Now look into the night sky. Our first experiment works best if you move to Alaska, Canada, Iceland, or northern Europe. That’s where you can see the nearest naturally glowing gas — the famous northern lights, or aurora borealis. Though central to our story, the aurora’s mysteries drove physicists bonkers during the 19th century and well into the 20th. What exactly is it, and how does it emit its distinctive green color? Spectroscopes, which separate an object’s light into its component colors, revealed that most auroral light had a precise wavelength of 557.7 nanometers. But, weirdly, comparing this to various glowing gases in the laboratory didn’t yield any answers. Nothing matched.
Physicists were in an uproar. Every explanation proved to be incorrect. Early in the 20th century, German astronomer Julius Scheiner concluded that, “The auroral spectrum is absolutely identical with the cathode spectrum of nitrogen.” Beep, wrong! English meteorologist Marshall Watts was equally firm in an antithetical opinion that, “There seems now little doubt that the [glow of] the aurora must be assigned to krypton.” Beep! The wrong ideas flowed in torrents. Just a few years later, German spectroscopy expert Heinrich Kayser threw up his hands in exasperation: “We know nothing at all about the chemical origin of the lines of the polar light.”
Still, everyone had an opinion. German researcher Alfred Wegener, soon to be famous for his theory of continental drift, published a major work on the atmosphere in which he suggested the auroral glow came from a new “geocoronium” gas. This notion of an undiscovered element producing a green light was not new. For nearly a century, scientists widely attributed the odd green glow of nebulae — which shine at an emerald wavelength of 500.7nm — to a substance called “nebulium.” The cosmos seemed awash with elements not found on Earth.
Decades passed. Lars Vegard, a Norwegian expert in the physics of the aurora, was sure he’d solved the green puzzle in 1924. As he wrote in Nature, “The typical auroral spectrum is emitted from solid [dust particles of] nitrogen.”
They were all wrong. As it turned out, the element nebulium does not exist, and neither does geocoronium. All along, the source of the green light was ordinary oxygen.
In both places — deep space and high in our atmosphere — the mystery arose from the near-vacuum conditions. You see, as Mrs. Wombat used to point out, oxygen’s electrons have certain allowable orbits around the nucleus. But when excited by solar electrons or a star’s high-energy ultraviolet radiation, the electrons jump to energetic yet unstable positions where they can’t stay. Near Earth’s surface where the air is thick, these excited atoms hit others so quickly that they dissipate their extra energy before they can emit it as light.
But in the rarefied upper atmosphere and also in the hard vacuum of deep space, oxygen’s electrons can linger in a “metastable” state before falling to a lower orbit and emitting photons. They thus give off colors never produced under more earthly conditions. This “forbidden radiation,” as it started to be called, comes in a precise shade of yellow-green at 557.7nm in the northern lights and blue-green at 500.7nm for planetary nebulae. (The latter emission originates from doubly ionized oxygen, which has lost two of its normal complement of electrons.) Oxygen. Nothing exotic, after all that trouble. Mystery solved at last.