How do we know the Milky Way is a spiral galaxy?
We don’t know exactly how the Milky Way would look to an outside observer because we are within the galaxy itself. However, by mapping the location, distances and velocities of the stars, we can determine a lot about the Milky Way and its structure, including any spiral arms – regions with large amounts of dust and gas which can fuel the formation of new stars. We can then reconstruct what the Milky Way would look like to an outside observer.
Even without telescopes we are given clues about the Milky Way. Looking up into the night sky, you will see a thin ribbon of stars stretching across, implying that the galaxy is a thin, disc-like shape. With telescopes we can go much further by accurately mapping the positions and velocities of these stars. From this we see that stars are orbiting about the galactic centre in a similar way to the planets in our Solar System orbiting the Sun. Mapping the stars in this way, we can also identify the location of any spiral arms. In the case of the Milky Way, current data shows the presence of four main spiral arms.