The Milky Way Galaxy is famous for its spiral arms that whirl around a thin disk. But how did our galaxy get its distinctive spiral feature?

According to a new theory published Wednesday, the Milky Way Galaxy collided with a dwarf galaxy not once but twice in the last 2 billion years to create the distinctive spiral arm structure.

The rogue galaxy that crashed into the Milky Way Galaxy is named the Sagittarius Dwarf Elliptical Galaxy, located on the other side of the Milky Way near the plane of the disk. It was classified as a galaxy only in 1994.

"Sagittarius was among the largest of the Milky Way's dwarf satellites before it began to be torn apart by galactic tides, but objects on that scale are still relatively small details in the eyes of many galaxy-formation theorists," said study lead author Chris Purcell, an astrophysicist at the University of Pittsburgh. "It had always been assumed that the Milky Way had evolved relatively unperturbed over the past few billion years in terms of its global structure and appearance."

Astronomers looking at the bits of the Sagittarius galaxy believe that it is smaller now because it crashed into the Milky Way, sending streams of stars away from both galaxies. As a result of the impact, the stars were eventually pushed outward by the Milky Way's rotation to form the spiral limbs we see today.

"That first impact triggered instabilities that were amplified and quickly formed spiral arms and associated ring-like structures in the outskirts of our galaxy," said Purcell.

The new view about the formation of the Milky Way Galaxy could forever alter the view that our galaxy resides in the quiet part of the universe.

"We may well learn to generally view intermediate-scale spirality in systems like the Milky Way as transient symptoms of recent impacts involving satellites too faint to see," Purcell told SPACE.com. "Future observations of nearby galaxies may even begin to discern the brightest of such companions, which will help solidify the phenomenological link between minor mergers and spiral arms."