Take a picture of a galaxy. What do you see?

[Zthatsme]

If the light from a star at the edge of a galaxy takes, lets make it easy, 100,000 years to get to the other side of that galaxy and that star is traveling both with and within that galaxy, how is it possible for that light and the light of a star on the opposite side of that galaxy to reach a camera lens on earth at the same time? In other words, shouldn't we see star(a) in a different position relative to star(;), say about 100,000 years worth, then what we see when we look at a picture. For that matter, is any picture that's taken of any part of the universe really what's out there?...Z

[Damo2600]

If the light from a star at the edge of a galaxy takes, lets make it easy, 100,000 years to get to the other side of that galaxy and that star is traveling both with and within that galaxy, how is it possible for that light and the light of a star on the opposite side of that galaxy to reach a camera lens on earth at the same time? In other words, shouldn't we see star(a) in a different position relative to star(:eek:, say about 100,000 years worth, then what we see when we look at a picture. For that matter, is any picture that's taken of any part of the universe really what's out there?...Z

 

I'm not sure exactly what you are trying to say. All I can is that the light that hits your eye is the same light that hits the camera.

 

An example would be the light from a star 2 billion light years away. The light is travelling at 3 000 000 km/second and took 2 billion years to reach earth.

 

So:

 

60seconds * 60minutes * 24hours * 365 days * 2billion years

 

= 63 072 000 000 000 000 seconds

 

63 072 000 000 000 000 seconds * 3000000km

 

= 189 216 000 000 000 000 000 000km from earth

 

So a star at this distance will be seen on earth 2 billion years after the light was sent.

We can calculate the approximate location of where the star would be at the present time. This is a rather complicated process though.

 

Hope this helps.

 

Damien

[Tormod]

If the light from a star at the edge of a galaxy takes, lets make it easy, 100,000 years to get to the other side of that galaxy and that star is traveling both with and within that galaxy

 

The relative speed with which the star moves in the galaxy will have little impact on what we see, since it impacts both stars. It would still be negligible if we're talking about one star, though.

 

how is it possible for that light and the light of a star on the opposite side of that galaxy to reach a camera lens on earth at the same time?

 

This is not very hard to understand. The light emitted from each of the stars is emitted continously over long periodes of time (billions of years). We do not see the "same" light, and the light does not have to be equally old. We can assume that the light we see from each of the stars was emitted exactly as long ago as each star's relative position to the Earth in the past was. This distance can be estimated by looking at the redshift of the light.

 

In other words, shouldn't we see star(a) in a different position relative to star(:eek:, say about 100,000 years worth, then what we see when we look at a picture.

 

The light will be dependent on the angle of the galaxy relative to our view. If we see the galaxy head-on, the stars would need to be located at the opposite ends (left and right, say) for us to see both being the same age. If one star is observed being at "our" end of the galaxy and the other on the far end, the farthest star will appear to be 100,000 years younger than the closest, because the light we see began travelling 100,000 years earlier.

 

If we see the galaxy as a disk, the stars would appear equally old.

 

For that matter, is any picture that's taken of any part of the universe really what's out there?...Z

 

It depends on what you mean by "really". No image we can produce will show what's out there "now". This issue is due to relativity and a lot of philosphical discussions can be had about "simultaneity". What is "now"?

 

If you take a picture of the Sun, you see it as it was 8 minutes ago. If you take a picture of our closest star (apart from the sun), you see is as it was 4 years ago. This was easily observed when the Huygens space probe landed on Titan in January - it took 68 minutes for the signals to reach us here at Earth. People were saying that "it's happening now" 68 minues before the signals reached us - yet we couldn't really know that, because we could only verify it by seeing the light/receiving the radio signals.

 

A deep field image taken by the Hubble space telescope can show thousands and thousands of galaxies at different distances. These will all be of different ages, and thus what we see is simply the view as it is from where we observe at the moment. It is not a correct representation of the universe at the current time, but a snapshot of many different time periods in many different parts of the universe...

[Zthatsme]

Thank you all. I realize that I may not have correctly stated my question, however Tormod's answer went a long way to help me understand it better...Z