NGC 6240 is a pair of galaxies merging together located about 400 million light years away towards the constellation Ophiuchus. It is surrounded by a huge cloud of gas, as massive as 10 billion Suns and 300,000 light years across. The cloud likely formed from the gas of both galaxies, agitated by the collision, with additions of elements from supernova explosions, slowly expanding and mixing with cooler gas already present.
The galaxies of NGC 6240 are large spiral galaxies, similar to the Milky Way, and will probably form an elliptical galaxy over the next few million years. Both contain supermassive black holes, which could also merge as they spiral towards each other. The collision has fostered an extended burst of star formation with shorter bursts as well. Some of these new stars were massive enough to collapse into supernovae relatively quickly, adding new elements to the surrounding medium.
Image and information from NASA.