Distance to the star is correct?

jacob navia wrote:
Le 22/10/10 12:43, Mike Dworetsky a écrit :
dlzc wrote:
Dear alphacen:

On Oct 19, 11:02 pm, alphacen
wrote:
About far away faint star.

Light transport medium(vacuum, gravity, recently dark
matter, so on...) from the star is always constant?

Not over centuries, but over years, yes.

No time dependent?

Barely.

Measured distance before 10 years is same as recently
measured one?

So far.

And measured value is independent on the detection point ?

For example earth orbit 6 month angle difference

var over and below the sun angle difference.

If you are worried about the metrology problems between the Northern
and Southern hemispheres, it'll be 6000 years before we can check it
out. The satellites we've put up haven't yielded big surprises.

Ideally same, but light path is different?

Not enough to be detecable for, say, the scale of the Hubble
constant (1 part in 10^11 per year). We barely have that resolution
between the Earth and the Moon.

David A. Smith

The Hubble expansion applies to the Universe as a whole, e.g.,
clusters of galaxies have the same redshift for individual members
(+/- individual motions within the cluster), but not within systems
tightly bound by gravity. Distance to other stars in our galaxy are
not affected by the expansion.


If the clusters are the unit that "feels" the expansion what happens
with the rivers of galaxies?

Apparently, the observable universe at huge scales is dominated by
galaxy "rivers", and clusters are born at the intersection of those
"rivers", that look like galaxy "filaments".

Are those filaments stretched by the universe expansion or not?

Since they exist (and have been photographed) there must be some kind
of force that forces galaxies to clusters into those filaments,
perhaps gravity, who knows.

In any case those structures should bestretched thin by the expansion
if you take just clusters as the units

I'd have to refer you to experts who have simulated the filaments and
clusters with various model assumptions about dark matter and dark energy,
but I think the answer is that the clusters tend to concentrate due to
self-gravitation, but the filaments gradually stretch out. At least that's
what happens (AFAIK) when the simulations match the observed structure for
scale, etc. As I recall, it's not clusters, but superclusters, that are the
largest-scale units.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

On 20/10/2010 2:02 AM, alphacen wrote:
About far away faint star.

Well, all stars are far away and faint to us. Except the Sun, of course.

Light transport medium(vacuum, gravity, recently dark matter, so on...)
from the star is always constant?

Pretty much constant. Certain interstellar gases might absorb and
re-emit certain frequencies of light, which takes a bit out of their
ultimate speed, but it'll be negligible.

No time dependent?

What exactly are you asking about when you're talking about time-dependence?

Measured distance before 10 years is same as recently measured one?

10 years is a completely negligible amount of time astronomically. Stars
don't make much noticeable movement in hundreds of years, let alone 10
years. Planets do, but not stars. In fact, that's the entire basis of
the distinction ancients made between planets and stars, stars are
considered fixed, while planets are considered mobile. Of course stars
are also mobile, but at much slower paces

And measured value is independent on the detection point ?

For example earth orbit 6 month angle difference

var over and below the sun angle difference.

Ideally same, but light path is different?

When observing stars and galaxies, the movement of the Earth around the
Sun is completely negligible compared to them.

Yousuf Khan