Galaxies merging.

(Space) [...] the Hubble Space Telescope captured an image of a peculiar group of interacting galaxies that contains a "cosmic fountain" of stars, gas and dust that stretches about 100,000 light years.

[...] The new image of a trio of galaxies, called Arp 194, looks as if one of the galaxies has sprung a leak. The bright blue streamer seen in the image is really a stretched spiral arm full of newborn blue stars. This stellar activity typically happens when two galaxies interact and gravitationally tug at each other. (source)