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I have 2 questions related to spiral galaxies.

Firstly, how did the arms form? Why would the stars accumulate into those specific areas? And secondly, why are they still intact?

The inner parts of the arms must be revolving more rapidly than the outer parts, so surely the would become stretched and then no longer resemble arms, but simply a disk around the galactic center.

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  • $\begingroup$ I enjoyed this short video yesterday, by Sixty Symbols : What makes spiral arms in some galaxies - and what is "pattern speed'? Professor Mike Merrifield explains. youtube.com/… $\endgroup$ – xdze2 Sep 1 '14 at 19:17
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Actually, the stars and nebulae that make up the spiral arm are only temporarily part of that spiral arm. Spiral arms are more like sound waves where individual particles move around a more or less stationary position. (Look for instance at the animation of longitudinal waves from Dan Russel, the red dots move a bit to the left and to the right around a stationary position). Dust, gas and stars move towards or away from another just as longitudinal waves. Where the dust, gas and stars come close together (and where, therefore, the density increases), spiral arms can be seen as more stars are close together increasing the brightness at that position in the galaxy.

This effect is, furthermore, much increased because the increased density of dust and gas in the spiral arm causes protostars to form. The brightest stars burn up their energy so fast that they will cease to exist even before the longitudinal wave (the spiral arm) has passed. These very bright stars only exist for a small portion of their orbital period around the galaxy's centre, and only while they are in the spiral arm. The large majority of stars exist much longer, but are also much dimmer and contribute only little to the overal brightness of the galaxy.

This causes the spiral arms to be so much brighter than the rest of the disk, where also a lot of stars exist. But these can hardly be seen as they are much dimmer.

Of course, the stars do not revolve around a stable position in the galaxy (as the red dots in the wave animation) but follow their own orbits around the centre of the galaxy. Sometimes a bit faster, and sometimes a bit slower depending on the position relative to the spiral arms.

Because the spiral arms are waves, it does not matter that stars near the centre move faster than the stars at the edge. It just means that they will be part of the spiral arm for a shorter period of time.

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    $\begingroup$ This is probably, in fact, the single most amazing thing I have learned on the internet. Thanks for this! $\endgroup$ – Fattie Nov 25 '16 at 14:57
  • $\begingroup$ Sent bounty for thanks and to promote the site! $\endgroup$ – Fattie Dec 2 '16 at 14:15
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To add to Dieudonné's excellent answer, I'd like to say that spiral arms are only really prominent in the blue part of the spectrum (massive stars tend to be blue and short-lived), while in infrared wavebands, for example, spiral arms only appear as mild over-densities of 10-20%.

Some galaxies have clear arms winding for almost 360$^\circ$ or even more (grand-design spirals), but most spirals arms much are shorter and less clear. Many galaxies in fact have only so-called flocculent spiral arms, which look like short pieces of spiral arms all over the galaxy, but which cannot be linked together into one large coherent spiral pattern.

The situation for the Milky is not so clear, as we don't have an external view and measuring accurate distances to objects within the Milky Way is notoriously difficult. The hope is that ESA's Gaia satellite will give us some more accurate picture.

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