How did Hubble use Cepheids to measure the distance to M31 and other galaxies?

Answer 1

Cepheids are variable stars. That means they vary in brightness cyclicly over some period of time (a few days). By observing cepheids that are close enough so that their distance can be detemined by parallax methods, it was found that the period of the brightness cycle directly relates to the absolute brightness of the star. Distant galaxies are too far away to determine their distance by parallax, but when cepheids are found in them, their cyclic period can still be measured. This way their absolute brightness can be determined. Once absolute brightness is known, distance can be determined from the observed brightness, knowing that the observed brightness falls off as the square of the distance.

Answer 2

Just to add to what gp4rts posted, Edwin Hubble used the 100 inch telescope to make observations of Cepheid variables in the Andromeda Galaxy (well nebula as it was then). Up to that point it hadn't really been possible to resolve the individual stars. Since the observed period of the variables gave the absolute brightness, the apparent brightness could then be used to infer the distance.

Answer 3

ask an astrophyisist--i wouldn't know how to describe the equation structure