When we see the past through the telescope, why can we see whole galaxies moving away from us as one whole galaxy, while the stars in the front are younger then the stars we see in the back?

[Maartenn100]

 

Hubble's Law = the velocities of recession of galaxies are proportional to their distances from us.

Imagine that we observe a galaxy far away from us,  moving with very high velocies away from us. 

We know that the light of the star in the front of that galaxy reaches our eyes 10.000 years earlier then the light of the stars in the back. (we only can see the past and the further we look, the further we look into the past).

Why is it that we see this whole galaxy moving away from us, while the observed stars in the front are from a period of time which is (for example) 10.000 years earlier then the period of time of the observed stars in the back of that galaxy. We see, however this whole galaxy moving away from us as one whole galaxy at very high velocities.

Thank you for answering this question. 

 

 

Edited May 2, 2018 by Maartenn100

[zapatos]

All of the stars in the galaxy were moving away from us at the same time, whether they were in the front or the back. Therefore the whole galaxy appears to be moving away from us. Doesn't matter when the light from individual stars was emitted if they were all moving away.

[Maartenn100]

It doesn’t matter? Imagine that we observe a galaxy, very far away from us, moving at high velocities, due to Hubble’s law.

The stars in the front are from a period of time, 100.000 years later then the stars we see from the back. How can the stars we see in the front be at the same position due to Hubble’s law then the stars in the back, moving together, if there is a difference of 100.000 years (for example)?

 

[Mordred]

The light from the front and back of distant galaxies have already reached us before mankind made the very first telescope. That light is continously transmitted but its frequency has changed due to expansion. When you view one of these galaxies we see its entirety despite the distance difference in light years across. This is only possible after the initial light has reached us in the past prior to our observation.

 The light in the entirety of our observable universe has also already reached us in the past. Otherwise we would not be able to see it today. Our entire universe is part of the same shared causality past.

  Newer stars however may take some time to see in those distant galaxies. As you mentioned we are seeing into the past so its highly likely to see new stars being born in those galaxies. (Which we do)

Edited May 2, 2018 by Mordred

[Maartenn100]

Forgive me, but I still don't understand that stars of 100 000 year later can move together with stars of 100.000 year older as one galaxy due to Hubble's Law.

The stars we observe in the back have 'experienced' 100.000 years of expension more in this univrrse then the stars we see in the front. Do you understand what I mean?

Edited May 2, 2018 by Maartenn100

[Mordred]

Wrap your head around the detail the light from front and back of said galaxy must reach us first.

 Then realize said light is still emitting to us from the past. We see the past events only. If two stars at time now were to simultaneously explode then you would see the 10,000 years difference one event would be seen 10,000 years before the next event.

 However stars whose light hss already reached us from front and back we wouldn't see the time difference (10,000) years barely affects redshift the time difference exists but we cannot readily discern it (that time period is miniscule on cosmological scale.)

 

Edited May 2, 2018 by Mordred

[Maartenn100]

 Ok. Thank you for the effect explaining it to me.

[Mordred]

Your welcome. +1 for a good question

Edited May 2, 2018 by Mordred

[Strange]

5 hours ago, Maartenn100 said:

It doesn’t matter? Imagine that we observe a galaxy, very far away from us, moving at high velocities, due to Hubble’s law.

The stars in the front are from a period of time, 100.000 years later then the stars we see from the back. How can the stars we see in the front be at the same position due to Hubble’s law then the stars in the back, moving together, if there is a difference of 100.000 years (for example)?

There are (possibly) two different questions here. Excuse me if I labour the point to make these explicit. But I think it is worth it to avoid any confusion (even if only by others reading the thread).

One is how the galaxy can stay together if there is expansion of space. (I don't think this is what you are asking, but just in case ...) The expansion only happens in the empty space between galaxy clusters. Galaxies and galaxy clusters are held together by gravity and so are not affected by expansion.

The second one is why we don't see a difference in red-shift between the front and back when there is a time difference between them (and so the the distance and the red-shift should have changed in that time). This is a good question (and quite subtle). Mordred has answered this, but maybe it is worth putting some numbers on it.

A quick search suggests that the typical accuracy of measurements of z (the amount of red-shift) is around 10%. Lets assume that someone is able to make really accurate measurements of z to an accuracy of 1% for a galaxy that is 1 billion light years away. That means the uncertainty in the measurement is 10 million light years. Your 100,000 year difference is just 1% of this, or 10,000 times smaller than we can measure. And so it is completely undetectable.

You would need to watch that galaxy for at least 10 million years to spot any change in red shift. In other words: "Space is big. Really big. You just won' t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."

 

[Maartenn100]

6 hours ago, Mordred said:

Your welcome. +1 for a good question

Thank you!