Spiral galaxies like the Milky Way can be thought of as thin disks. The disk is believed to be fundamentally axisymmetric (circular symmetry in the plane). When viewed at an angle other than face-on (from above or below the plane exactly), the circular disk appears elliptical. The greater the ellipticity, the more the galaxy's disk in inclined with respect to our line of sight.
In practice, we measure the ellipticity from examining the contours of equal surface brightness (isophotes) in an image.
Here are four images of spiral galaxies viewed at different inclinations, although these galaxies probably have very similar structure. Notice how the ellipticity changes. (Notice too that the face-on one isn't perfectly symmetric!)
|The galaxy directly below is viewed nearly face-on.|
|The galaxy directly above is viewed nearly edge-on.|
Of course, it is not possible to observe a single galaxy at different inclinations!
These images were taken by Riccardo Giovanelli and Martha Haynes using the 0.9m telescope at the Kitt Peak National Observatory.
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