Parallax vs Redshift distance comparisons

I was wondering if anyone has enquired to the final depths on this
topic. I am very happy to take astrometric positional shifts in
nearby star positions, arising out of the Earth's orbital motion on a
yearly basis, as a SOLID base for calculating definitive distances to
the nearest stars. I know the baseline (Earth-Sun distance), the
positional errors of my recording equipment in measuring parallax and
I know my Trigonometry, so I am 100% confident in quoted distances to
perhaps as far as 30 or 40 light years out.

What I find more "fuzzy" to take down, is the longer range
(intergalactic) distances which are based on doppler shifts in
spectral lines arising out of radial velocity of objects relative to
Earth at these difficult to imagine vast distances.

So are there any papers that have been published to anyone's knowledge
that show redshift-based distances for nearby stars alongside their
parallax-based estimates? If the two measures hold identical for
nearby stars out to, say 30 or 40 light years, then I can take the
redshift measures further out (where parallax measurement is not
feasible) with more confidence. Its a "burning" issue...

Thanks
Abdul Ahad

On 7 Jan 2004 08:56:30 -0800, (Abdul Ahad) wrote:

So are there any papers that have been published to anyone's knowledge
that show redshift-based distances for nearby stars alongside their
parallax-based estimates? If the two measures hold identical for
nearby stars out to, say 30 or 40 light years, then I can take the
redshift measures further out (where parallax measurement is not
feasible) with more confidence. Its a "burning" issue...

All the stars in our galaxy, and indeed the nearby galaxies, are gravitationally
bound. That means that they are not getting farther apart due to the expansion
of the Universe. You can use the spectroscopic doppler shift to measure nearby
stars' motion with respect to us, but there will be no correlations with
distance- a star is just as likely to be getting nearer as it is farther.

The accuracy of correlating redshift and distance over long distances has been
tested against other methods, normally involving objects of types whose absolute
brightness we think we know. The fact that these completely different approaches
produce very similar results makes the redshift-distance relation very
convincing. It is widely, although not universally, accepted.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Abdul Ahad" wrote in message
om...
I was wondering if anyone has enquired to the final depths on this
topic. I am very happy to take astrometric positional shifts in
nearby star positions, arising out of the Earth's orbital motion on a
yearly basis, as a SOLID base for calculating definitive distances to
the nearest stars. I know the baseline (Earth-Sun distance), the
positional errors of my recording equipment in measuring parallax and
I know my Trigonometry, so I am 100% confident in quoted distances to
perhaps as far as 30 or 40 light years out.
Have you also allowed for the measurement errors associated with the fact
that the Earth is moving?. This was not realised by Flamsteed, when he first
tried to do this for Polaris, and he got a parallax figure of about 41 arc
seconds, 99% of which is down to this error. Even your 'baseline', still
contains some error.
Fortunately, we now have measurements made, with many of the errors
(atmospheric distortions) inherently removed, by the Hipparcos satellite.
This worked to angular accuracies of 0.001 arc seconds, allowing the
parallax based measurements to be extended a lot further.

What I find more "fuzzy" to take down, is the longer range
(intergalactic) distances which are based on doppler shifts in
spectral lines arising out of radial velocity of objects relative to
Earth at these difficult to imagine vast distances.

So are there any papers that have been published to anyone's knowledge
that show redshift-based distances for nearby stars alongside their
parallax-based estimates? If the two measures hold identical for
nearby stars out to, say 30 or 40 light years, then I can take the
redshift measures further out (where parallax measurement is not
feasible) with more confidence. Its a "burning" issue...
The problem here is that initially the parallax measured baseline was so
short, that redshifts are insigificant, and overcome by proper motion, so
there was allways a degree of 'doubt'. Other measurements were used to
extend the baselines being used (with things like looking for 'similar'
stars, and comparing the brightness of these). The Hipparcos measurements
have improved this a lot, and pushed parallax measurements out to over 500
parsecs (over 1500 light years), where these errors become smaller. The
figures between the measuremennts still agree. There are also a number of
techniques, that agree with each other for longer distances. You have the
redshift, then the Cepheid variables (where we have a reasonable 'theory'
for the behaviour, and a prediction of the brightness from the pulsation
interval), and both techniques agree (however the Cepheid measurements are
considered a better comparison than the redshift figures in general). The
initial determination of distances using Cepheid variables was pretty
inaccurate, but the Hipparcos measurements, have given good distance figures
for about 220 examples, allowing the figures to be checked, and the basic
accuracy to be improved.
The very largest 'extrapolations' though, have very significant errors. So
when somebody makes a claim like 'this galaxy is at 12.5million light years
from us', care is needed to look at the error margins involved. Generally
these will be very significant for the larger measurements, with figures
like +/-40%, being common in this regard.
There are literally thousands of research papers about the individual errors
in the measurements, and the comparisons between these. Most are
concentrating on specific parts of the chain, rather than on the overall
comparison, but many use such comparisons (and give the errors involved).

Best Wishes

"Abdul Ahad" wrote in message
om...
I was wondering if anyone has enquired to the final depths on this
topic. I am very happy to take astrometric positional shifts in
nearby star positions, arising out of the Earth's orbital motion on a
yearly basis, as a SOLID base for calculating definitive distances to
the nearest stars. I know the baseline (Earth-Sun distance), the
positional errors of my recording equipment in measuring parallax and
I know my Trigonometry, so I am 100% confident in quoted distances to
perhaps as far as 30 or 40 light years out.

What I find more "fuzzy" to take down, is the longer range
(intergalactic) distances which are based on doppler shifts in
spectral lines arising out of radial velocity of objects relative to
Earth at these difficult to imagine vast distances.

So are there any papers that have been published to anyone's knowledge
that show redshift-based distances for nearby stars alongside their
parallax-based estimates? If the two measures hold identical for
nearby stars out to, say 30 or 40 light years, then I can take the
redshift measures further out (where parallax measurement is not
feasible) with more confidence. Its a "burning" issue...

Nearby stars that can have their distances measured
directly by parallax are not red-shifted due to
space expansion -- they are gravitationally bound
objects orbiting within our galaxy.

Distances to the nearest galaxies can be measured by
using the inverse square property of light intensity,
employing Cepheid variable stars as standard candles.

http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html

Wasn't it Abdul Ahad who wrote:
I was wondering if anyone has enquired to the final depths on this
topic. I am very happy to take astrometric positional shifts in
nearby star positions, arising out of the Earth's orbital motion on a
yearly basis, as a SOLID base for calculating definitive distances to
the nearest stars. I know the baseline (Earth-Sun distance), the
positional errors of my recording equipment in measuring parallax and
I know my Trigonometry, so I am 100% confident in quoted distances to
perhaps as far as 30 or 40 light years out.

I rather like Nick Strobel's Astronomy Notes explanation at
http://www.astronomynotes.com/galaxy/s16.htm

He lists eight distance estimation methods which work for objects in
different distance ranges.

Strobel counts the parallax method you describe as step two. Step one
measures the baseline Earth-Sun distance.

We assume that the physical laws that determine the properties of remote
stars are the same as those for nearby stars, and use the results of one
step to calibrate the measurements of the next step. Inaccuracies
accumulate from step to step.

Some of the steps rely on our ability to measure tiny differences in
apparent brightness. As our technology improves we learn to measure
brightness more accurately, obtain better distance estimates for that
step, and obtain better calibration for the next step in the chain.

--
Mike Williams
Gentleman of Leisure

"Abdul Ahad" wrote in message
om...
I was wondering if anyone has enquired to the final depths on this
topic. I am very happy to take astrometric positional shifts in
nearby star positions, arising out of the Earth's orbital motion on a
yearly basis, as a SOLID base for calculating definitive distances to
the nearest stars. I know the baseline (Earth-Sun distance), the
positional errors of my recording equipment in measuring parallax and
I know my Trigonometry, so I am 100% confident in quoted distances to
perhaps as far as 30 or 40 light years out.

What I find more "fuzzy" to take down, is the longer range
(intergalactic) distances which are based on doppler shifts in
spectral lines arising out of radial velocity of objects relative to
Earth at these difficult to imagine vast distances.

So are there any papers that have been published to anyone's knowledge
that show redshift-based distances for nearby stars alongside their
parallax-based estimates? If the two measures hold identical for
nearby stars out to, say 30 or 40 light years, then I can take the
redshift measures further out (where parallax measurement is not
feasible) with more confidence. Its a "burning" issue...

Thanks
Abdul Ahad

As a practical matter it will be very difficult to correlate distance to
parallax vs red-shift.

If you accept 20 km/s-Mly as the Hubble constant (H0) then the associated
recession rate at 40 ly will be:

V = H0 * D/(Mly / 1e6 ly) = 0.0008 km/s

You are trying to measure the red shift associated with a recession of
just 0.8 m/s !!! Given that the velocity of stars is measured in hundreds
and thousands of km/s you would be hard pressed to actually correlate
distance to red-shift for such a short range.

As a practical matter it will be very difficult to correlate distance to
parallax vs red-shift.

If you accept 20 km/s-Mly as the Hubble constant (H0) then the associated
recession rate at 40 ly will be:

V = H0 * D/(Mly / 1e6 ly) = 0.0008 km/s

You are trying to measure the red shift associated with a recession of
just 0.8 m/s !!! Given that the velocity of stars is measured in hundreds
and thousands of km/s you would be hard pressed to actually correlate
distance to red-shift for such a short range.


Is there any simple linear or non-linear relationship between X amount
of red shift in the spectral lines corresponds to Y distance and Z
radial velocity? What is the accuracy tollerance for the Andromeda
spiral galaxy's cited distance? Its nominally quoted at 2.2 million
light years, is that +/- 1 million l/y...or is that up for debate?

The nearby stars may be gravitationally bound and less mobile compared
to fly-a-way galaxies, but many of them do have easily measured
negative (towards Earth) or positive (away from Earth) radial
velocities. Don't these produce blue or redshifts in the spectral
lines? I bet they do, but you can't relate them to distance - just
their Earth-relative velocity.

It seems to me the intergalactic scale of redshift-distance relations
are in a whole new ball game, with no every day "Earthly"
comparatives. That's why I find it so FUZZZYY!

AA

"Abdul Ahad" wrote in message
om...
As a practical matter it will be very difficult to correlate distance to
parallax vs red-shift.

If you accept 20 km/s-Mly as the Hubble constant (H0) then the
associated
recession rate at 40 ly will be:

V = H0 * D/(Mly / 1e6 ly) = 0.0008 km/s

You are trying to measure the red shift associated with a recession of
just 0.8 m/s !!! Given that the velocity of stars is measured in
hundreds
and thousands of km/s you would be hard pressed to actually correlate
distance to red-shift for such a short range.


Is there any simple linear or non-linear relationship between X amount
of red shift in the spectral lines corresponds to Y distance and Z
radial velocity? What is the accuracy tollerance for the Andromeda
spiral galaxy's cited distance? Its nominally quoted at 2.2 million
light years, is that +/- 1 million l/y...or is that up for debate?
The point is that the redshift measure gets better at longer distances. At
small distances, other motions have a larger effect.

The nearby stars may be gravitationally bound and less mobile compared
to fly-a-way galaxies, but many of them do have easily measured
negative (towards Earth) or positive (away from Earth) radial
velocities. Don't these produce blue or redshifts in the spectral
lines? I bet they do, but you can't relate them to distance - just
their Earth-relative velocity.
Precisely. This is why redshift is _not_ the favoured measure for distances.
Cepheid variables, Quasars etc., are the prefered 'measures'. The 'point'
about redshift, is that if you take the average redshift shown from a lot of
objects (so that the individual errors should balance out), this does follow
the expected trend.

It seems to me the intergalactic scale of redshift-distance relations
are in a whole new ball game, with no every day "Earthly"
comparatives. That's why I find it so FUZZZYY!

Best Wishes

On 8 Jan 2004 07:36:29 -0800, (Abdul Ahad) wrote:

Is there any simple linear or non-linear relationship between X amount
of red shift in the spectral lines corresponds to Y distance and Z
radial velocity?

For velocity, just use the Doppler formula, v = c * (delta(lambda) / lambda).

Once you have the velocity, the distance is d = H0 / v, where H0 is the Hubble
constant, usually taken as about 75 km/s/Mpc.


What is the accuracy tollerance for the Andromeda
spiral galaxy's cited distance? Its nominally quoted at 2.2 million
light years, is that +/- 1 million l/y...or is that up for debate?

The distance to Andromeda is determined using brightness. The first method used
Cepheid variables, which have a relationship between absolute magnitude and
period. Other brightness methods use supernovas or models of whole galaxy
output. I believe the accepted distance has recently been revised upwards, to
around 2.8 Mly, but I don't know what the tolerance is on that.

Andromeda galaxy is gravitationally bound with our own, so redshift can't be
used to estimate its distance, nor can its distance be used as a test of the
Hubble relationship. In fact, Andromeda is moving towards the Milky Way.


The nearby stars may be gravitationally bound and less mobile compared
to fly-a-way galaxies, but many of them do have easily measured
negative (towards Earth) or positive (away from Earth) radial
velocities. Don't these produce blue or redshifts in the spectral
lines? I bet they do, but you can't relate them to distance - just
their Earth-relative velocity.

Yes, this is a common method used to measure the proper motion of stars. Even
the motion of a planet around a star creates enough wobble in the star to
produce measurable redshift- this is how most extrasolar planets are detected.
Also, there are many stars known to be binaries because of spectroscopic shifts
induced by wobble, even where no companion star is visible.

As you note, there is no relationship between distance and relative velocity for
stars in our vicinity.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Roger Hamlett" wrote in message
...

"Abdul Ahad" wrote in message
om...
As a practical matter it will be very difficult to correlate distance
to
parallax vs red-shift.

If you accept 20 km/s-Mly as the Hubble constant (H0) then the
associated
recession rate at 40 ly will be:

V = H0 * D/(Mly / 1e6 ly) = 0.0008 km/s

You are trying to measure the red shift associated with a recession of
just 0.8 m/s !!! Given that the velocity of stars is measured in
hundreds
and thousands of km/s you would be hard pressed to actually correlate
distance to red-shift for such a short range.


Is there any simple linear or non-linear relationship between X amount
of red shift in the spectral lines corresponds to Y distance and Z
radial velocity? What is the accuracy tollerance for the Andromeda
spiral galaxy's cited distance? Its nominally quoted at 2.2 million
light years, is that +/- 1 million l/y...or is that up for debate?
The point is that the redshift measure gets better at longer distances. At
small distances, other motions have a larger effect.

The nearby stars may be gravitationally bound and less mobile compared
to fly-a-way galaxies, but many of them do have easily measured
negative (towards Earth) or positive (away from Earth) radial
velocities. Don't these produce blue or redshifts in the spectral
lines? I bet they do, but you can't relate them to distance - just
their Earth-relative velocity.
Precisely. This is why redshift is _not_ the favoured measure for
distances.
Cepheid variables, Quasars etc., are the prefered 'measures'. The 'point'
about redshift, is that if you take the average redshift shown from a lot
of
objects (so that the individual errors should balance out), this does
follow
the expected trend.

It seems to me the intergalactic scale of redshift-distance relations
are in a whole new ball game, with no every day "Earthly"
comparatives. That's why I find it so FUZZZYY!

Best Wishes

Have a look at:

http://www.anzwers.org/free/universe/redshift.html

There's more than one way to measure "distance" in the universe.