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Comment by PhasmaFelis | Hacker News Reader

...Huh. I always assumed that most stars kept approximately the same position relative to the rest of the galaxy. (Barring "up/down" oscillation if the orbital plane isn't perfectly aligned with the galaxy's.) There must be some kind of overall structure to maintain distinct spiral arms, right?

Does the Solar System actually make a complete circuit of the bulk of the Milky Way? Is that unusual?

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The spiral arms are not fixed masses. If they were, they would quickly wind themselves up tighter and tighter, as the inner areas are orbiting faster than the outer parts. Within a short time the spiral arms would be dispersed.

Instead, you can think of the spiral arms as being like the highest-density part of a traffic jam. In a traffic jam, cars are slowing down and bunching onto the back of the queue, while other cars are finally trickling off the front of the queue and speeding back up. In this way the traffic jam itself stays in a stable position, while individual cars constantly move through it.

In other words, it isn't the same stars that make up the spiral arms over time. As stars orbit the galactic center, they move in and out of the spiral arms.

Put a different way, a galaxy's spiral arms are like waves.

Mind == blown

A good explanation. I think one detail may be corrected: a traffic jam typically goes backwards, with respect to the direction of the cars forming the jam, at a speed much slower than the speed of the traffic. Perhaps a similar behavior is observed with the galactic arms?

The author explains it pretty well in the Batchelor interview:

https://audioboom.com/posts/4827661-mass-extinction-and-the-...

>I always assumed that most stars kept approximately the same position relative to the rest of the galaxy.

That was the original assumption. However, basic [0] orbital mechanics says that objects with smaller orbits orbit faster. If this were applied to the galaxies spiral arms, then as they continue to orbit, the inside ones would orbit faster than the outside ones, and the galaxy would become increasingly spirally. By this point, the galaxy would be so tightly wound that we would not even be able to see the spiral any more.

A more modern hypothesis is that the arms are more like waves of high density. Individual stars move through the the arm.

Since this high density region exerts gravity, stars that are moving out of it are slowed down, and stars that are moving into it are sped up. The end result is that orbiting stars tend to spend a little bit more time in the arm region then they would under a pure orbit. In doing so, they contribute to the mass of the arm, causing other stars to also spend more time in the arm then they otherwise would. The end result is a stable wave as all the individual stars move into and out of it throughout their orbits.

https://en.wikipedia.org/wiki/Density_wave_theory

[0] As in, just applying Newtonian gravity.

Right, density waves. But as rtkwe and the Wikipedia article note, there's also increased star formation in the higher-density arms. So more stuff hitting Earth also seems plausible.

a visualization of the current model orbital path http://m.imgur.com/Eup0Yoa

It's hard to be sure. But observing both the Wiki animation and this GIF, it appears that stars don't orbit at a constant radius from the galactic center. Rather, they appear to move significantly closer and further.

Does that seem right? If so, that's much different than planets orbiting our solar system. To a first approximation our orbits appear circular. Yes I know there's a certain eccentricity but for most planets it's not much.

I don't like the idea of our solar system getting too close to the galactic center. It gives me a vague sense of dread, like "bad things can happen there".

The orbit in the model are supposedly eccentric, but the period for a complete prbit is quite long, allegedly around 220milliom years, and yes the paper is all about bad things happening periodically as we change our local neighboors. But not something we'll have to worry about in quite a while

I think that's just a side effect of the compressed scales used by the animation. If they didn't do that then you would not be able to see the wave effect as clearly.

That's an exaggerated, simple model of spiral density waves; it's not a model for the Sun's orbit, which probably only varies in radius by about 10-15%.

> If this were applied to the galaxies spiral arms, then as they continue to orbit, the inside ones would orbit faster than the outside ones, and the galaxy would become increasingly spirally. By this point, the galaxy would be so tightly wound that we would not even be able to see the spiral any more.

I thought that problem was the same as the galaxy rotation curve problem? If we solve (erm) that issue by adding in some dark matter - can't we then also accept a spiral structure to rotate with the galaxy without "winding up" the spiral?

The Spiral arms aren't really cohesive bunches of stars but just bunches of relatively new stars that burn brighter than the older stars outside the spiral arms.

http://www.space.com/24642-spiral-galaxies-milky-way-shape-e...

No, in many spiral galaxies the spiral arms are present in older stars as well; that's why they're considered density waves in the stars. The contrast is stronger for younger stars, but it's there in old stars, too.

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