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When seeing artist's impressions of galaxies they always look somewhat like this:

Artist's impression of the milky way _

Even in school we have been taught that such a type of galaxy consists of spiral arms.

According to Kepler's laws a body in a circular orbit has a higher orbital period the greater the radius of the orbit, which indeed is observable for objects orbiting around the earth. (ISS: 90 min; Moon: 1 month)

Now, given that the spiral arms consist of many different stars and other bodies moving more or less independentley from each other, how come those structures are still visible and haven't smeared into a flat gradient?

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  • $\begingroup$ BTW: Galaxies come in other structures, but there are a couple of reasons that you see a lot of depictions of spirals: (a) the other 'kinds' are generally less visually interesting (i.e. lots of undifferentiated ovoids) and (b) our own galaxy is a (possibly barred) spiral. $\endgroup$ – dmckee Sep 26 '16 at 16:12
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The spiral arms of high density region of a density wave. They do not consist of the same stars always. Individual stars governed by Kepler's law enter and leave the arms. For details: https://en.wikipedia.org/wiki/Density_wave_theory

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  • $\begingroup$ @MarkusAppel You are welcome! $\endgroup$ – velut luna Sep 26 '16 at 12:08
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When seeing artist's impressions of galaxies they always look somewhat like this:

They also look like this, although we might disagree over the word "somewhat" :)

enter image description here

A mixture of shapes, as galaxies collide, evolve over time, and get pulled apart by other galaxies.

The reason spiral shaped galaxies maintain their shape is possibly because there are density waves passing through them.

From: Spiral Arms Wikipedia.

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

Since the 1960s, there have been two leading hypotheses or models for the spiral structures of galaxies:

  1. Star formation caused by density waves in the galactic disk of the galaxy.

  2. The SSPSF model – star formation caused by shock waves in the interstellar medium.

These different hypotheses do not have to be mutually exclusive, as they may explain different types of spiral arms.

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