Nebulia: Forbidden [O III] 500.7 nm Line
Cosmology & the Sky arc, page 149 of 193. Back to The Pixelated Physics Universe
Who they are
Nebulia, a veiled oracle in deep-navy robes, holds a ring of doubly ionised oxygen whose green light fills a planetary-nebula halo.
Nebulia is a character of the Cosmology & the Sky arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
The physics
The bright green nebular lines at 5006.84 Å and 4958.91 Å (air wavelengths) were long blamed on a hypothetical element, “nebulium”. In 1927 I. S. Bowen identified them as forbidden transitions of doubly ionised oxygen, O²⁺ (spectroscopic notation O III), from the metastable ¹D₂ level of the 2p² ground configuration down to the ³P₂ and ³P₁ levels. Because both levels share the same configuration and parity, an electric-dipole (E1) transition is forbidden; the lines go mainly by the much slower magnetic-dipole (M1) channel. In a lab gas, collisions empty ¹D₂ long before it can radiate. In a nebula they do not, because the density sits far below the level’s critical density.ncrit = Aul / qulλ = hc / ΔEul ncrit: critical electron density, where collisional de-excitation equals radiative decay (m⁻³) Aul: Einstein spontaneous-emission coefficient from upper level u to lower level l (s⁻¹) qul: collisional de-excitation rate coefficient (m³ s⁻¹) λ: emitted wavelength (m) h: Planck constant, 6.626 070 15 × 10⁻³⁴ J s (exact) c: speed of light in vacuum, 299 792 458 m s⁻¹ (exact) ΔEul: energy gap between the two levels (J) Below ncrit nearly every excitation ends in a photon, so a weak transition can still be one of the brightest lines in the spectrum. William Huggins first saw these lines in 1864 in NGC 6543.
No single equation is shown for this character; the physics above is stated in words because it is an observation or a piece of history, and we do not invent formulas.
Honest caveat: “Forbidden” means strongly suppressed, not impossible. Quoted wavelengths are in air (500.68 nm); the vacuum value is slightly longer.The glow is the approximate line colour: a pure spectral colour lies outside the sRGB range a print can show.In characterI was called a new element for sixty years. I was oxygen all along, speaking only where the dark is quiet enough to hear me.
Crossovers
Nebulia meets Flipwarden (21 cm Hydrogen Spin-Flip Line). Flipwarden and Nebulia both shine with forbidden or highly suppressed transitions. Those transitions only survive in thin gas, where atoms wait a long time between collisions. In a dense lab gas, collisions would empty the states first.
Nebulia meets Sulphra ([S II] 671.6/673.1 nm Density Doublet). Nebulia's [O III] lines and Sulphra's [S II] doublet glow in the same nebulae. Sulphra's line ratio reveals the electron density, and Nebulia's lines are used to estimate temperature.
Wear the lore
Concept keywords: Forbidden [O III] 500.7 nm Line, Nebulia, Cosmology & the Sky, physics lore, Pixelated Physics