Do arms of galaxies rotate synchronously?

How can spiral galaxies have coherent “arms” if the gravity toward the center of the galaxy depends on distance? If you look at the Solar System, for instance, the closest planets rotate much quicker than the farther out ones.

How can the arms of a galaxy maintain coherency, or are they not really all that stable and coherent after all? The only answer I could come up with was that maybe they are held together by gravitational attraction to other members of the arm, but I wouldn’t expect this to be able to hold an arm together for billions of years.

This is precisely one of the observations that led cosmologists to formulate the Dark Matter theory. The way galaxies rotate is apparently not at all consistent with the way visible matter appears to be concentrated in them. The rotation of galaxies is believed to only make sense if there were some form of unseen, or “dark”, matter spread more or less uniformly through the galaxies, and making up the bulk of their mass. This is from reading a few years back, so some details may have changed, but you get the general idea, I hope.

You’re quite right; observers quickly figured out that spiral arms would wind themselves up in just a few million years if they were structures imbedded in the galactic disk.

The arms of galaxies are now thought to be spiral waves moving through the galaxy, creating bursts of star formation as they go. Thus, spiral arms are marked by bright blue stars. These big blue stars quickly burn through their fuel and become supernovae, so as the density wave moves on, only the longer lived yellow and red stars remain behind.

So how does this apply to our own galactic situation? Since our sun is yellow and remained behind after the last wave, does this mean there’s another wave of new, blue stars bearing down on us? Or do we simply get pushed/dragged along, between the two arms, by all the dark matter?

There are two different things you can mean by “same speed” or “synchronously”. All of the stars in a galaxy do move at about the same number of kilometers per second, which is why we know we have dark matter (as Q.E.D alluded to). However, the ones closer to the center have less distance to travel per orbit, so they’ll still get out of synch with their neighbors at a different distance. But as Podkayne says, the arms are not moving at the same speed as the stars that make them up, so the arms don’t actually wind up (or at least, not as much as the stars do).

And I used to think I was quite knowledgeable about the universe. Now, the more I learn, the more weird it gets.


What are these spiral waves? Stars are simply living and dying, to give impression of arm “movement”? To keep the arm coherent, wouldn’t they have to live and die faster near the center of the galaxy than they do at the limb of the galaxy (because the stars are physically traveling faster)?

… in terms of degrees of arc per unit of time as the star travels around the center.

If you picture water swirling around in a toilet, with the water going faster at the center, you’ve got a picture of the motion of the stars and other matter. Now imagine there are waves, which don’t have to move at the same speed and don’t necessarily have to maintain some specific appearance. Like the waves on a river, maybe in a boat’s wake, can move very differently from the water itself.

I guess one disadvantage of the toilet picture is that you don’t generally see radial waves in toilets. Another disadvantage might be the theological implications.

I’m afraid to say much more, 'cause I’m a lowly planetary astronomer, and I don’t understand this stuff very deeply, but here’s a page with lots of detail and diagrams (but not a lot of math.)

The way I learned it a number of years ago, they are density waves in the gas and dust. The arms actually move more slowly than the general rotation of the galaxy, so that stars and gas overtake them. Since they are dense, the gas falls into them faster than it leaves, thus perpetuating the density wave. And also causing new star formation.

It should also be noted that what you see in pictures of spiral arms is mainly bright nebulae. Bright stars are mainly found in the spiral arms and their UV radiation excites the gases in the nebulae and cause them to glow.

The sun will pass through spiral arms and indeed has done so quite a few times since it was born. There are several spiral arms in the galaxy, and the sun orbits the galaxy every 200 to 250 million years. So in the 4.6 billion years the sun has existed, it’s orbited the galaxy some 20 or so times.

Podkayne, your link was very helpful. Thank you.

Interesting. Do we have any idea how fast the density waves (arms) propogate (rotate)? If so, we’d know how many arms our sun has passed through.

Related questions: how do they calculate how rapidly our sun orbits the center of the galaxy? and how would they calculate how rapidly the arm rotates?

This topic has bugged me since the third grade. I used the the whirlpool of a bathtub drain as a metaphor, and added bits of math and physics as I learned them. Within five years, I was convinced that it had to be an illusion because gravity simply didn’t work the way the starfish cartoon image of galaxies seemed to suggest. I didn’t have the answer, of course, but I was pretty sure it was an illusion like the spiral radial waves that seem to extend outwards from a bathtub or toilet whirlpool.

In all the years since (and there have been more than I care to admit), I’ve been quite convinced that you do see spiral radial waves extending from the center of a whirlpool (and that they are pretty good physical analogy for the compression waves that create the illusion of glactic arms) I even went and flushed my toilet to check they were really there (okay, maybe I just needed to offload all that coffee, but still…) Did I miss something? Is my dry wit detector on the fritz?

And what are the theological implications? Please don’t misunderstand, I’m sincerely curious.

Haven’t the foggiest. For all I know, they could go backwards. Or not rotate at all. We can measure the relative speed of physical objects like stars and emission nebulae via doppler shift, but that doesn’t work for non-physical objects like density waves.

I don’t think we even know how many spiral arms the galaxy has. Some galaxies have simple two arms while others have arms that spread out into multiple sub-arms. I believe they’ve found at least 4 arms in our general area, so it looks like our galaxy is one of the latter.

I’ll let someone else handle these.

Thanks for your response dtilque. Chronos? Are you out there?

This page and this one might be helpful.

Note that we don’t measure the orbital period directly, but rather infer it from our absolute speed and our distance to the galactic center, which are measured separately.

Ain’t no such thing. Velocity is always relative to something.

Well yes, I guess I worded that rather poorly. Sorry.

I meant “absolute” in the sense of “not relative to the galaxy’s size”.

Bytegeist, from your second link:

This doesn’t seem to provide enough information.

Sol <-----(distance x)------> Sagittarius A <-----(distance y)-----> “star far beyond”

Assuming Sagittarius is at the center of the Milky Way (which by itself generates all sorts of questions I’ll conveniently ignore about how they determined that fact), doesn’t one need a “fixed” point “far beyond” like another galaxy?

I fail to see how another star on the other side of Sagittarius and also moving within our Milky Way is helpful. If you know neither the relative velocity of Sol nor of the “star far beyond”, I don’t see how this could give you the rotational velocity of Sol around the galactic center.

This article explains Reid’s observation a little better:

The key point (not covered in the earlier article) is that Sagittarius A* is assumed to be at the galactic center, and at rest in that reference frame.

Ahhhh. That makes sense. The “star far beyond” is a distant quasar. If one assumes that the quasar is so far away to be essentially motionless (over a 6 month timeframe) then I can see how our solar system’s velocity around our galactic center can be calculated. Thanks.

I’m still curious about whether or not the galaxy “arms” rotate, and at what speed.

How could one determine if the “arms” rotate? Any redshift measurements would be of individual stars, not the density wave that defines the “arm”.