Britannica Sources used in this answer

Why do H II regions glow as emission nebulae?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

H II region Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.

H II regions glow as emission nebulae primarily because they are composed of hydrogen gas that has been ionized by intense ultraviolet radiation from nearby, massive, hot stars.12

The process of emission occurs through two main mechanisms:

Recombination: Ions capture free electrons into high energy levels. As these electrons cascade down to lower energy states, they emit photons at specific wavelengths, creating observable emission lines.4
Collisional Excitation: Energetic electrons collide with ions, exciting them to higher energy levels. When these ions return to lower energy states, they release light. In the low-density environment of interstellar space, atoms can remain undisturbed long enough to emit "forbidden lines"—highly improbable transitions that are typically suppressed in denser laboratory conditions on Earth.34

Because these ionized gases are transparent to visible light, the photons emitted throughout the entire depth of the nebula contribute to its overall brightness, allowing us to observe these regions as glowing clouds.3