Why doesn't the light from galaxies appear stretched?

Question

Maybe it's my ignorance of astrophysics/cosmology, but I have been wondering this:

Why do galaxies not appear stretched when we observe them? Assuming a galaxy that we observe is 100,000 light years in diameter, and we are viewing at an angle that is almost sideways, but enough to see its shape, there would be a 100,000 year delay between what we see at the front of the galaxy versus the far end of the galaxy. So wouldn't it look all jumbled up instead of a perfect spiral?

At first, I thought that the Big Bang would explain this, because the light from that matter would have been there from the start, but that doesn't make sense when we consider that it took hundreds of thousands of years for the universe to clear up enough for photons to travel far in any direction without bouncing of of something. Also, if inflation theory is true, wouldn't that just reinforce the confusion about the delay in light, being that the universe expanded faster than light for an instant?

It's confusing, but if someone has an answer for this I'd appreciate the enlightenment.

This is a good question, but the big bang/inflation aspect is something of a confusing tangent. Galaxies had not formed by the time of recombination, and in fact most images of galaxies are from at most a few billion light years away. — , Aug 22 '14 at 03:40
Also, I understand that stars would be red-shifted as they move away, so there would be continuity of light, but that's assuming that the light was uninterrupted during the first few 100,000's of years when nothing was visible. — JavaScript_Guru, Aug 22 '14 at 03:41
@Chris white Yeah I was just using inflation as an example. But my real conundrum is the continuity of light either reflected or emited from the matter at the "far end" even before they formed. — JavaScript_Guru, Aug 22 '14 at 03:45
possible duplicate of How distorted does the Andromeda Galaxy appear to us due to the speed of light? — John Rennie, Aug 22 '14 at 08:09

score 3 · Answer 1 · edited Aug 22 '14 at 15:45

Galaxy rotation happens at a very slow rate (compared to the speed of light). Let's suppose you are observing a galaxy edge-on that the delay from the farthest point is $\Delta t = d/c$, where $d$ is the galaxy diameter. If we take the lag from one extreme point to the other as D:

$D = vt = \frac{v}{c}t$

(where $v$ is the rotational speed).

You can see that, to have an appreciable effect, your velocity must be a fraction of the speed of light. The Milky Way rotates at a speed of $\sim10^2$ km/s, which is $\sim10^{3}$ times less than $c$. This means that your lag from one point to the opposite one of the galaxy is exceedingly small.

Of course, it is possible that for larger galaxies (see, e.g., Hercules A) you can have faster rotation, but you still need a coincidence of factors like distance, angle, and size of the galaxy, to appreciate a small difference in the lag. And this, as far as I know, has never been observed yet.

NeutronStar · Accepted Answer · 2018-05-23T01:00:52.723

Galaxies would appear stretched along the line of sight, not jumbled. Let's say a galaxy (specifically, the closest side of it) is ten million light years away and, as you proposed, is 100,000 light years across and we see it nearly edge on. The front of the galaxy will appear to us as it did ten million years ago and the back of the galaxy as it did 10,100,000 years ago. Thus, if the galaxy is moving towards us it will appear bigger than it actually is due to the delay between light from the front of the galaxy and the rear of the galaxy reaching us. If the galaxy is moving away from us, it will appear smaller than it actually is (compressed along the line of sight).

As far as the effects of this time delay on viewing the rotation of the galaxy, the length of the cosmic year (the length of time it takes for the sun to travel around the center of the Milky Way) is 225 to 250 million years. So, as many other people have already pointed out, the rotation period of a galaxy is small compared to the time it takes for light to travel across the galaxy and there is no jumbling effect.

Fattie · Answer 3 · 2014-08-22T13:52:51.283

I think the very short answer is:

galaxies are very small, indeed tiny, compared to how far away they are, and secondly, compared to the size of the universe.

The answer to the spirit of your question, is that simple! "Galaxies are tiny."

You're used to hearing "galaxies are 100,000 light years across!" but that's a piffle compared to either the size of the universe, or, how far away they are from us.

Note that in your question it's somewhat unclear if you mean the seeming paradox due to redshift (in which case the answer is: galaxies are incredibly small compared to the universe), or, if you mean a seeming paradox something to do with spinning/perspective (in which case the answer is: galaxies are incredibly small compared to our distance to them).

Note that with galaxies very close to us, as I understand it, it is indeed commonplace for Astronomers to be able to measure different qualities on one side or the other (again: your question is somewhat unclear "which" seeming-paradox you mean, but as I say, indeed for "nearby" (lol) galaxies, in fact yes, it's an everyday thing to measure "bending" and other stuffs).

Note: piffle is a new SI unit with the definition "quite a many to damned many orders of magnitude".

score 0 · Answer 4 · answered Aug 22 '14 at 06:05

I suspect that the nice rotational formations of galaxies we see come from galaxies that are oriented in parallel with our solar plane so that the light arrives almost synchronously.

Considering we are talking of gravitational forces the "almost" could cover a large window, the distortions not being too great to lose track of the shape. A galaxy whose plane is perpendicular to the solar plane would not show the rotational curves anyway, due to superposition. I do not know whether the shape of the infrared spectrum could be characteristic, i.e. gives some hint that the rotational curves exist.

Why doesn't the light from galaxies appear stretched?

4 Answers4