What is an emission nebula (H II region)? See Figure 11-3 in Comins,Discovering the Essential Universe,6th ed.

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An emission nebula, also known as an H II region, is a region of ionized hydrogen gas that emits light due to the presence of one or more hot O- and B-type stars in the region. These stars emit large amounts of ultraviolet radiation that ionize the surrounding gas, causing it to glow. The ionized hydrogen gas is denoted by the symbol H II, where the Roman numeral II represents the fact that the hydrogen atoms have lost their electrons and are ionized. The emission nebulae are some of the most photogenic objects in the night sky and are often referred to as H II regions because the ionized hydrogen gas is the prominent feature of these objects.

What Is an Emission Nebula (H II Region)? See Figure