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

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.

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.

Read more:

• How many stars are in the Milky Way?
• How do we know what the Milky Way looks like?
• If the Universe is expanding, why is the Andromeda galaxy on course to collide with the Milky Way?
• How do we calculate distances to other galaxies?

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