How do we know the Milky Way is a spiral galaxy? © Getty Images

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

The clues lie in the stars…

Since we live inside the Milky Way it is difficult to see its spiral form. There are some clues though. First, there is a concentration of stars along the galactic plane and particularly in the constellation of Sagittarius. This implies the Milky Way is disc-shaped with a central bulge, just as we see in other spiral galaxies.

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Second, measurements of the velocities of stars and clouds of gas reveal that their motion is not random but follows a rotational pattern – just like those we see in other spiral galaxies.

Most convincingly, measurements of the distances of these objects show clearly that they are concentrated along the arms of a spiral. Conclusion: the Milky Way is a barred spiral galaxy with four arms.

Asked by: Dileep Bagnall (Lancashire)

Want to know more about our galaxy? Check out our complete astronomy for beginners UK guide.

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