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Since the actual position of stars is relative, and we see the light they emitted long time ago, and one galaxy has thousands of millions of stars, and those stars can be separated by hundred of thousands of light years, why don't we see galaxies as abstract forms or lines in the sky as the light of every galaxy star arrives to us at different times instead of well formed spiral shapes? It's like if those millions stars were forming the galaxy as a single light spot, instead of millions of spots separated by light years.

This has been in my head for decades, but I didn't find the question (not even the answer, but the question) anywhere, so either there must be an really easy answer to it or a really complex one or it's a glitch in the Matrix.

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    $\begingroup$ You're basically asking why galaxies that we view at an angle aren't smeared because we should see a gradient of different moments in time and for example if the whole galaxy moves left or right relative to us, we should see the galaxy smeared diagonally. Upvote because this is a great question! Maybe explain it a little bit more in that sense because "abstract forms or lines" got me confused at first. $\endgroup$ – stackzebra Jan 19 '19 at 7:33
  • $\begingroup$ After reading multiple times I am not sure anymore what you would expect. At the end there is certainly a smearing in the sense that at emission the relative position was certainly different than that apparent at reception . This is negligible within the galaxy as @Rob Jeffreis explained and more generally to a far away observer. We can see an airplane cruising relative to sun not because its speed is comparable to c but merely because it is close to us. $\endgroup$ – Alchimista Jan 19 '19 at 14:52
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Basically, because the relative speeds of stars within galaxies are much less than the speed of light.

Imagine a structure with dimension 100,000 light years (about the size of a galaxy). Now suppose the components of that structure move with speeds relative to each other of around 100 km/s (or 0.03% of the speed of light). In the time it takes light to cross the structure (100,000 years of course), the components within it will have moved by just 30 light years (0.03% of the size of the structure).

There would therefore be no significant "blurring" of our snapshot of a galaxy caused by the differing light travel times from its structural components.

The other thing you may have not understood, is that the lives of stars are generally much longer than 100,000 years. So the stars that emit the light on the far side of a galaxy are still in existence 100,000 years later, when their light passes stars that are present at the near side of a galaxy.

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  • $\begingroup$ Thank you, but I still don't understand. Even if there's a distance of 1 'light minute' between two stars in a galaxy, the light of the furthest star should arrive to us 1 minute later. Is there any reason why the light of both stars separated by 1 light-minute would arrive to us at the same time? And by then, the relative position of the galaxy would be different, just taking in consideration the earth rotation during that minute. $\endgroup$ – Mbbc Jan 19 '19 at 0:39
  • $\begingroup$ @Mbbc Because they are not single flashes $\endgroup$ – Alchimista Jan 19 '19 at 8:46
  • $\begingroup$ @MBbc Because, as I explained, the lifetime of a star is much longer than their separation in light years. $\endgroup$ – ProfRob Jan 19 '19 at 10:24
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    $\begingroup$ @Mbbc - it doesn't arrive at the same time. We see the light from a star 1 light minute further away from us that was generated 1 minute earlier. But in 1 minute, that galaxy doesn't do much, the relative movement is negligible, as Rob said - so we don't see any change. $\endgroup$ – Rory Alsop Jan 19 '19 at 10:24
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I am not rewriting what nicely said in another answer. I just point out, as it is just implicit there, that even the closest stars in our galaxy are really far away! Not too say other galaxies. If, as I suppose, you aren't surprised by the fact that the stars look as fixed over millennia, then considering distance should fix your question.

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  • $\begingroup$ Can I ask for what the down vote. If we we were to look at close distance the fact that the speed of stars is negligible to that of light won't suffice to answer the question. Just think earth with an hypothetical extra sun. We will surely see them changing their separation. $\endgroup$ – Alchimista Jan 19 '19 at 11:42
  • $\begingroup$ To the down voter: it is why 30 ly aren't a significant "blurring". $\endgroup$ – Alchimista Jan 19 '19 at 11:50
  • $\begingroup$ And a further messages to the OP as it than mentioned earth rotation in a comment. To get a not blurred image of stars and galaxy you must account for earth rotation, sure. That is however a different point and it will be there even with a truly point like source. $\endgroup$ – Alchimista Jan 19 '19 at 12:06
  • $\begingroup$ Stop voting down without thinking plz! $\endgroup$ – Alchimista Jan 26 '19 at 9:15

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