The term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly … See more

The precursor to an H II region is a giant molecular cloud (GMC). A GMC is a cold (10–20 K) and dense cloud consisting mostly of molecular hydrogen. GMCs can exist in a stable state … See more

Physical properties
H II regions vary greatly in their physical properties. They range in size from so-called ultra-compact … See more

As with planetary nebulae, estimates of the abundance of elements in H II regions are subject to some uncertainty. There are two different ways of determining the abundance of metals (metals in this case are elements other than hydrogen and helium) in nebulae, … See more

A few of the brightest H II regions are visible to the naked eye. However, none seem to have been noticed before the advent of the See more

Stars form in clumps of cool molecular gas that hide the nascent stars. It is only when the radiation pressure from a star drives away its 'cocoon' that it becomes visible. The hot, blue stars that are powerful enough to ionize significant amounts of hydrogen and form … See more

Notable Galactic H II regions include the Orion Nebula, the Eta Carinae Nebula, and the Berkeley 59 / Cepheus OB4 Complex. … See more