How do astronomers know the Milky Way has spiral arms if we live inside it?

Because Earth sits inside the Milky Way’s disk, we cannot photograph the whole galaxy from outside. Astronomers instead combine infrared surveys, which can see through dust, with radio observations of hydrogen gas and precise distance measurements from missions like Gaia. They also track the motions of stars and gas to reconstruct the Galaxy’s overall shape. This shows that the Milky Way is a barred spiral galaxy with multiple arms or arm segments. Our view has improved dramatically over the past century as telescopes and astrometry became more precise.