How do we know the Milky Way is a spiral galaxy? (Intermediate)

How do astronomers know the shape of our galaxy (the Milky Way), even though it is not possible to take a photograph of it because to do that we would have to go away from it?

The clues we have to the shape of the Milky Way are:

1) When you look toward the Galactic Center with your eye, you see a long, thin strip. This suggests a disk seen edge-on, rather than a ellipsoid or another shape. We can also detect the bulge at the center. Since we see spiral galaxies which are disks with central bulges, this is a bit of a tipoff.

2) When we measure velocities of stars and gas in our galaxy, we see an overall rotational motion that differs from random motions. This is another characteristic of a spiral galaxy.

3) The gas fraction, color, and dust content of our Galaxy are like other spiral galaxies.

So, overall, it's a pretty convincing argument. Of course, we have to assume our galaxy is not completely unlike the other galaxies we see—once a civilization has accepted that it does not occupy any special place in the Universe, arguments about similarity seem sensible.

This page was last updated January 28, 2019.

About the Author

Sara Slater

Sara is a former Cornell undergraduate and now a physics graduate student at Harvard University, where she works on cosmology and particle physics.

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