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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 distribution of magnitudes of red-clump stars (which have very nearly the same luminosity) is split towards the Galactic centre, as expected from a boxy/peanut bulge (which is always associated with a bar).

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 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
    Commented Dec 29, 2013 at 19:01
  • $\begingroup$ Bar at Milky Way's heart revealed (August 2005): newscientist.com/article/… $\endgroup$
    – Wayfaring Stranger
    Commented Sep 13, 2014 at 22:52
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    $\begingroup$ 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. $\endgroup$
    – ProfRob
    Commented Jun 9, 2015 at 6:05
  • $\begingroup$ @RobJeffries Look at my article and any quoting it. $\endgroup$
    – Walter
    Commented Jun 10, 2015 at 9:50
  • $\begingroup$ Ah that "Walter"! I'll check it out - I am interested in young moving groups and similar kinematic structures. $\endgroup$
    – ProfRob
    Commented Jun 10, 2015 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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The comment that the picture is an artists`s impression is strictly valid. But as more major surveys of stars are performed and analysed (Gaia in the visible, with distances but difficulties due to the thick dust in the Galactic plane, and VVV in the infrared, which allows dust penetration but distances have to be inferred) our picture of the bar becomes slowly clearer.In fact it does not look unlike the bar in this picture, which was originally based on the GLIMPSE survey of stars in the infrared by the Spitzer satellite (whose mission finished yesterday).

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