As you know, the Milky Way is a spiral galaxy… a majestic pinwheel in space. But how did astronomers map out the spiral structure of the Milky Way from our position on the far edge of the galaxy? They used a special type of radio wave made by massive clouds of hydrogen atoms spread throughout the spiral arms of our galaxy.
(Note: This article is a little more technical than most, but try stay with it until the end… you’ll be able to follow along.)
A hydrogen atom is a single proton surrounded by an electron. Each of these charged particles has a “spin” which can be in one of two states, up or down. When the spin states of the proton and electron in hydrogen match, the atom has a slightly higher energy than when the spin states are opposite. Once in a while, the electron flips from the “spin aligned” state to “spin opposite” state and spits out extra energy in the form of a special radio wave with wavelength of 21 cm (and a frequency of 1420 MHz).
The hydrogen “spin flip” is a rare event… each atom might flip once every 10 million years. But there are so many atoms in the massive interstellar clouds in the galaxy that the total intensity of the radio waves is big enough to detect with small radio telescopes on Earth. Here’s the best part: much of the galaxy is transparent to these 21 cm radio waves, so they give us a way to see clearly across the entire galaxy even when visible light is blocked by gas and dust in the spiral arms.
When a radio telescope points into the plane of the galaxy, a strong 21-cm radio signal means there are big clouds of hydrogen along the line of sight. As it turns out, the hydrogen clouds congregate in spiral arms that wind around the centre of the galaxy.
But how do astronomers tell which cloud of hydrogen belongs to which spiral arm?
By figuring out the speed at which the cloud moves away from us. The slight shifts in the frequency of the 21 cm line reveal the speed of the clouds relative to the Earth. Fast moving hydrogen clouds are closer to the center of the galaxy, slower-moving clouds are further out. The kind of map astronomers first produced looked something like this:
A rough map of the Milky Way, showing the location of the spiral arms relative to the sun.
The first observations of 21-cm radiation from the Milky Way was no accident, but it was accomplished on the cheap. These radio waves from the Milky Way were first detected in 1951 by E. M. Purcell and his student Harold Ewen. While working part-time on weekends with a $500 grant, Ewen built a radio detector and horn-shaped antenna which he pointed out the window of his lab at Harvard. Passing students enjoyed tossing snowballs into the antenna during the winter months.
Now, a motivated amateur can now easily detect 21-cm waves from the Milky Way from a backyard observatory. Some amateurs and professionals even search for signals from extraterrestrial civilizations, which presumably know all about the 21-cm radiation from hydrogen.