Ever hear of nebulium? It was the element that was discovered in 1864, and nearly busted the periodic table of elements. It didn't fit in anywhere! And that made sense — when you consider that the element that everybody believed was responsible for forming giant nebulae never actually existed.
Here's the strange story of how the universe faked an element.
In the mid-eighteen hundreds, when people were finding new and incredible ways to use light to uncover the secrets of the universe, one of the most important methods of the time was spectroscopy. A spectroscope was a relatively cheap piece of equipment that centered on a length of film, or a prism, that would separate light out into its component wavelengths. Each element both absorbs and emits lines of a certain wavelength. When sodium, for example, heats up, it emits a particular orange-yellow light by which scientists can identify it. This was one of the major ways to spot new elements.
Spectroscopy was also one of the major ways to understand the composition of stars or heated gases in space. We separated out the light coming from them into lines, and each set of lines told us what the stars were made of.
Which is why, at first, no one said anything in 1864 when Sir William Huggins looked at the spectroscopic lines associated with a nebula and found a strong set of lines at 4959 and 5007 Å wavelengths. These two green lines couldn't be identified. For lack of a better explanation, he attributed them to a new element and called this new element nebulium. There was soon a problem. Why don't we know of this element today? Because years passed and nebulium did not fit in anywhere on the periodic table of elements. No one seriously thought that the table was wrong, but the spectroscopic lines kept showing up in nebula after nebula.
The case for nebulium was both strengthened and weakened in 1869, when PC Janssen observed the sun through a spectroscope and found other clear lines that indicated a strange, yet incredibly widespread element not found on Earth. Named for Helios, the sun god, helium was the first element discovered in space before it was discovered on Earth. Unlike nebulium, helium has nestled comfortably into the periodic table.
The mystery continued into the next century, with scientists everywhere scratching their heads as to what the element, which showed up in one third of nebulae, could be. At last, in 1927, Ira Bowen solved the mystery. The mysterious element, in the words of a fellow astronomer, "vanished into thin air." Literally. It was oxygen.
This oxygen was simply emitting an — equally literally — unearthly light, because it was in a state that could never exist on Earth. Oxygen, when ionized, is a tenacious grabber of anything around it. Take away one of its electrons and it will reach out and nab anything it can with an electron to spare. The oxygen in the nebulae, however, was doubly ionized. Light generally occurs when an electron on an atom gets bumped up a little, because the atom has absorbed energy, and then falls back down to its original state. Nebulae produce these conditions because of the vacuum of space, and because the concentration of gas inside them is so low that it's nearly impossible for humans to induce, even under laboratory conditions.
Now we know of a lot of elements that exist in space, under conditions that we'd never see naturally on Earth. Coronium, another example of an early pseudo-element, turned out to be iron missing thirteen electrons. When these atoms are raised to an excited state — when they take on energy — their only way to reach a lower energy level is to emit a photon that they wouldn't otherwise emit. If they were any denser, they'd knock into each other, and lower their energy that way.
Even today, the hardest vacuums that people can create generally can't force these atoms to reach the low density they experience in the nebulae. For this reason, the spectral lines they emit are actually called "forbidden lines," and are found only in space or in the upper atmosphere of Earth. Yet another reason to take up astronomy. You could see the forbidden light that shines nowhere on Earth.