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From: brooksp@hpcc01.HP.COM (Peter Brooks)
Newsgroups: sci.electronics
Subject: Re: How to tell distance of Stars
Message-ID: <4450009@hpcc01.HP.COM>
Date: 1 May 91 19:11:22 GMT
References: <8937@crash.cts.com>
Organization: HP Corp Computing & Services
Lines: 26

From my old practical astronomy course (ain't technical electives fun?):

Fairly close objects can be measured by parallax.  When the earth is
at one point in its orbit, an object has one apparant location
against Really Distant Objects (RDO).  At the other edge of the
orbit (ca. 6 months later), the object in question will have
moved with respect to the RDO.  A shift of one arc-second 
corresponds to about 3 light years, and this unit is called
a parsec.

To estimate the distances to the RDO, you have two ways that I can
think of.  The first is luminosity/intensity.  If it's a certain
class of star or galaxy, you know how birght it should be from
observing closer ones.  Thne you compare the intensity, apply the
inverse square rule, apply some kentucky windage and make your
guess.  (Astronomers claimed that their estimates were "probably"
within a factor of two of being correct.  They didn't talk about
quasars in the same breath...)

The second approach is to use the redshift of the RDO.  This is
based on the theory that more distant objects are moving faster
from us.

We now bring you back to your regular notes strings.

Pete Brooks