From: utzoo!decvax!cca!LRC.Slocum@UTEXAS-20@sri-unix
Newsgroups: net.space
Title: red-shift & distance measurement
Article-I.D.: sri-unix.2580
Posted: Mon Aug  9 13:52:02 1982
Received: Tue Aug 10 03:44:57 1982

From: Jonathan Slocum 
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.
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