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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.astro
Subject: Re: Hubble's constant
Message-ID: <786@utastro.UUCP>
Date: Thu, 15-Nov-84 10:37:02 EST
Article-I.D.: utastro.786
Posted: Thu Nov 15 10:37:02 1984
Date-Received: Sat, 17-Nov-84 19:46:33 EST
References: <646@watdcsu.UUCP>
Organization: UTexas Astronomy Dept., Austin, Texas
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[Herb Chong]
>What's the current best value for the Hubble constant?
     The canonical range of 50-100 km/sec/Mpc has not been
significantly narrowed in recent years.  Part of the problem
is that each of the extreme values has a small group of
ardent partisans who continue to get their favorite value
(with small error bars) each time they try a new method.
One of the best efforts in recent years by an unprejudiced
group of observers found that their best estimate could be
varied from 65 to 90 by changing the local calibration of
the Cepheid luminosity. (These are variable stars with a
period - luminosity correlation.)  The distances to the local
Cepheids are known through a variety of indirect distance estimators.
This particular uncertainty may only disappear when accurate
parallax measurements become possible on these stars.  There
are many other sources of uncertainty.    
     I take the arithmetic mean myself.
                         
"I can't help it if my     Ethan Vishniac
    knee jerks"         {charm,ut-sally,ut-ngp,noao}!utastro!ethan
                           Department of Astronomy
                           University of Texas
                           Austin, Texas 78712