I am always on the lookout for an enquiring mind in the angry swarm that is my family. The kid with a brain (under a goulag hiarcut) is at present slowing my plunge into senility.
During a discussion about measuring stellar distances, short and long, we soon progressed to the nature of galaxies and their rotation. Now I explained the Doppler shift to him (he got it straight away, the little bastard) and how galaxies seen edge on have a red shift one side and a blue shift the other, proving that they are spinning. He then asked how we know that our galaxy is spinning; I said that in astronomy if things are alike in many ways we assume them to be alike in many others- we are in a galaxy, all the other galaxies spin, so ours does as well. I was’nt satisfied with this answer either.
We could’nt see how Doppler shift could help because all the stars in a galaxy fly in formation around the centre, keeping a constant distance between each other.
I said that maybe stars in our galaxy appear to move over the years against the background of distant galaxies although I could not remember reading about it.
He asked ‘’ When are we going to have to rename the Andromeda Galaxy?’’
I said it was time for my nap and sent him home.
Before I nodded off I had two thoughts:
1.Those gravitational lens pictures of very distant galaxies magnified and distorted as thier light passes by nearer ones- do they change over time?
2.Dopers know eveything
Doppler shift isn’t going to be very effective looking at stars at the same distance from the center of our galaxy as ourselves, as we should maintain roughly the same distance from them all the time. But looking at stars that are further and closer to the center should be informative.
The inner stars should be traveling faster around the center than we should. Looking towards the center should reveal roughly the same pattern as seeing an entire rotating galaxy from the outside of it. If you understand the Doppler relationship in your example, this should be just more of the same.
Looking to the stars further from the center than us should reveal exactly the opposite Doppler pattern. Stars farther along in the direction of galactic rotation will be blue shifted as we overtake them in a smaller orbit. Stars behind us and to the outside will be red shifted.
Also recognize that stars do not stay in the arms. In fact as I understand the arms move in the opposite direction to the stars that make them up.
Anyway this arms have a lot of mass so stars accelerate as they head into them and deccelerate as they head out. And we can see this in their spectrum.
We can also look at groups of stars born in the same burst of formation and see that when young they are grouped together but as they age they get strung out.
I don’t know of any gravity lenses shifting but they must since the geometry that makes them is extremely delicate. But I have never heard anyone talk about it.
Well except for people in the ol’ MACHO camp. The MACHO theory held that most of the dark matter existed in objects larger then Jupiter but too small to ignite their core. They ran experiments looking for lensing between us and (the LMC) I believe. They caught a few lensing events but not enough to support their theory. Anyway that’s very close to us so not really what you were asking but it’s as close as I can come.
We know our Galaxy spins from measuring the Doppler shift of clouds of gas. Radio waves travel clear across the Galaxy relatively unimpeded, so we can use radio telescopes to look at gas clouds all over the place. I found a rather technical paper about it here (it’s a PDF), but it seems to have everything right as far as I looked.
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The stars on the edge of the galaxy have the highest speed. Those near the center the slowest. Longer orbital period but actually moving faster.
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If a galaxy is somewhat edge on to us then the left edge is moving away from us and the right edge is moving towards us (or vice versa). That gives you your doppler shift. Aim at one edge, run it thru a spectrograph. Aim at the other edge, etc. The stars at the front and back edges are moving sideways to us and won’t show much doppler shift. If the galaxy is flat faced to us, we are out of luck. (Globular galaxies are never flat faced to us.)
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Even at the speeds these stars are moving, it takes huge periods of time for the position of the stars in another galaxy to change from our perspective. Far too long since accurate photos of stars in galaxies have been made to detect any difference. Speed is determined by doppler shift (and other neat tricks).
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If the stars in our galaxy weren’t in orbit, Bad Stuff would happen real soon (10’s of millions of years), all the stars would pull each other into the center, a little Gib Gnab. No different than if all the planets weren’t orbiting the Sun. They’d all start heading into the Sun.
This is completely wrong. In any orbital system, objects closest to the center not only complete orbits in less time, they must travel faster to maintain their orbit.
For example:
Our moon’s orbit covers roughly 1.57 million miles in a bit less than a month. This gives an orbital speed of roughly 2300 miles per hour.
A satellite in low earth orbit (say a 200 mile orbit) travels roughly 25000 miles per orbit. But, it does this orbit several times a day (as often as once every 1.5 hours) for a velocity of 17,000 miles per hour.
That is more than 7 times faster.
scotth, you’re assuming that all the mass is in the center. For a galaxy, that’s not true.
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scotth, what you say is true if the bulk of the matter around which the system is orbiting is located in the center. In a galaxy, however, it is most concentrated in the center, but mass is distributed all over. The result is that the velocity versus distance-from-center curve is not the nice power-law that it would be for a solar system. I believe that beyond 8 kpc (the Sun’s distance) the error bars on the velocity are enormous, but consistent with a constant velocity.
But I also think that ftg misspoke.
I considered it.
Spiral and barred spiral galaxies behave pretty similarly to a system that has a simple central mass. The mass of our galaxy is dominated by core and central halo. While the rest of the mass skews the gravitational gradient, it isn’t enough to overturn the premise that inner orbits are faster.
Ah, so we can prove that our galaxy spins by observing stuff inside it. The kid with a brain will be impressed when I show him what awsomely smart friends I have. I’ll see if I can interest him in working out for ourselves the date that the Andromeda Galaxy has to be renamed the Pegasus Galaxy or whatever. Lets see, we need the rotation period, the relative angle of M31 and the boundaries of the constellations. Google awaits.
Thanks chaps.
Why else would I feel so dizzy all the time?
The peculiarities of galactic rotation curves gave rise to the theory of dark matter, or missing mass, back in the 1930’s. For the local galaxies to spin in the way they do, they must contain several times more mass than anyone has been able to find. Here’s a brief intro to the subject, and here’s a bit more about how the dark matter must be distributed to account for constant rotational speed of the disk stars.
Would it be immodest of me to link to a couple of my Staff Reports, and subsequent discussions?
As to the renaming of the Andromeda Galaxy, the constellations will have distorted to the point of unrecognizability long before M31 will have moved far enough. For that matter, in current use, the word “Andromeda” by itself more often refers to the galaxy than to the constellation, so it’s not necessarily the galaxy which will be renamed.
Dangit Chronos, put your pants back on ! We’ve already got enough former mods who’re willing to fake jellyfish injuries for immodest purposes. 
Oops!