6563

jethereal mentioned to me in private conversation that the only slightly initiated might think I was referring to NGC 6563. To those who have studied astronomy, 6563 is an unmistakable reference to the Hydrogen-alpha line in the Balmer series - recall if you can your Bohr atom. In Hydrogen you've got one proton in the center, and one electron around it. The electron can be in the lowest unexcited state (n=1), or excited (n=2), or more excited (n=3), and so on. If it goes down from an excited state (say n=4) to a less excited state (say n=1), it emits a photon, a particle of light, of just the right energy or equivalently just the right color. Going down from n=anything to n=1 is called the Lymann series after its discoverer, and the lines tend to be in the ultraviolet. Going from n=anything to n=2 is the Balmer series and the lines start in the visible and go on to UV. Going from anything to n=3 is Paschen, and there's something with an H for ones that end on n=4. The specific line n=3-2 is the first one in the visible spectrum, and if you look at a tube of excited hydrogen gas you see it as the brightest color - bright red. Its wavelength is 6563A, or 656.3nm.

ETA:
A more lengthy post on it can be found on my other blog.