Visible light spans roughly 400nm (violet) to 700nm (red). Stars emit a continuous spectrum, but nebulae glow at discrete emission lines set by their chemistry: hydrogen-alpha (Hα, 656nm, deep red), doubly-ionized oxygen (OIII, 501nm, teal), and singly-ionized sulfur (SII, 672nm, red). Because these lines are narrow and specific, you can isolate them with narrowband filters — capturing nebula light while rejecting almost all light pollution. This is also the basis of false-color palettes like SHO (the Hubble palette).