An emission nebula is a celestial cloud of high-temperature, ionized interstellar gas and dust that emits its own visible light. Unlike reflection nebulae, which scatter light from nearby stars, emission nebulae are ionized—typically by ultraviolet (UV) radiation from nearby hot, young stars or stellar remnants—causing the gas to fluoresce and glow.

Key Characteristics

Composition: Primarily ionized hydrogen (often referred to as H II regions), along with helium, oxygen, and nitrogen.

Color: Often appear red or pink due to the strong emission of hydrogen-alpha light. Other colors, such as green or blue, can indicate the presence of ionized oxygen or helium.

Ionization Source: The gas is energized by UV photons from nearby massive, hot stars, causing electrons to jump to higher energy levels. When they fall back to lower levels, they emit photons (light).

Location: Often associated with stellar nurseries (star-forming regions), such as the Orion Nebula, where young stars are still surrounded by the gas from which they formed.

Note: Emission nebulae often coexist with dark nebulae (dust clouds that block light) and reflection nebulae (clouds that scatter light), sometimes appearing together in the same region, such as the Trifid Nebula.