<p>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. </p>

<p>This branch of astronomy is called <a href="https://en.wikipedia.org/wiki/Astrometry" rel="nofollow noreferrer">astrometry</a>. This is done from Earth, but in recent years we've had 2 satellite missions, <a href="https://en.wikipedia.org/wiki/Astrometry" rel="nofollow noreferrer">Hipparcos</a> and <a href="http://sci.esa.int/gaia/28820-summary/" rel="nofollow noreferrer">Gaia</a>, 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. </p>

CC BY-SA 4.0
2019-04-16T11:26:54.080Z
2019-04-16T11:26:54.080Z
Hobbes
<p>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).</p>

CC BY-SA 4.0
2020-01-29T08:22:37.010Z
2020-01-29T08:22:37.010Z
John Beckman
<p>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:</p>
<p>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.</p>
<p>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).</p>
<p>The observed gas velocities show velocities which are &quot;forbidden&quot; in an axisymmetric or near-axisymmetric (spiral arms only) galaxy. These velocities occur naturally from the orbits of gas in a barred potential.</p>
<p>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.</p>
<p>The extent, pattern speed, and orientation of the bar is consistent between all of these.</p>

CC BY-SA 3.0
2013-12-29T19:01:31.633Z
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.
Alexey Bobrick
CC BY-SA 3.0
2014-09-13T22:52:33.997Z
Bar at Milky Way's heart revealed (August 2005): http://www.newscientist.com/article/dn7854-bar-at-milky-ways-heart-revealed.html#.VBTKHOeItFQ
Wayfaring Stranger
CC BY-SA 3.0
2015-06-09T06:05:33.800Z
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.
ProfRob
CC BY-SA 3.0
2015-06-10T09:50:25.380Z
@RobJeffries Look at [my article](http://iopscience.iop.org/1538-4357/524/1/L35/) and any quoting it.
Walter
CC BY-SA 3.0
2015-06-10T10:06:58.450Z
Ah that "Walter"! I'll check it out - I am interested in young moving groups and similar kinematic structures.
ProfRob
CC BY-SA 3.0
2015-06-10T12:04:21.130Z
@RobJeffries psst about that walter here.
Walter
CC BY-SA 4.0
2013-12-29T16:25:31.440Z
2021-09-28T13:12:54.733Z
2021-09-28T13:12:54.733Z
Walter
<p>In reference to the question, <a href="https://astronomy.stackexchange.com/questions/1225/how-can-we-tell-that-the-milky-way-is-a-spiral-galaxy">"How can we tell that the milky way is a spiral galaxy?"</a></p>

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

<p><strong>Question:</strong> 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?</p>

<p><strong>Note:</strong> 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.</p>

<galaxy><milky-way>
How do we know Milky Way is a 'barred' spiral galaxy?
Cheeku
# How do we know Milky Way is a 'barred' spiral galaxy?

(Asked by: username_0 on 2013-12-29)

In reference to the question, ["How can we tell that the milky way is a spiral galaxy?"](https://astronomy.stackexchange.com/questions/1225/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.

**Tags:** galaxy, milky-way

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## 3 answers

### Answer by username_3 on 2013-12-29. Score: 22 | Accepted answer

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.

> **Comments:**
> **username_4**: 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.  
> **username_5**: Bar at Milky Way's heart revealed (August 2005): http://www.newscientist.com/article/dn7854-bar-at-milky-ways-heart-revealed.html#.VBTKHOeItFQ  
> **username_6**: 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.  
> **username_3**: @RobJeffries Look at [my article](http://iopscience.iop.org/1538-4357/524/1/L35/) and any quoting it.  
> **username_6**: Ah that "Walter"! I'll check it out - I am interested in young moving groups and similar kinematic structures.  
> **username_3**: @RobJeffries psst about that walter here.  

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### Answer by username_1 on 2019-04-16. Score: 2

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](https://en.wikipedia.org/wiki/Astrometry). This is done from Earth, but in recent years we've had 2 satellite missions, [Hipparcos](https://en.wikipedia.org/wiki/Astrometry) and [Gaia](http://sci.esa.int/gaia/28820-summary/), 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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### Answer by username_2 on 2020-01-29. Score: 2

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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