Crimson & Blue & Magenta

Crimson is the hydrogen-alpha — the deep vivid crimson of the hydrogen-alpha emission line (Hα — the first line of the Balmer series of hydrogen emission — at 656.28 nanometers wavelength — approximately in the middle of the visible red spectrum, producing a specific vivid deep red-to-crimson glow). In nebula astrophotography: the H-alpha emission is produced when electrons in the outer shell of hydrogen atoms fall from the third to the second energy level (the n=3 to n=2 transition in the Bohr model of the hydrogen atom — releasing a photon of exactly 656.28 nm wavelength — the most abundant and most widespread emission in the universe, because hydrogen is by far the most abundant element). H II regions: the most spectacular hydrogen-alpha emitters are H II regions (pronounced 'H two' — from the astronomical notation for ionized hydrogen: H⁺ or H II) — clouds of ionized hydrogen surrounding newly formed massive stars — the ultraviolet radiation from the hot young stars (O and B class stars — the hottest, brightest, and most massive stars, with surface temperatures above 10,000 K — the Sun's surface temperature is approximately 5,778 K) ionizes the surrounding hydrogen gas, creating the most spectacular and most visually extraordinary nebulae in the galaxy. Most celebrated Hα nebulae: the Eagle Nebula (M16 — Messier 16 — the 'Pillars of Creation' — the most iconic Hubble image of 1995, showing three columns of dense molecular hydrogen gas and dust that are simultaneously sculpted by the radiation of nearby young massive stars and forming new stars within their tips — the deep vivid crimson-to-pink of the Hα emission dominates the image); the Orion Nebula (M42 — the most easily visible naked-eye nebula, 1,344 light-years from Earth, containing approximately 3,000 young stars in the process of forming); and the Carina Nebula (NGC 3372 — the largest and most spectacular H II region visible from the Southern Hemisphere, containing the most luminous and most massive star visible from Earth: Eta Carinae — a hypergiant star approximately 100-150 times the mass of the Sun, near the luminosity limit for a stable star). Blue is the OIII emission — the pure electric blue of the [OIII] forbidden emission line at 500.7 nm (doubly ionized oxygen — oxygen that has lost two electrons — O++ or O³⁺ — the most characteristic emission of the hottest and most energetically excited nebular regions — particularly the planetary nebulae surrounding dying white dwarf stars and the innermost regions of the most energetically excited H II regions). The [OIII] line: the square brackets in '[OIII]' indicate a 'forbidden' transition (an electron transition that is forbidden by quantum mechanical selection rules under laboratory conditions — in very low density nebular gas, however, where collisions are rare enough that excited atoms can remain in their excited states for seconds or hours before colliding with another particle, these forbidden transitions do occur, producing the characteristic emission lines). The specific pure electric blue of [OIII] at 500.7 nm: sits exactly at the wavelength of maximum sensitivity of the human eye's dark-adapted cone receptors, making it the most visually vivid and most immediately impactful nebular emission. Magenta is the composite — the pure magenta of the combined Hα + [OIII] composite channel, which appears where both hydrogen-alpha (red/crimson) and doubly-ionized oxygen (blue) emission is present simultaneously — the specific additive combination of vivid red and vivid blue light produces magenta (the CMY primary — the color directly opposite green on the RGB color wheel — a color that does not exist as a single spectral wavelength but only as the perception produced by simultaneous stimulation of the long-wavelength and short-wavelength photoreceptors with no stimulation of the medium-wavelength receptors).