Emission nebula

An emission nebula is a cloud of ionized gas that emits light after energetic radiation excites its atoms, often from nearby hot stars.

Photoionization: The removal of electrons from atoms or molecules by absorbed photons. Ultraviolet photons from hot stars ionize the gas that makes an emission nebula glow.
Orion Nebula: A bright star-forming nebula in the constellation Orion, also cataloged as Messier 42. It is a nearby example of an H II region shaped by newly formed massive stars.
Reflection nebula: A dust cloud visible because it scatters light from nearby stars. Unlike an emission nebula, it shines mainly by redirecting starlight rather than emitting excited-gas lines.
Nebular emission-line diagnostics: Methods that infer physical conditions in ionized gas from ratios of its spectral emission lines. Line ratios in emission nebulae estimate temperature, density, and chemical abundances.
Stellar wind: A continuous outflow of charged particles from a star. Winds from massive stars carve cavities and compress gas around emission nebulae.
H II region: A region of ionized hydrogen surrounding young, hot stars. Many emission nebulae are H II regions, with hydrogen ionization driving their visible light.
Eagle Nebula: A star-forming region in Serpens that contains the Pillars of Creation. Its pillars show how radiation and stellar winds sculpt gas inside an emission nebula.
Dark nebula: A dense cloud of gas and dust that appears as a silhouette against brighter background objects. It obscures light instead of glowing from ionized gas.
Metallicity: The abundance of elements heavier than helium in an astronomical object. Nebular emission lines help measure how enriched a galaxy’s gas is in elements made by stars.
Radiation pressure: The force exerted when electromagnetic radiation transfers momentum to matter. Starlight can push dust and gas, helping reshape the nebula around its ionizing sources.

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