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When I see pictures of the Milky Way they show a spiral shape. Why is this?

artist depiction of Milky Way from above

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2 Answers 2

up vote 8 down vote accepted

Density waves cause the build up of galactic material in to spiral arms. An object does not have to stay in a spiral arm; it can join or leave it as the density wave passes through the galaxy. However, the likelihood of an object being in a spiral arm is greater.

The shape of the arms does not relate to matter falling in; this is an illusion. Objects perform elliptical or near-circular orbits around the center of the galaxy.

The origin of these density waves is not known conclusively, though there are many simulations that re-create the effect through satellite galaxy perturbations.

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The objects in a galaxy orbit it's center as a planet does with it's star, they keep the same distance. They look like a spiral for something that is called density waves that make some parts of the galaxy (the arms) more dense.

You have to imagine a star system like ours orbiting the galaxy in your picture following those radial lines, so sometime the system will be inside of and arm and sometimes outside of it.

Thanks to this extra density in the arms is where most of the star formation takes place.

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two answers seem to have come in at the same time so I +1 both of them, thanks –  esé May 23 '14 at 14:11

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