Milky Way's Spiral Arms Caused by its Crashes with Dwarf Galaxy

Scientists stated that the two powerful collisions with a dwarf galaxy in the past two billion years was the cause of the spiral arm structure of the Milky Way. The new findings were published in the journal Nature. In trying to explain the shape of our own galaxy, the Milky Way, with its prominent spiral arms rooted in a central bar, scientists had earlier dismissed the influence of outside forces.

However, in the new study, scientists focused on the nearby Sagittarius dwarf galaxy, much of which had been ripped apart by the gravitational pull of the Milky Way, leaving debris that formed a large but very faint stream of stars around our galaxy.  This dwarf galaxy might have once been 100 times more massive.

 This dwarf galaxy's collision with the Milky Way triggered the formation of our galaxy's spiral arms, caused the flaring seen in the outermost disk and influenced the growth of its central bar.  The impact could have generated ringlike structures wrapping around the Milky Way, similar to ones actually seen in our galaxy, such as the Monoceros ring.