In the early nineteenth century, two scientists -- Joseph von Fraunhofer and Charles Wheatstone -- independently observed something strange; if you heated up samples of various elements, they emitted a light spectrum that contained strong peaks at certain frequencies, showing up as bright lines instead of a continuous rainbow of colors.
It quickly became obvious that this property could be used to identify the presence of different elements in mixed samples. In fact, helium was discovered when French astronomer Georges Rayet found emission lines in the solar spectrum that didn't correspond to any other known element, making it the only element in the periodic table first detected somewhere other than on Earth. (The name helium comes from the Greek Ἥλιος, meaning the Sun.)
Figuring out why this phenomenon occurred, though, took almost a hundred years. The explanation, due in large part to the work of Danish physicist Niels Bohr, has to do with the fact that the electron shells in atoms are quantized -- there are only certain allowed energy levels, so an atom has to absorb a particular frequency of light in order for one of its electrons to jump to the next level (or, conversely, to drop to a lower level, the atom has to emit a photon of a particular frequency). This simultaneously explained the specificity of emission spectra and the odd phenomenon of absorption spectra, where broad-spectrum light passing through transparent substances shows dark lines where certain frequencies are absorbed, effectively subtracting them from the beam.
So each element has its own distinctive "fingerprint" of spectral lines, which is how researchers here on Earth can determine the chemical composition of distant stars, and even the constituents of the atmospheres of exoplanets.
However -- as usual -- even this rather complex model has some unexpected twists.
Very rarely, the electrons in atoms will undergo forbidden transitions, resulting in light being emitted that should not be possible from the element in question. (A simple analogy is if you were climbing a staircase, and somehow were able to go up by one-and-three-quarters steps.) These transitions are highly unstable (just as your attempted ascent would be), and the electron almost instantaneously collapses back into one of the allowed energy states, but when it does so the atom emits a frequency of light you wouldn't expect. So these aren't so much forbidden as they are extremely improbable; in ordinary situations, their contribution to the light spectrum is vanishingly small.
But in very high energy conditions, where the electrons are bouncing all over the place millions of times per second, you begin to see a significant contribution from forbidden transitions.
The reason this comes up is because of a study of a Seyfert galaxy named MCG 01-24-014. Seyfert galaxies, named after American astronomer Carl Keenan Seyfert who studied them extensively, look superficially like ordinary spiral galaxies, but have an active galactic nucleus. This latter name is a massive understatement, mostly because astronomers shy away from calling something "Holy Shit This Thing Is Super Bright, No Really You Have No Idea How Bright It Is." The center bit of a Seyfert galaxy has a luminosity equal to the luminosity of all the stars of the Milky Way put together, and is thought to be the result of large quantities of material falling rapidly into a supermassive black hole. Most of the light emitted is outside of the visible spectrum -- thus their ordinary appearance through a telescope -- but when viewed in other frequency ranges, it becomes obvious how weird they are.
And MCG 01-24-014 is really peculiar -- emitting far more light from forbidden transitions than even an average Seyfert galaxy would. So whatever is powering its galactic core is running full-throttle.
The forbidden light of Seyfert galaxies provides us with yet another example of "you think you understand, then nature throws you a curve ball."
Sometimes you hear the criticism levied at scientists that all the technical details somehow take away from the wonder of simply looking up and delighting at the beauty of the night sky. I can't speak for anyone else, but for me, the exact opposite is true. I can still go outside on a clear winter's night and look up at my favorite naked-eye astronomical object -- the Pleiades -- and fully appreciate how lovely it is, but my enjoyment is increased further by knowing that it's a cluster of recently-formed hot blue supergiant stars inside the wispy strands of a reflection nebula.
Understanding and appreciation shouldn't be inversely proportional. The more I know, the more I wonder at the beauty, complexity, and strangeness of this universe in which we live. The only frustrating part about it all is the limitation of my mind in comprehending it all.