Sulphra: [S II] 671.6/673.1 nm Density Doublet
Cosmology & the Sky arc, page 153 of 193. Back to The Pixelated Physics Universe
Who they are
Sulphra, a two-faced porcelain oracle, breathes two unequal ribbons of red light into a balance that tips toward the brighter one.
Sulphra 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
Singly ionised sulphur, S⁺ (S II), emits a forbidden red doublet at 671.6 nm and 673.1 nm (air 6716.44 / 6730.82 Å) from the ²D°5/2 and ²D°3/2 levels down to the ⁴S°3/2 ground level. The two upper levels lie at almost the same energy, so temperature hardly matters, but they have different radiative rates. That makes the line ratio a gauge of electron density.R = I(671.6) / I(673.1)ne → 0: R → g(²D5/2)/g(²D3/2) = 6/4 ≈ 1.5 ne → ∞: R → (g A)5/2/(g A)3/2 ≈ 0.44 R: intensity ratio of the doublet (dimensionless) I: line intensity (W m⁻² sr⁻¹) ne: electron number density (cm⁻³) g: statistical weight 2J + 1 of the upper level A: Einstein spontaneous-emission coefficient of each line (s⁻¹) At low density every collisional excitation radiates, so R follows the collision strengths (proportional to g). At high density the levels reach Boltzmann populations and R follows gA. In between, roughly 50 to 2000 cm⁻³, R measures ne: gas more than ten million billion (10¹⁶) times thinner than air.
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: The limiting ratios (about 1.45 to 1.5 and 0.44) depend on the atomic data adopted and weakly on temperature, so density values carry real systematic uncertainty.The glow is the approximate line colour: a pure spectral colour lies outside the sRGB range a print can show.In characterAsk one tongue and you hear a colour. Ask both and compare, and I will tell you how crowded the dark is.
Crossovers
Sulphra meets Nebulia (Forbidden [O III] 500.7 nm Line). 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.
Sulphra meets Nightglowe (OH Airglow). Sulphra's red doublet and Nightglowe's airglow are both faint emission lines that observers must know by wavelength. One maps the density of a distant nebula; the other is foreground glow from our own upper atmosphere.
Wear the lore
Concept keywords: [S II] 671.6/673.1 nm Density Doublet, Sulphra, Cosmology & the Sky, physics lore, Pixelated Physics