6563

Time now to explain the wonderful 6563. It was pointed out to me that some might think it referred to NGC 6563, but in fact it has a very specific meaning to astronomers and those familiar with spectra.

Bohr atom
Most everyone learns sometime in middle school or high school about the Bohr model of the atom (and then forgets it). To remind you, in the center is the nucleus, containing only the protons (positive particles) and neutrons (neutral). Around it somewhat like planets around the sun are the electrons (negative). The whole atom is so small that you cannot see one with normal microscopes. All the pictures you see in textbooks or online are either artists' illustrations, or using electron microscopes, which are special microscopes that don't use light and you can't look through them like normal microscopes.

Although the analogy of comparing the Solar System to the Bohr model is a good start, it is an imperfect analogy. The most important difference is where the orbiting things can go. In the Solar System there is no real physical law saying where the planets can be located. (Astronomers Bode and Titius once thought they had found such a law, but they turned out to be wrong.) Electrons on the other hand have to be in specific places called orbitals. The innermost one is called n=1 and can contain up to two electrons. The next is n=2 and can hold up to 8, then n=3 and so on. (I forget how many electrons n=3 and greater can hold, but for Hydrogen it's irrelevant as there's only one electron except in some freaky circumstances.)

Emission lines
Each orbital or level also has a corresponding energy, so that the lowest (n=1) is called the rest state, or unexcited, and n>1 are successively higher energies and are called excited states. Exactly how much energy each state has in Hydrogen is very well known, and so when an electron moves from an excited state to a lower, less excited state, we know just how much energy is released -- E=R*[(1/n2)^2-(1/n1)^2] if you care, where R is a constant whose value doesn't really matter for argument's sake here.

Dropping down from any level to any other level produces a packet of energy in the form of a photon - yes, that's what light is made out of. The color of the light (or frequency and wavelength of the photon) depends on precisely how much energy was released. Dropping from any level to the lowest (unexcited, rest, n=1) state produces a photon of ultraviolet light, and these photons are always of set UV colors - one for 2-1, one for 3-1, one for 4-1, ... The specific corresponding colors are called the Lyman series. The first (n=2-1) is called Lyman-alpha, after the first letter in the Greek alphabet, then Lyman-beta (n=3-1), Lyman-gamma (n=4-1), etc. Dropping from anything down to n=2 are the Balmer series. These start off with n=3-2, then n=4-2, n=5-2, etc. Since they were actually the first discovered observationally, they're just called Hydrogen-alpha, Hydrogen-beta, or H-alpha, H-beta, etc. for short. After n=*-2 is n=*-3, the Paschen series, mostly in the infrared, and then Brackett, Pfund and Humphreys, though no one (except IR astronomers) ever remembers those names.

Hydrogen-alpha turns out to be a bright red color, and if you took a look at a tube of hot hydrogen gas through a diffraction grating (think of those kids' fake glasses that make all lights look like rainbows or fireworks) you'd see a bright red streak, then some teal, then blue, and maybe if you have good eyes some purple too. If you do the calculations or measurements and determine the precise energy and wavelength of the photon, you get 0.0000006563 meters. Since astronomers don't like writing out all those zeros, you can instead write it as 6.563*10^-7m. Or 656.3nm (nano=10^-9), or we even came up with a new unit for optical astronomy to have convenient units: 6563Å (1 Ångstrom = 10^-10 meters).

Why Hydrogen?
So what, who cares about hydrogen? It's not like there's that much of it.

Not true. Here on the Earth, just about every molecule out there has hydrogen in it. Water, sugars, carbon chains, DNA, and all other molecules involved in life certainly do. But off the Earth it becomes even more important. In the whole universe, some 98% of the particles out there are Hydrogen. Just under another 2% are Helium. The remaining stuff, less than 0.2%, is everything else. Oxygen, Carbon, Nitrogen, Silicon, Lithium, Uranium, whatever, it's all less than 1 in 500 of the things in the universe. When astronomers point a telescope in the sky, they see hydrogen. It's pretty damned hard to see anything else.

Hydrogen is present in all stars. The outer (Jovian, gas giant) planets of the solar system are primarily hydrogen. Vast clouds hundreds and thousands of times larger than our solar system are made (almost) entirely out of hydrogen. When we look at other galaxies we see hydrogen first. Hydrogen, hydrogen, H. If the universe were God, hydrogen would be His Word, and H-alpha, 6563Å, is the Bible where we can read His Word.