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From: mll@hpfcso.FC.HP.COM (Mark Luce)
Newsgroups: sci.electronics
Subject: Re: How to tell distance of Stars
Message-ID: <7480021@hpfcso.FC.HP.COM>
Date: 3 May 91 23:50:50 GMT
References: <8937@crash.cts.com>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 51

/ hpfcso:sci.electronics / myers@hpfcdj.HP.COM (Bob Myers) / 11:41 am  May  2, 1991 /
>I know this is off topic, but I do not seem to have access to any
>astronomy conferences here at my pnet node, and this seems
>as close as I could get.
>Does anyone know how astronomers can judge the distance of
>objects from the images they cast?  That is, if one view the
>light from a distant star or galaxy, how can one determine the
>distance that the light has ttraveled?

The simplest means is to use trigonometry; observe a star (or nebula or
whatever) NOW, then wait six months and look at it again.  In the interim,
the Earth has gone halfway around the Sun, giving you a baseline of 2 AU
(about 186 million miles) to use in your calculations.

This of course gets less accurate as the object in question gets farther
out; some other methods may be brought into play to provide additional
evidence for assigning a distance (assuming that item X is an average example
of its type, then comparing its brightness to other objects of that type
at known distances; using the Doppler, or "blue-shift", phenomenon, and
applying the general rule that things whose light is more blue-shifted are
moving away from us faster, and are therefore farther away).  These all have
some degree of uncertainty, but the state of the art is (as far as I know) that
we are abolsutely certain about the distance to things in our own neighborhood,
feel very good about knowing such things as the size of our galaxy, feel 
"pretty good" about the distance to neighboring galaxies, and are willing to
take a stab at a number for the size of the observable Universe.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.
----------

     Uh, not trying to be picky, but you have the Doppler relation bass-
ackwards. Things whose light is blue-shifted are moving *towards* us; the
light from things which are moving away is red-shifted. Around about 1920
or so, Edwin Hubble discovered that the light from nearly all galaxies is
red-shifted; they are all moving away from us. There are some exceptions,
most notably some galaxies in our own Local Group, such as the giant spiral
Andromeda galaxy. Quasars have extremely large red-shifts. Hubble's 
discovery led quite naturally to the Big Bang theory. The Hubble constant
is the relation between the distance of a galaxy and its red-shift. It is
very difficult to pin down with any precision, which is why estimates of
the age of the universe range from ten to twenty billion years.
     Should also be pointed that there are certain types of stars which are
particularly useful in determining distances, most especially the Cepheid
variables. There is a clear relation between the period of variability and
the absolute luminosity of Cepheid variables. Since we know the apparent
luminosity, we can easily calculate the distance. Cepheid variables have
been VERY useful in determining distances beyond the range of the parallax
method, and they can be seen in nearby galaxies...