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marqueemark
2009-Mar-18, 11:16 PM
If one were to look at an image of a spiral galaxy almost but not quite edge on so that the far arms are visible, shouldn't the image appear distorted somewhat since the stars on the far side are appearing where they were give or take 100 000 years earlier than the stars at the closer edge?

Hornblower
2009-Mar-18, 11:27 PM
If one were to look at an image of a spiral galaxy almost but not quite edge on so that the far arms are visible, shouldn't the image appear distorted somewhat since the stars on the far side are appearing where they were give or take 100 000 years earlier than the stars at the closer edge?Since we are looking at less than 1/1000 of a revolution, I don't think it would be conspicuous.

astromark
2009-Mar-19, 04:33 AM
NO. and welcome here. You are right in noting the diameter of the galaxy would be distorted by the 100 000 L/Y of its width...., But when that number is taken into context as the distance from you might be thousands of millions of light years. By the time we flew there it would catch up with reality as we reached it. Nothing is were we see it., but it is all relative.

geonuc
2009-Mar-19, 08:25 AM
NO. and welcome here. You are right in noting the diameter of the galaxy would be distorted by the 100 000 L/Y of its width....,
For the reason Hornblower gave, I believe. The back half hasn't rotated much in that 100,000 years as compared to the front.


But when that number is taken into context as the distance from you might be thousands of millions of light years. By the time we flew there it would catch up with reality as we reached it. Nothing is were we see it., but it is all relative.
:confused: Not sure what you're saying. The distance from us is not important in this question, given sufficient telescope resolution to see the details of the whole galaxy (and thus the ability to discern any distortion if it were there).

astromark
2009-Mar-19, 08:45 AM
Not sure what you're saying.... Thats fine, nor do I. All I am eluding to is that the image we see is the image that reaches us as it does. Yes the 100 000 l/y different distance will have some effect on the image. But its only a image. The reality would soon become apparent as we get near to such a place... there would be some movement detectable across the distance you ask of. As a near by thread has covered the fact that you me and us can not expect to see light moving any velocity other than c. any time ever... what was the question.?


marqueemark; is no relative of astromark... if you say mark this often, you sound like a dog with a hair lip.:)

Jeff Root
2009-Mar-19, 08:49 AM
Mark,

Hornblower and geonuc have it right. The distance from the galaxy to
us isn't relevant.

-- Jeff, in Minneapolis

Jeff Root
2009-Mar-19, 09:10 AM
Back in 1970, between eleventh and twelfth grades, I took some summer
classes, including a filmmaking class. The first thing on the first day, the
teacher, Jack Barkla, asked each of the students (maybe seven in total)
to say our names. One boy must have thought he wasn't clear the first
time he said it... or something funny just happened in his head... He said
"Mark, Mark." Jack responded, "Sounds like a dog with a harelip."

-- Jeff, in Minneapolis

Maddad
2009-Mar-19, 05:47 PM
The distortion of the image front to back is a curious thought. It would be very little; probably not measurable. Consider our own Milky Way.

We take a quarter billion years to orbit the galaxy once. That is 2,500 times as long as the 100,000 year difference between front and back. The maximum distortion would be proportion to that ratio, or 0.04%.

rommel543
2009-Mar-19, 07:59 PM
On that same note would it be possible get gravitational lensing when looking at the far side like that.

astromark
2009-Mar-19, 09:19 PM
0.04%... so its not a great deal of distortion is it ? 0.04% is hardly detectable and even a blink comparison would not show a lot of movment.... What You seem to have missed is that the galaxy is not where its image is any longer.

marqueemark
2009-Mar-19, 11:03 PM
thank you all, this has been helpful, I hadn't realized the timescale of a rotation of the galaxy.

geonuc
2009-Mar-20, 08:22 AM
0.04%... so its not a great deal of distortion is it ? 0.04% is hardly detectable and even a blink comparison would not show a lot of movment.... What You seem to have missed is that the galaxy is not where its image is any longer.
I don't think anyone missed that fact, it just wasn't relevant to the OP's question. The question was about the lack of distortion in the image we see now.

Cougar
2009-Mar-20, 01:31 PM
If one were to look at an image of a spiral galaxy almost but not quite edge on so that the far arms are visible, shouldn't the image appear distorted somewhat since the stars on the far side are appearing where they were give or take 100 000 years earlier than the stars at the closer edge?

Yeah, galaxies just don't rotate very fast, so the effect is, as Hornblower says, not conspicuous. But suppose a galaxy did rotate considerably faster so the effect was more obvious. Everyone has just mentioned "distortion." But obviously this distortion would be very well defined. It would be easy to program a computer to demonstrate the effect. I'm just having a bit of a difficult time picturing this massively-multibody problem in my head. I'm thinking even with a fast rotation, the effect would be fairly subtle.... :think:

Hornblower
2009-Mar-20, 01:35 PM
Yeah, galaxies just don't rotate very fast, so the effect is, as Hornblower says, not conspicuous. But suppose a galaxy did rotate considerably faster so the effect was more obvious. Everyone has just mentioned "distortion." But obviously this distortion would be very well defined. It would be easy to program a computer to demonstrate the effect. I'm just having a bit of a difficult time picturing this massively-multibody problem in my head. I'm thinking even with a fast rotation, the effect would be fairly subtle.... :think:
Even if the apparent positions of stars on the far side lagged by a few degrees, the effect might not be conspicuous, simply because the undistorted patterns of spiral arms and other such things often are rather ragged and irregular.

01101001
2009-Mar-20, 05:58 PM
Even if the apparent positions of stars on the far side lagged by a few degrees, the effect might not be conspicuous, simply because the undistorted patterns of spiral arms and other such things often are rather ragged and irregular.

I think yes. Distortion would be in the form of lack of rotational-symmetry: the back side doesn't look like the front side rotated 180 degrees, right? Does any galaxy have such perfection of symmetry that it would be noticeable? I'd expect 10, 20 even 30 degrees would be hard to spot.

For instance our current artist impression (http://www.nasa.gov/mission_pages/spitzer/multimedia/20080603a.html) of the Milky Way doesn't have much rotational symmetry at the outer edges.