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In reference to the question, "How can we tell that the milky way is a spiral galaxy?"

The answers there clearly sum up the question asked. But Milky Way is not just a spiral galaxy. It is further classified as a barred spiral galaxy.

Question: Which particular feature in the distribution of stars, or in general a feature in observations led us to believe that it is a barred galaxy?

Note: The edge-on picture is not sufficient to establish it, since a dense distribution at smaller radii could also arise from a non-uniform density on the spiral disk if modeled to fit the observations. We don't have any data from other angles or orientations.

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There are several different lines of evidence which together form a coherent picture: that of a barred galaxy. Moreover, as most disc galaxies are barred, we should expect the same from the Milky Way. The various evidences are:

The observed light distribution (2MASS) shows a left-right asymmetry in brightness and the vertical height. This is explained by the near end of the bar being located on that side.

The observed gas velocities show velocities which are "forbidden" in an axisymmetric or near-axisymmetric (spiral arms only) galaxy. These velocities occur naturally from the orbits of gas in a barred potential

The velocity distribution of stars in the Solar neighbourhood shows some asymmetries and clumping which is most naturally explained by orbital resonance with the bar rotation.

The extent, pattern speed, and orientation of the bar is consistent between all three of these.

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  • $\begingroup$ Nice answer! Can you add a few references to articles or instruments concerning at least some of these things you mention: gas velocity measurements, solar neighbourhood assymetries and clumping, bar parameters fitting. $\endgroup$ – Alexey Bobrick Dec 29 '13 at 19:01
  • $\begingroup$ Bar at Milky Way's heart revealed (August 2005): newscientist.com/article/… $\endgroup$ – Wayfaring Stranger Sep 13 '14 at 22:52
  • $\begingroup$ Can you provide something that backs up the item about velocity clumps in the solar neighbourhood are due to resonance with the bar - genuinely interested. $\endgroup$ – Rob Jeffries Jun 9 '15 at 6:05
  • $\begingroup$ @RobJeffries Look at my article and any quoting it. $\endgroup$ – Walter Jun 10 '15 at 9:50
  • $\begingroup$ Ah that "Walter"! I'll check it out - I am interested in young moving groups and similar kinematic structures. $\endgroup$ – Rob Jeffries Jun 10 '15 at 10:06
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We can only see edge-on, but that gives us enough information to establish the shape of our galaxy. For many stars, we don't just have their position in our sky, we also know their distance and relative motion.

This branch of astronomy is called astrometry. This is done from Earth, but in recent years we've had 2 satellite missions, Hipparcos and Gaia, which have given us unprecedented levels of precision for more than a billion stars, which is plenty to give us a good idea of what our galaxy looks like.

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If you look at a recent picture of our galaxy (top view), then you'll be able to see that the stars are distributed in not so much a spiral in the middle, but in the outer arms. The middle is shaped like a bar, which then extends into the two spiraling outer arms. It is because of this particular arrangement that we say that it is a barred spiral galaxy. Top View

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    $\begingroup$ There are no top view pictures of the Milky Way. $\endgroup$ – HDE 226868 Jun 8 '15 at 15:26
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    $\begingroup$ Someone should frame this. $\endgroup$ – Rob Jeffries Jun 8 '15 at 18:17
  • $\begingroup$ Ouch... $\endgroup$ – pela Jun 8 '15 at 21:26
  • $\begingroup$ This is not an image of the Milky Way, as I said before. $\endgroup$ – HDE 226868 Jul 19 '15 at 16:00
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    $\begingroup$ @RohitNutalapati Of course it's an artist's impression. But that should be clearly indicated. And it's not proof, because the picture is only accurate because it's based off of the idea that the galaxy is a barred spiral galaxy. You're using circular logic here. $\endgroup$ – HDE 226868 Jul 22 '15 at 20:50

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