LRC.Slocum@UTEXAS-20@sri-unix (08/09/82)
From: Jonathan Slocum <LRC.Slocum at UTEXAS-20> Using red-shift to compute distance does not work within the Local Group (= Milky Way, Andromeda, Magellanic Clouds, etc.) -- in other words, not anywhere close to here -- because these galaxies are gravitationally bound, pursuing orbits around a common center, and hence are not expanding in the sense defined by Hubble. Cepheid variables (whose intrinsic luminosities are mathematically related to their periods of variation) are used to calculate distances in these ranges. The discovery that there are two different types (technically, "populations") of Cepheid variables is what led to the revision of the "measured" distance to Andromeda, among others, some years ago. -------