Dreiecksnebel
This violent collision formed most of the galaxy's (metal-rich) galactic halo and extended disk. During this epoch, its rate of star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda and the Triangulum Galaxy had a very close passage 2–4 billion years ago. This event produced high rates of star formation across the Andromeda Galaxy's disk — even some globular clusters — and disturbed M33's outer disk.