hydrogen emission region

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A region in interstellar space containing hot hydrogen gas which emits a characteristic red spectral line at 656.3 nm; such a region is the visible counterpart of an H II region. The 656.3 nm red line is the brightest line in the optical spectrum of hydrogen, and is known as the Hα (hydrogen-alpha) line (see balmer series). It gives nebulae their characteristic red colour in photographs.

Subjects: Astronomy and Astrophysics.

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