An emission nebula is a glowing cloud of ionized gas that emits its own light when energized by nearby hot stars, particularly their ultraviolet radiation which strips electrons from hydrogen and other elements, causing the gas to shine in distinctive colors such as red from H‑alpha emissions.
These nebulae typically form in regions where massive, young O‑ or B‑type stars have recently been born; their intense radiation both sculpts the gas into features like cavities and pillars and can trigger further star formation through pressure waves.
They belong to a broader class known as H II regions—vast stretches of ionized hydrogen—that mark active stellar nurseries, seen in iconic examples like the Orion Nebula and NGC 1858 in the Large Magellanic Cloud.
Famous emission nebulae include the Bubble Nebula (NGC 7635), where a central massive star creates a spherical bubble in the surrounding gas, and the Trifid and Crescent Nebulae, which are rich laboratories of stellar evolution and gas dynamics.
In visible-light observations, emission nebulae often display vivid red hues from hydrogen emissions, alongside greens and blues from ionized oxygen and sulfur; recent imagery from NASA and ESA missions like Hubble and Webb reveal even more complex structures and infrared details.
These nebulae are not just static clouds—they are dynamic and evolving, as ongoing stellar winds, radiation pressure, and gravity interact to form new stars, carve intricate shapes, and disperse gas back into the galaxy.