... A Sky Full Of Hydrogen . ... Interstellar space is filled with extremely tenuous clouds of gas which are mostly Hydrogen. ... Mercury, the closest planet to the Sun, remains the most mysterious of the Solar System's inner planets. ... Nebula Nova Cygni Turns On . ... Light flooded the local interstellar neighborhood, illuminated this existing gas cloud, excited the existing hydrogen, and hence caused the red emission. ... Here a very distant QSO happened to be placed right behind a massive galaxy. ...