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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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FYI, as Gabriel noted, this isn't a picture. – HDE 226868 Jun 21 '15 at 20:13
@HDE226868: Depends on what you mean by "picture". It's definitely not a photograph taken from outside the galaxy; it's an artist's conception. – Keith Thompson Jun 22 '15 at 18:31
@KeithThompson Sorry, I meant photograph. – HDE 226868 Jun 22 '15 at 18:32
up vote 9 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

Just to clarify a concept here: there are no pictures of the Milky Way other than the ones taken from within the Solar system, and they all look somewhat like this:

enter image description here

Everything else you see are artists' depictions of the Milky Way based on the latest research. This is the currently most accepted shape:

enter image description here

i.e.: a spiral galaxy with four main arms.

There is an ongoing discussion about whether our Galaxy is made up of four arms or just two. See for example: Vallée (2014), which sadly is not available online :(

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Norma? I suppose they had to pick something. – Wayfaring Stranger Jun 22 '15 at 13:51
@WayfaringStranger: It's named after the Norma constellation. – Keith Thompson Jun 22 '15 at 18:32

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