Britannica
Sources used in this answer
How dense is the gas in a typical H II region?
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.
In a typical H II region, the gas density generally ranges from 10 to 100,000 particles per cubic centimeter. Specifically, a typical H II region within the Milky Way Galaxy has an average density of approximately 10 atoms per cubic centimeter.1
It is important to note that these regions are not uniform; they often contain dense condensations or "knots" of matter. Furthermore, there are specialized, very young, and small structures known as "ultracompact" H II regions, which exhibit significantly higher densities of about hydrogen atoms per cubic centimeter.12