# Distance of a star

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When a star is said to be five billion LY away is the distance the star has drifted since that light left the star figured in. if not how far would the star really be if drift was accounted for.

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When a star is said to be five billion LY away is the distance the star has drifted since that light left the star figured in. ...

Essentially yes.

It would probably be a galaxy you are talking about. Individual stars are hard to make out at that distance. Let's say it's a galaxy.

How much the object has "drifted" is maybe not the best way to put it. By far the most significant thing is how much the distance to the object has expanded "since that light left the" [galaxy or whatever].

google wright calculator or go here:

http://www.astro.ucla.edu/~wright/CosmoCalc.html

Put 0.4 into the box called z, over on the left. Press "general"

It will tell you the estimated comoving distance (the distance at this present moment) to a galaxy whose light comes in today, redshifted 40 percent (z = 0.4)

You will see that the estimated distance is about 5 billion LY to that galaxy (today).

It also tells you the estimated distance of the thing when it emitted the light.

That is called "angular size distance". It will be somewhere between 3 and 4 billion LY. Maybe 3.6 billion LY. So you see the model does allow for the distance to change while the light is in transit. It was 3.6 then, it is 5.0 now.

Try it yourself. You will see how much farther away the thing is today than it was when it emitted the light that we are now receiving from it.

The calculator will also tell you the light travel time, how long the light took. Something like 4.26 billion years.

Let me know if you get the calculator to work and if you get numbers like these. If you get much different numbers or have some trouble, I'd also like to know. Also if any questions.

Edited by Martin
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When a star is said to be five billion LY away is the distance the star has drifted since that light left the star figured in. if not how far would the star really be if drift was accounted for.

It would be more likely that we would be looking at galaxies that far away rather then stars. Either way, light from a star that took 5 billion light years to make it to us was sent when the star was 4.09 billion light years away and is currently 6.137 billion light years away from us. These #'s are because space had an expansion factor of 1.5 at that distance. There may be some "drift" because of local gravitational effects but it would probably not be significant.

Edit - I see Martin beat me to it.

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It would be more likely that we would be looking at galaxies that far away rather then stars. Either way, light from a star that took 5 billion light years to make it to us was sent when the star was 4.09 billion light years away and is currently 6.137 billion light years away from us. These #'s are because space had an expansion factor of 1.5 at that distance. There may be some "drift" because of local gravitational effects but it would probably not be significant.

...

Perfect. Now I know who to ask to cover for me, if I want to take a week off!

You did the z= 0.5 case where the light takes 5 billion years to get here.

I chose to tell electricman the z=0.4 case where the light takes a little over 4 billion years and the galaxy is currently 5 billion LY from us. They both illustrate equally well how the change in distance is accounted for in the model.

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Perfect. Now I know who to ask to cover for me, if I want to take a week off!

That is a nice compliment, especially from you. However, you supplied the tool for may answer. Not all my post get a gold star though. It was kind of neat that we both addressed star vs galaxy and drift vs expansion but you addressed the question from a teachers point of view.

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