied to Hubble's calculations to adjust that constant if it needs to be. It would be shame if Sandage's lifetime work was thrown out. They did apply a lot of mathematical corrections for atmospheric conditions and they did suspect Cephieds where shedding material as they moved threw space. They just had no proof. I imagine the Spitzer Scope has simplified the corrections some a deal. IE, atmospheric conditions no longer need be considered. One thing is for certain cosmologist are excellent in Math. Erik The Cepheid distance scale has required statistical help to give us an
accurate distance to M31. The initial value of the distance has been corrected by a factor of 2 from the time that the first Cepheid measurements were done with the 100 inch on Mount Wilson.
The best atmospheric resolution prior to adaptive optics was seldom less than an arc second. At the distance of M31 that means a cone of space 10 light years accross. That is wide enough to pick up stray stars within M31 that are in front of or behind the Cephied being studied. These strays are too dim to be resolved as companions but still bright enough to mess up the measurement of the Cepheid.
This was resolved by statistics. Take the distance value to M31 from dozens of different Cepheids and the numbers will give a broad range of values. But this range will have a hard edge at the upper end where you have a few samples with no stray light from optical companions.
It took awhile but that's how we resolved the present value of distance to M31. It's also the reason why the supernovae distance scale is based on thousands of sample points. So we can calibrate the scale to a much higher degree over time. It's also the reason that the SDSS is doing the spectrum on millions of galaxies not just a few dozen.
Statistics. It's where the rubber meets the road in science.
DT
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