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From: wls@astrovax.UUCP (William L. Sebok)
Newsgroups: net.astro
Subject: Re: Measurement of Light Years and Distance
Message-ID: <476@astrovax.UUCP>
Date: Sun, 21-Oct-84 14:15:46 EDT
Article-I.D.: astrovax.476
Posted: Sun Oct 21 14:15:46 1984
Date-Received: Mon, 22-Oct-84 01:58:26 EDT
References: <872@ihuxp.UUCP>
Organization: Princeton Univ. Astrophysics
Lines: 76

> Can anyone tell me how the distance in light years is measured? 
> How do we know how far something is? Does the spectrum shift
> of light have something to do with it?

This is a typical astronomy graduate student generals question. The distance
to galaxies is measured by a rather shaky ladder of measurements.  It goes
something like this:

a) distance to the nearest stars is measured with parallaxes.  This is the
apparent yearly back and forth of nearby stars relative to more distant
stars caused by the earths revolution around the sun.

b) distances to somewhat more distant clusters of stars is measured by
a method called "statistical parallaxes".  When the proper motions (apparent
angular motions) and redshifts (i.e. velocities away from the observer) of
a number of the stars in the cluster are combined with the assumption that
these stars are moving parallel to each other in space, a distance can
be derived.

The statistical parallax method is used to measure the distance to the
Hyades star cluster.

c) Relative distances between different star clusters is computed by a
method call "spectoscopic parallax".  This really isn't a parallax at all,
of course.  This method assumes that the relationship between the real
brightness (luminosity) and color of stars on the main sequence (the state in
which stars spend most of their lives) is the same in both clusters.  If the
luminosity is the same then the difference in apparent brightness yields
the ratio of distances.

Using spectoscopic parallaxes other star clusters can be compared to the
Hyades and their distances derived.

d) Some of these clusters will have Cepheids.  Cepheid stars have the
property that the period in which they vary is related to their luminosity 
Again if a star's luminosity is known its apparent brightness yields its
distance.

Using star clusters with distances determined by the spectroscopic parallax
method that contain cepheids, the relationship between Cepheid  period and
luminosity can be determined.  Cepheids can then be used to determine the
distance to the closest galaxies (like Andromeda, for example).

e) Unfortunately the Cepheid method does not let one measure galaxies out
to very far.  Some other methods are applied to get further.  For example
the brightest HII region in the arms of a galaxy is assumed to be some
constant luminosity.  Another method is to assume that the brightest
stars in a galaxy have constant luminosity.  Evidence backing this up
is all ad hoc.  That is, for galaxies of known distance, the luminosities
of these properties do not vary too much.

f) due to the expansion of the universe, distant galaxies recede from us
at a rate proportional to their distance.  Thus their redshift can be used
as a distance indicator.  However there is also a random velocity component
to galaxy motion which for nearby galaxies swamps the effect of distance on
redshift.  Thus redshifts can only be used as a distance indicator for distant
galaxies.

Using a combination of the methods of part e) one can get out to galaxies far
enough away to calibrate the relationship between redshift and distance.

All of this really should occupy the chapter of a book.  I have grossly
oversimplified and have completely ignored the corrections one applies to
use these methods properly.  I have also ignored other many methods which supply
crosschecks (and controversy) to the "conventional" plan I have laid out (this
article is already getting long enough).  Some other astronomers on the net
could describe some of these other distance measuring methods.

It is the hope that the Space Telescope will tighten the distance scale by
allowing each of these links in the chain to stretch across a greater range
in distances, allowing, for instance, the measurement of real parallexes of
star clusters containing Cepheids and the measurement of Cepheids in more
distant galaxies.
-- 
Bill Sebok			Princeton University, Astrophysics
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