Twice in the history of astronomy someone found a color in the sky that nothing on Earth could make, and twice they decided they had found a new element.
The first was nebulium. In 1864 William Huggins pointed a spectroscope at the Cat’s Eye Nebula expecting the continuous smear of a glowing cloud, and got instead a handful of sharp bright lines — two of them strong and green, near 5007 and 4959 ångströms, matching nothing in any laboratory on the planet. For the next sixty years the green of the planetary nebulae was assigned to nebulium, an element that existed nowhere but out there.
The second was coronium. During the total eclipse of 1869, with the Sun’s disk blocked, observers caught a bright green line in the corona at 5303 ångströms — again matching no known substance. Coronium. A second element that lived only in the sky.
Both were wrong, and wrong in the same way. In 1927 Ira Bowen showed the nebular green wasn’t a new element at all. It was ordinary oxygen, stripped of two electrons, making a transition the rules of quantum mechanics call forbidden — a jump out of a metastable state so improbable that in any normal gas the atom never gets to make it. Down here, an atom sitting in that state is knocked out of it by a collision long before it can emit; the light simply never happens. But a planetary nebula is emptier than the best vacuum we can build. There, the atom is left undisturbed for seconds — sometimes much longer — and the forbidden jump, given enough time, finally happens. Henry Norris Russell’s verdict was that “nebulium has vanished into thin air,” which is funnier and more exact than he may have meant. Thin air is the whole answer.
The same green burns in the aurora. Atomic oxygen at 557.7 nanometers, another forbidden line, with a radiative lifetime around three-quarters of a second — an eternity by atomic standards. It only appears above about 100 kilometers, where an oxygen atom travels four or five kilometers between collisions and gets touched, on average, once every seven seconds. Lower down, in thicker air, the same atoms are jostled out of the state before they can shine, and the green just doesn’t come.
Coronium took longer — Grotrian in 1939, Edlén in 1943 — and the answer was stranger still. The green corona line was iron, stripped of thirteen electrons, making its own forbidden transition. To tear an iron atom that bare you need a gas hotter than a million degrees. So the fake element turned out to be the discovery that the Sun’s outer atmosphere is hundreds of times hotter than its surface — a fact we still can’t fully explain.
In both cases the thing no laboratory could reproduce was never a substance. It was a condition. Oxygen and iron are common; you can hold them in your hand. What you can’t build on a bench is the emptiness — a vacuum thin enough, or a plasma both searing and rarefied enough, that a familiar atom is left alone long enough to do the forbidden thing. They didn’t find new elements in the sky. They found places, and named the places as if they were things.
The coronium line was finally made in a lab in 2013, a hundred and forty-four years after that eclipse — iron ions held in an electron beam trap, the forbidden green measured at 530.2801 nanometers. The resolution, when it came, wasn’t discovering an element. It was learning to manufacture the circumstance.