I Why are pictures of galaxies so clear?

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zuz

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When a photo of a nearly edge on galaxy, say Andromeda, is taken. Why are the stars on the far side of the galaxy so clear? The light from those stars took an additional 220,000 years to reach us. Those stars have moved quite a lot from their positions since the light from the foreground stars left them. Shouldn't the background be fuzzy?
 

Orodruin

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How long they have moved since the light left them is completely irrelevant. What matters is the angular motion over the exposure time, which is completely negligible.
 
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When a photo of a nearly edge on galaxy, say Andromeda, is taken. Why are the stars on the far side of the galaxy so clear? The light from those stars took an additional 220,000 years to reach us. Those stars have moved quite a lot from their positions since the light from the foreground stars left them. Shouldn't the background be fuzzy?
To add to Orodruin's comment, if you had an exposure time of, say, 1,000 years then you would get fuzziness. Most photographers aren't willing to wait that long.
 

russ_watters

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When a photo of a nearly edge on galaxy, say Andromeda, is taken. Why are the stars on the far side of the galaxy so clear? The light from those stars took an additional 220,000 years to reach us. Those stars have moved quite a lot from their positions since the light from the foreground stars left them. Shouldn't the background be fuzzy?
The stars didn't move much during the exposure, they moved long before it.
 

zuz

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Thank you russ_watters. That is the what I thought.
 

davenn

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The stars didn't move much during the exposure, they moved long before it.
they are STILL moving 😉


Thank you russ_watters. That is the what I thought.
The stars in the galaxy haven't stopped moving

You should really be thanking @Orodruin with the added bit from @phinds for the better answer :smile:

How long they have moved since the light left them is completely irrelevant. What matters is the angular motion over the exposure time, which is completely negligible.

Dave
 

zuz

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Exposure time is usually measured in minutes, with multiple exposures staked up. Even if you had 1000 years to spare, no one would risk that much time on a single photograph and have a single airplane ruin it.
 
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Exposure time is usually measured in minutes, with multiple exposures staked up. Even if you had 1000 years to spare, no one would risk that much time on a single photograph and have a single airplane ruin it.
? Why do you think a single airplane going past the lens for maybe a second would "ruin" a 1,000 year exposure ?

Don't you think the rotation of the Earth, just as one example, would have WAY more effect?

Anyway, your question has been answered.
 
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A single second of sunlight would ruin a 5 year integrated exposure of a magnitude 15 galaxy with the same area in the sky (would lead to the same amount of light coming from the galaxy and the Sun). That's not how you take these long-term pictures, of course. You make many pictures with much shorter exposure times and add them, if one of them is ruined you discard that.
 
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Exposure time is usually measured in minutes, with multiple exposures staked up. Even if you had 1000 years to spare, no one would risk that much time on a single photograph and have a single airplane ruin it.
I was looking for something once, using google satellite view. As I zoomed in on the subject I noticed a weird fuzzy spot in the image... zoom zoom, sure enough, there was a slightly blurred, half transparent airliner. Not sure how common that is.
 

sophiecentaur

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. Not sure how common that is.
Looking in the other direction, it is extremely common for an aircraft to fly across the Moon when you live in the UK and spend any time Moon-viewing in a telescope. There are hundreds of those things up there and they subtend a degree of angle or more. They are always leaving streaks (dotted lines) over astrophotography frames but clever stacking progs will eliminate them from a stack of a dozen or more.
 
Because tired light theories are wrong? :)
 

sophiecentaur

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Don't you think the rotation of the Earth, just as one example, would have WAY more effect?
Star trails due to Earth's rotation are a real bind for the ill-equipped astrophotographer but they can be dealt with with a motorised mount that rotates the camera / telescope about the polar axis (tracking). You can improve on this with a guiding arrangement that will lock onto a brightish star and follow it, allowing you to get sharp images of a more faint object of interest.
However, all stars are moving relative to each other and we, on Earth get parallax effects as we move around in our orbit (well worth Googling). Nearby stars can be seen to wander back and forth over the year. Also there is so called 'Proper Motion which can also be seen with some closer stars. This link shows the results obtained by a genuine amateur astronomer (a hero of mine aamof). He observed parallax motion and also the proper motion of a nearby star against the more distant stars (which we used to call FIXED, until we knew better).
 

Janus

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When a photo of a nearly edge on galaxy, say Andromeda, is taken. Why are the stars on the far side of the galaxy so clear? The light from those stars took an additional 220,000 years to reach us. Those stars have moved quite a lot from their positions since the light from the foreground stars left them. Shouldn't the background be fuzzy?
As already pointed out, "fuzziness" would have to involve really long exposures during which the stars were moving. But this would effect both the near side and far side stars equally as they both would be moving during the exposure.
Perhaps, you are thinking more along the lines as to why the galaxy doesn't look "distorted". If we are seeing near side stars where they were at a time 220,000 years later than than where we are seeing the far side stars, shouldn't the galaxy look kind off "bunched up" on one side and stretched out on the other?

To answer this we need to compare the galactic rotation rate against that time delay. Our sun is about 27,000 light years away from the center of the Galaxy. Which puts its orbital diameter at ~ 54,000 ly. It also takes roughly 230 million years to complete 1 full orbit around the center. This means that in the 54,000 yrs it takes for light to cross its orbit, it will have only traveled 1/4259 of a complete orbit, or 1/12 of a degree. This is how much of a "shift" you would expect to see in stars on opposite sides of this orbit when viewed edge on.

If we are looking at a galaxy with a typical flat rotation curve, then looking across greater diameter of the galaxy won't make this visual angular shift larger. A flat rotation curve means that the orbital velocity of the star remains the same as you move outward. Moving outward means traveling in a larger circle. traveling a larger circle at the same speed takes more time. So at 110,000 ly from the center, or about twice the distance of the Earth from the center, a star would take about twice as long to complete a orbit, and the fact that it takes the light twice as long to cross the diameter results in us seeing the same 1/12 of degree shift.

1/12 of a degree is just too small a angle to cause any noticeable distortion in the image when seen by eye alone.

Another way to look at it is that if a star moves at ~ 200 km/sec then in 220,000 years it will have moved 1.4e15 km in 220,000 years. This is ~150 ly.
If we were looking at a photo of a galaxy 220,000 ly across, and and that photo has a resolution of 4000 pixels in width and the galaxy fills the image, each pixel in that photo represents 55 ly or a bit over 1/3 the distance moved by the star in the time it took light to cross the galaxy. This equates to less than 3 pixels worth of apparent shift in that 4000 pixel wide image.
 

sophiecentaur

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This is how much of a "shift" you would expect to see in stars on opposite sides of this orbit when viewed edge on.
I'm trying to make sense of this idea.The only reason for seeing something different in the layout of the distant stars would (surely?) be if there was significant expansion or some other change but if you are just looking at a uniform wheel, how would the extra delay from the distant stars give a detectable effect? We would basically be looking that a different set of stars occupying any particular angular distance but with the same density. Galaxies are not all that big, compared with their distance so the time differences across their diameter would not be that huge.
An easy model is the fairground roundabout with uniformly spaced seats. If you took two separate photos with the near and far sides illuminated at different times, the spacings wouldn't look different - with a big enough delay, you could get some riders appearing in both photos.
 

zuz

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Thank you Janus. That was an excellent explanation.
 
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I'm trying to make sense of this idea.The only reason for seeing something different in the layout of the distant stars would (surely?) be if there was significant expansion or some other change but if you are just looking at a uniform wheel, how would the extra delay from the distant stars give a detectable effect?
You will still see a disk, but with stars on the approaching side much brighter - which also means you see more of them if you don't correct for that effect.
This gets more complicated within our own galaxy.
 

sophiecentaur

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You will still see a disk, but with stars on the approaching side much brighter - which also means you see more of them if you don't correct for that effect.
This gets more complicated within our own galaxy.
Yes - nothing about the perceived motion / streaking /spacing, though. Except perhaps the density of visible stars would be less - if, indeed you could actually distinguish the front from the back. Talking of that, the side that's coming towards us might have different red shift from the other side but how to tell which ones are at the back, I wonder. Perhaps Cepheids would give a clue.
 

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