Most of the gas in the interstellar medium, some 90 percent, consists of the simplest element, hydrogen. Helium, the second simplest element, accounts for another 9 percent. The remaining 1 percent consists of other elements. And the gas is mostly cold. Recall that hydrogen consists of a proton and an electron. If there is enough ambient energy, the electron gets bumped up the energy ladder into an excited state. But most of these clouds of hydrogen are far from energy sources—stars—and emit no detectable visible light.

But they do emit at radio wavelengths.

Dutch astronomers were the first to appreciate, in the 1940s, that radio waves would travel unimpeded through clouds of dust, and that hydrogen would produce a radio frequency spectral line. The way it happens is this: Both the proton and the electron can be pictured to be spinning like tops. They are either spinning in the same direction or in opposite directions. If they are spinning in the same direction, the hydrogen atom has a little more energy than if they were spinning in opposite directions. Every so often (it takes a few million years), an electron will spontaneously go from the high energy state to the low energy state (flip its spin), and the atom will give off a photon with a wavelength of 21 cm. This photon travels unimpeded through the Galaxy to our radio telescopes.

Since the 21 cm line (or HI—pronounced H-one—line) has a particular frequency, it tells us not only where the gas is, but how its moving. As we'll see in the next chapter, the HI line has been invaluable in mapping our own Milky Way Galaxy.

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