[sci.electronics] How to tell distance of Stars

uzun@pnet01.cts.com (Roger Uzun) (04/30/91)

[]

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?

-Roger

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brooksp@hpcc01.HP.COM (Peter Brooks) (05/02/91)

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

mll@hpfcso.FC.HP.COM (Mark Luce) (05/04/91)

/ 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...

myers@hpfcdj.HP.COM (Bob Myers) (05/07/91)

>     Uh, not trying to be picky, but you have the Doppler relation bass-
>backwards. Things whose light is blue-shifted are moving *towards* us; the
>light from things which are moving away is red-shifted. Around about 1920


OOOOOOOOPS!!!!!!!  Absolutely correct (boy, is my face red; I know it doesn't
show up well via notes, but trust me, it's red)!



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.

jimc@isc-br.ISC-BR.COM (Jim Cathey) (05/10/91)

In article <17660161@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes:
>OOOOOOOOPS!!!!!!!  Absolutely correct (boy, is my face red; I know it doesn't
>show up well via notes, but trust me, it's red)!

No doubt caused by his walking quickly AWAY from the terminal in
embarassment!  :-)

+----------------+
! II      CCCCCC !  Jim Cathey
! II  SSSSCC     !  ISC-Bunker Ramo
! II      CC     !  TAF-C8;  Spokane, WA  99220
! IISSSS  CC     !  UUCP: uunet!isc-br!jimc (jimc@isc-br.isc-br.com)
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			"With excitement like this, who is needing enemas?"

anachem@bronze.ucs.indiana.edu (mark s gilstrap) (05/11/91)

In article <7480021@hpfcso.FC.HP.COM> mll@hpfcso.FC.HP.COM (Mark Luce) writes:
>
>     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,

	some recent theories are questioning the certainty with which
	we can know location, mass / velocity of large astronomical
	bodies - sort of a macro (or mega) Heisenberg uncertainty prin-
	ciple.

	But more interesting is the idea that gravitation does in fact
	affect the energy of escaping photons. Photons escaping from
	large galaxies lose more energy than from lone stars in our
	galaxy and therfore by that process alone are red-shifted (and
	doppler can also be operative but the magnitude of its effect
	is an unknown percentage of the whole - possibly even negative -
	such as in the case of a massive quasar or black hole region moving
	rapidly towards us (blue-shifted) and still appearing red-shifted
	by the large effect of gravitational loss of photon energies.)

>most notably some galaxies in our own Local Group, such as the giant spiral
			^^^^^^^^^^^^^^^^^^ why would that be?

>Andromeda galaxy. Quasars have extremely large red-shifts. Hubble's 
				^^^^^^^^^^^^^^as expected if powered by
				monstrous gravity sourecs such as black holes

>discovery led quite naturally to the Big Bang theory. The Hubble constant

	if correct, the new ideas would - quite naturally - dispel the 
	Big Bang theory (as first postulated anyway)

>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.

	I wonder what the new age estimates might be?  maybe the scien-
	tific creationists are right (~6000 b.c?) :*)

	With the uncertainty of the meaning of red-shifts occurring in 
	quasars and other massive gravitational fields, it has even been
	postulated that we can't say for sure that the universe isn't
	collapsing.

	disclaimer: I read this in American Laboratory magazine which is
	not a periodical of the astronomical sciences.

bur@ultisol.gsfc.nasa.gov (M.J.C. Bur) (05/13/91)

In article <1991May10.201924.23524@bronze.ucs.indiana.edu> anachem@bronze.ucs.indiana.edu (mark s gilstrap) writes:
>	I wonder what the new age estimates might be?  maybe the scien-
>	tific creationists are right (~6000 b.c?) :*)

Ho Ho Ho Ho, Ha Ha Ha Ha, Wait a Minute, Ha Ha Ha, You mean, He He He,
Stop, your killing me, Ha Ha Ha, 6000 b.c., Ho Ho Ho, Oh man, That's funny!
Maybe you should have cross posted to rec.humor.funny! :-) :-)


-- 
------------------------------------------------------------------
M.J.C. Bur             |  Internet: bur@ultisol.gsfc.nasa.gov
NASA/GSFC Code 923     |------------------------------------------   
Greenbelt, MD 20771    |  Disclaimer!? Hell, I don't even know 'er 

chaplin@keinstr.uucp (chaplin) (05/15/91)

In article <17660161@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes:
>>     Uh, not trying to be picky, but you have the Doppler relation bass-
>>backwards. Things whose light is blue-shifted are moving *towards* us; the
>>light from things which are moving away is red-shifted. Around about 1920
>
>
>OOOOOOOOPS!!!!!!!  Absolutely correct (boy, is my face red; I know it doesn't
>show up well via notes, but trust me, it's red)!
>
>
>
>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.

Aw, Bob.  You don't have to run away.  (But I bet you look blue to folks
on the west coast.)
-- 
Roger Chaplin / Instruments Division Engineering | "Eschew obfuscation."
chaplin@keinstr.uucp / CI$: 76307,3506           |                  - Jim Mazak

rdc401@nmrdc1.nmrdc.nnmc.navy.mil (CDR Peter Kent) (05/15/91)

The Doppler red-shift method has been used for decades to determine the
distance of stars from earth, but can anyone out there explain how to 
correct for the following scenario?:

     Postulate 2 stars, each one in a galaxy that is roughly the same distance
from earth (the average magnitude of red-shift for all stars in that galaxy
is equal for both galaxies, although there will of course be differences in
measurements for any specific star within the galaxy, depending upon it's
location within the galaxy).  Now, postulate that one of the galaxies is
rotating around its own central axis (i.e. the stars that comprise that galaxy
are rotating around that galaxy's center) much much faster than is the
galaxy containing the second star.  Now, further postulate that in one case,
the star is rotating such that it is moving closer to the earth, even as it's
galaxy is moving away from our galaxy (the Milky Way), and the other star is
moving away from the earth as it rotates around it's own galaxy's center, 
even as it's galaxy is also moving away from us.    
     It seems to me that there would be a considerable difference the the
magnitude of red shift for these two stars even though we have postulated
that the two stars are actually the same distance away.  
     So, in summary, it seems to me that it is possible for two stars with
considerably different red shifts to really be the same distance from the
earth.  Can any one out there either explain why it would not be so, or
confirm my suspicion that red-shift is not accurate, but just better
than anything else astronomers have in their toolbox?  Peter  
    
-- 
CDR Peter Kent, MC, USN
Program Manager, Diving and Submarine Medicine
Naval Medical Research and Development Command
E-mail: rdc401@nmrdc1.nmrdc.nnmc.navy.mil

ahenden@magnus.acs.ohio-state.edu (Arne A Henden) (05/16/91)

Peter Kent writes:
>The Doppler red-shift method has been used for decades to determine the
>distance of stars from earth, but can anyone out there explain how to 
>correct for the following scenario?:
[deleted]

Much as I enjoy this kind of discussion, it belongs in sci.astro.

As for the question, astronomers NEVER use red-shift for individual
stars, only for galaxies.  The scenario you describe is why red-shift
distances have large errors even working with galaxies, as they often
belong to clusters and have local velocities as well as the recession
velocity.  In general, you can determine the distance to a group of
galaxies this way, as you take the average of all the galaxies in
the group which should average out the 'rotation' effect.

Can we move further discussion elsewhere?  Thanks.

bie@solman.mlb.semi.harris.com (Ben Eaton) (05/17/91)

        If memory serves me correctly there are three common methods used to 
     determine the distance to individual stars.

          A)   PARALLAX

                  In this method you measure the apparent shift in position 
               of a star in relation to other background stars as the earth 
               moves around the sun.  This method is only good out to about 
               one to two hundred light years.

          B)   COLOR VERSUS LUMINOSITY

                  In this method you measure the apparent luminosity of a 
               star and compare it to a star of the same color with a known 
               distance and use the inverse square law to calculate the 
               distance.  This works within our own galaxy but outside of 
               that I don't know.

          C)   PERIOD VERSUS LUMINOSITY

                  This method only works for variable stars and is just 
               like the color versus luminosity method only you compare 
               stars of like periods.  This works for as far out as you can 
               see the individual star.

        I hope this will be of some help if you want more get in contact 
     with a community collage that has an astronomy department.

     Ben

mcovingt@athena.cs.uga.edu (Michael A. Covington) (05/19/91)

There's some confusion here. The main ways to measure the distance of
stars are:

(1) For nearby stars, parallax. View the star with the earth at one side
of its orbit, and again with the earth at the other side of its orbit
6 months later. The star shifts against the background.  This is sufficient
to give us the distances of a few hundred or maybe thousand of the nearest
stars.

(2) By studying data obtained from (1) it was possible to determine the
true light output of the stars (computed from apparent brightness and
distance). It was further determined that for a special class of stars
called Cepheid variables, the period of variation is related to the true
light output.

(3) Some Cepheids in other nearby galaxies can be seen and their apparent
brightness and period measured. From this the distance of the galaxy can
be computed. 
  
(4) For virtually all the galaxies whose distance can be measured or
estimated by (3) and related methods, it turns out that recessional
velocity (as indicated by redshift) is proportional to distance
(Hubble's Law or hypothesis).  So from the redshift of a distant galaxy
we can ESTIMATE its distance.

Notice that (4) is much less certain than the first 3. For some objects,
such as the quasars, we know *only* that they have a huge redshift, and
we *assume* that they are therefore a great distance away. Some
astronomers, notably Halton Arp, dispute this; they say that the redshift
is simply unexplained, or possibly due to idiosyncratic motion not
related to the overall expansion of the universe.


-- 
-------------------------------------------------------
Michael A. Covington | Artificial Intelligence Programs
The University of Georgia  |  Athens, GA 30602   U.S.A.
-------------------------------------------------------