Description:
NGC 6819 is a rich open star cluster located in the constellation Cygnus.
The faint red glow behind NGC 6819 is part of the widespread hydrogen-alpha emission nebulosity that fills much of Cygnus. It comes from thin clouds of ionized hydrogen gas, which emit strongly at the deep-red Hα wavelength of 656.3 nm when energized by ultraviolet radiation from nearby hot stars.