Asked by Jamie Sullivan
Firstly, we shouldn’t assume that our galaxy isn’t too dissimilar to those we see just outside of it. Other than that, there are three characteristics the Milky Way possesses which imply that it’s a spiral.
Looking towards the galactic centre, you’re able to see a long, thin strip that suggests that we’re looking at a disc seen edge-on. This rules out an ellipsoidal or any other shape of galaxy. We can also detect a bulge at the centre – and, from our studies of other spiral galaxies in the universe, this breed of galaxy usually has a central bulge.
Combine this with the way that the stars and gas in our galaxy are moving, along with the amount of gas and dust as well as its colour, and we’ve got pretty solid evidence that our galaxy is a spiral at the very least.
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