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

By mapping the locations, distances and velocities of the stars, we can determine a lot about the Milky Way and its structure, including any spiral arms. 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 accurately map the positions and velocities of these stars. We see that stars are orbiting the galactic centre in a similar way to the planets 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. Stephen Molyneux, European Southern Observatory