The element first called coronium
And later renamed to newtonium
Seemed to colour the rays
That the sun cast our ways—
But it's now mostly known as balonium.

In astronomy, we identify the various elements out there by looking at the light (that is, electromagnetic radiation, not just visible light) they emit. Different elements, and different ions of those elements, will emit different wavelengths of light. We know what is given off the various known elements in various states of excitement. So, when Charles Augustus Young observed 530.3nm light (a shade of green) from the Sun's corona during a solar eclipse, we were rather excited to realize that this had to be a new element. Nothing we knew emitted 530.3nm wavelength.

Incidentally, this is also how we discovered helium--analysis of the Sun's spectrum revealed an element unknown on Earth, which we named for the Sun (Helios). This new light, however, was only present in the corona, the atmosphere of our sun, so we named it Coronium

We knew nothing else about coronium: we didn't even know it's atomic number to place it on the periodic table. Those of you who have spent much time looking through said table may notice that it's still not present. In 1902, Dmitri Mendeleev (the guy who created the periodic table) trying to explain what aether was, theorized two elements lighter than hydrogen, and thought coronium to be the heavier of the two. At some point, he renamed it newtonium and assigned it an atomic weight of 0.4, which makes little sense from a modern perspective, but he did his best. Of course, newtonium isn't on today's periodic table either.

A good while later, in 1939--some time after our good friends Webster 1913 and EverybodysCyclopedia--Walter Grotrian and Bengt Edlén finally figured out what this mysterious element really was. Grotrian was the one who first suggested that coronium may, in fact, be iron, and after some calculations, Edlén determined that multiply-ionized iron, Fe13+, would indeed result in the observed spectral lines. This also allowed us to explain another coronal line as the result of ionized nickel.

It's also worth noting that Ira Sprague Bowen had paved the way for this line of investigation when, in 1927, he explained away nebulium by pointing out that it's just an oxygen ion, O2+, and Edlén's spectral extrapolation was based on Pol Swings' work. Shoulders of giants, all the way.

Initially, folks weren't too keen on this "it's iron" explanation, as such ionization would require a temperature of several million degrees Kelvin in the sun's corona, yet we knew the surface to be a mere 6 000 degrees Kelvin. Later observation finally confirmed these high coronal temperatures, and the question of coronium was finally settled: it's just iron. Really hot iron.

Sources/Further reaching

The chemical tree: a history of chemistry By William Hodson Brock (p. 326)
And what made me look up coronium in the first place, Neil deGrasse Tyson's excellent The Universe series.