Diffuse nebulae
What is HII?
Nebula
Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries. In visible light these nebulae may be divided into emission and reflection nebulae. Emission nebulae emit spectral line radiation from ionized gas (mostly ionized hydrogen); they are often called HII regions (the term "HII" is used in professional astronomy to refer to ionized hydrogen). Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them. Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called "dark nebulae". Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae.