Distance Measuring Errors
Recall that the errors in the initial determination of the distance scale
described in the left panel were associated
with several systematic errors. First, it was not
realized that there were two kinds of Cepheids with different period-luminosity
relations.
Second, in some cases RR Lyrae variables were confused with Cepheid variables. Third,
the early analysis failed to account
for the scattering of starlight by dust. Finally, in some of Hubble's original
analysis, he mistakenly identified what we now know are bright clusters of stars
and associated emission nebulae as single bright stars.