How is an H II region formed?
A) In a supernova explosion large amounts of hydrogen are ionized, and this forms the H II region.
B) They are remnants of the earliest period in the universe when only hydrogen existed. They were ionized when the cosmic microwave background radiation was released.
C) They are hydrogen clouds ionized by the radiation from hot O- and B- type stars.
D) They are hydrogen clouds ionized in the eruption of the T Tauri phase of the star that they surround.
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