It’s hard to map a forest when you’re standing in the middle of it. You see the trees that are close by, but most of the forest is blocked out. Astronomers have faced the same challenge when trying to map the Milky Way Galaxy. We’re right in the middle of it, surrounded by bright stars and dark dust clouds. So we can’t get an overall picture of the whole thing.

But nature has provided a way to see the forest through the trees: galactic radio. Big clouds of hydrogen gas emit radio waves at a wavelength of 21 centimeters – eight and a quarter inches. The radio waves pass through the intervening material, giving us a good outline of the structure of the Milky Way.

That wavelength is produced when hydrogen atoms get “bumped up” to a higher energy level. When the atoms drop back to their base level, they emit radio waves. This process plays out most commonly in clouds where new stars are being born.

Mapping the clouds revealed that the Milky Way is a spiral galaxy – a beautiful cosmic pinwheel. And measuring the motions of the clouds reveals how that pinwheel spins. So a lot of what we know about the Milky Way has come to us through the broadcasts of “galactic radio.”

The Milky Way arcs across the east as night falls. You need dark skies to see it. The center of the galaxy is in Sagittarius, which is low in the southeast. It’s easy to pick out because its stars form the outline of a teapot.

Script by Damond Benningfield