The Gum Nebula is an emission nebula that covers a large region of space in the Beta Quadrant portion of the Milky Way Galaxy's Orion Arm beyond the Scorpius Reach, approximately 1,200 light years (or 350 parsecs) from the Sol system. The greatly-expanded remnant of an ancient supernova, the nebula was discovered by Australian astronomer Colin Stanley Gum in the year 1955.
A roughly spherical bubble of hydrogen gas, the Gum Nebula contained a volume of space larger than the entirety of the United Federation of Planets and its neighboring powers combined. This region of space was first reached by the Beta Quadrant survey missions of the USS Excelsior in the 2290s and the USS Olympia in the 2360s. In 2379, Starfleet assigned two of the newly-launched Luna-class vessels, the USS Titan and the USS Ganymede, to commence comprehensive exploratory missions of the nebula and its environs. The Titan began her explorations in the coreward half of the nebula, while Ganymede started in the rimward half.
Early in her explorations, the crew of Titan discovered that the Gum Nebula was home to many of the Milky Way Galaxy's cosmozoan species such as the Star-jellies and the Branchers, among others. (Star Trek: Titan novel: Orion's Hounds)