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Spiral Galaxies

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I heard something recently that changed the way I think about spiral galaxies. I thought that in the arms of a spiral, all the stars in the arm moved together, and the individual stars moved at the same velocity (around the galactic centre) as the arm. But now I've heard that the individual stars move faster, so you get stars entering the arm and slowing down, and the stars leaving the arm speed up, the analogy being like a traffic jam on a motorway. Is this correct? If so what causes this effect?



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Two things are going on: rotation of disc stars (i.e. gravitational orbits) and pressure waves in the very low density interstellar medium of the disk.

In the case of gravitational orbit, stars closer to the centre move faster than ones at the edge. Imagine a line joining all the planets of our solar system at some time when the planets are all in a line. Run the clock forwards and this line will get twisted up because Mercury orbits a lot faster than Uranus. So gravitational orbit can't explain spiral arms: they'd get wound up too quickly.

The idea is that spiral arms are a density wave: this is the "traffic jam" idea. The spiral arm is a region of higher density, which slows down incoming gas, dust etc. This prompts the formation of new, bright stars, which we see as the spiral arms. Massive stars in the arms will eventually become supernovas, creating pressure which supports the continuation of the density wave. The process has been compared to striking a cymbal so that vibrations run round it.

There's an animated illustration here:

Density wave theory - Wikipedia, the free encyclopedia

Edited by acey

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Brilliant, this is what makes looking at the skies so interesting. It just adds to it all to have a small idea of what you are looking at.

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