Astronomy
How Astronomers Map the Milky Way's Spiral Structure
Quick fact
The first evidence for a spiral structure in the Milky Way came in the 1950s from radio observations of hydrogen gas—not from visible light, which is blocked by dust.
Why this is interesting
You’ve seen illustrations of the Milky Way as a majestic spiral galaxy, but we’ve never taken a picture of it from outside. So how do we know it’s a spiral at all?