A region of ionized hydrogen gas surrounding hot, young stars, appearing as bright emission nebulae. The intense ultraviolet radiation from massive stars strips electrons from hydrogen atoms, creating glowing plasma clouds.
Named using spectroscopic notation where 'H' represents hydrogen and 'II' indicates singly ionized hydrogen (having lost one electron). The Roman numeral system for ionization states was established in early 20th-century atomic physics.
H II regions are cosmic neon signs powered by the universe's most massive stars! The Orion Nebula, visible to the naked eye, is a nearby H II region where newborn stars 40 times more massive than the Sun are lighting up a gas cloud spanning 25 light-years with the energy of 40,000 suns.
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