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

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.

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

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.

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".

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?

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

Isn't only one of the mass extinctions (the dinosaur ending one) somewhat tightly linked to a comet impact? It seems as though it's finding a pattern that does no fit the data.

Also, I believe the dinosaur extinction was not just a comet impact, but a combination of an impact and a period of increased volcanic activity. Could the volcanic activity be triggered by comets?

I think the jury is still out, but it's an interesting field of study across many different fields (astronomy, biology, geology and probably more), and it has obvious implications for humanity's future on Earth, in space, and the chances of finding other life out there.

There is a local museum rigth next to the cliffs. A few years ago, I spoke to the leader there, a woman with a ph.d. in history, who more or less supplied the final death blow to any lingering respect I might have held for any kind of humanities: Her only take on Walter and Louis Alvarez and their world famous work in her own backyard was how of course they were so arrogant, they had raised hostility among scientists and everybody else the world over.

That's an interesting question. My completely unqualified opinion would be no, an comet couldn't trigger the massive and ongoing volcanic activity that a mass extinction requires. I'd like to hear from someone more informed though.