When the logarithm of one of the variables in this equation drops below about 0.4, this equation changes slope, which limits its usefulness for shorter timescales. The Key project on the Hubble Space Telescope measured the Hubble constant by exploiting this formula, and a major problem with using this formula is finding a reliable zero-point. This formula is slightly sensitive to metallicity, and before it can be used it must be adjusted for reddening. Once color is known, the fact that one of the observed variables in this equation does not depend on distance allows the determination of a distance scale by regarding 
When the logarithm of one of the variables in this equation drops below about 0.4, this equation changes slope, which limits its usefulness for shorter timescales. The Key project on the Hubble Space Telescope measured the Hubble constant by exploiting this formula, and a major problem with using this formula is finding a reliable zero-point. This formula is slightly sensitive to metallicity, and before it can be used it must be adjusted for reddening. Once color is known, the fact that one of the observed variables in this equation does not depend on distance allows the determination of a distance scale by regarding objects that obey this equation as standard candles. Originally discovered as an empirical rule by Henrietta Levitt, for 10 points, identify this relationship between the size and light output of a pulsating Cepheid variable.
When the logarithm of one of the variables in this equation drops below about 0.4, this equation changes slope, which limits its usefulness for shorter timescales.
period-luminosity relation