HII region

An HII region is a glowing bubble of gas lit up around a freshly born massive star. The 'H' is hydrogen and the 'II' is the chemist's and astronomer's label for hydrogen that has lost its electron, that is, hydrogen that has been ionized. So an HII region (say it 'H-two') is literally a region of ionized hydrogen, a pocket of cloud that a hot young star has stripped of electrons and set aglow. These are some of the most colourful and famous sights in the sky.

When a massive star forms, it is so hot, tens of thousands of degrees at its surface, that it floods its surroundings with ultraviolet light energetic enough to knock the electrons clean off hydrogen atoms. A growing sphere of gas around the star becomes ionized, a soup of free protons and electrons at about ten thousand degrees. When a stray electron rejoins a proton, the atom sheds the extra energy as light, much of it in a characteristic deep-red glow (the hydrogen-alpha line), which is why so many star-forming nebulae shine pink-red in photographs. The ionized bubble can be many light-years across, and its size and brightness are a direct measure of how many hot young stars sit inside.

HII regions matter as beacons and as forces. As beacons, they are the unmistakable signal of recent massive star formation; because such stars are short-lived, an HII region marks a place where stars were born just now, astronomically speaking. As forces, the same ionizing radiation that lights the bubble also heats and pressurizes the gas, helping to blow the leftover cloud apart, a key piece of the feedback that ends a burst of star formation. The Orion Nebula, visible to the naked eye, is the nearest grand example.