Our world orbits a sun located on one of the arms of the Milky Way, a galaxy of some 200 billion stars with spiral limbs that whirl around a thin disk.

But how did the galaxy develop those famous arms?

A new theory, aired Wednesday, says they formed after the Milky Way was whacked by a dwarf galaxy, sending cascades of stars flying to the galactic rim.

And the collision has happened not just once, but twice in the last two billion years -- and a third smashup is on the cards within the next 10 million years, a blink of an eye in cosmic terms, say a team of astronomers.

The intruder is named the Sagittarius Dwarf Elliptical Galaxy, or Sgr Dwarf, according to the paper, published in the British science journal Nature.

Sgr Dwarf does not get a high interest rating among astronomers.

In fact it was only awarded a "galaxy" category in 1994, and with reluctance, too.

It is located far on the other side of the Milky Way near the plane of the disk, which makes it hard to observe, and compared with other galaxies seems a bit of a tiddler.

But if it is small now, it's because it was once bigger. Fat clusters of stars trail from it, evidence of its once-greater size.

Looking at these strange remnants through high-powered telescopes and using computer simulations, a team of astronomers believe that the galaxy crashed into the Milky War through a rival orbital path.

The giant impact sent streams of stars from both galaxies. The stars were eventually tugged outward by the Milky Way's rotation into the limbs that we perceive today. In both episodes, what remained of Sgr Dwarf pierced through the Milky Way's disk.

Its punch came from a thick envelope of "dark matter", the poorly-understood invisible substance that constitutes most of the matter in the Universe, the scientists believe.

"It's kind of like putting a fist into a bathtub of water, as opposed to your little finger," said James Bullock, a theoretical cosmologist at the University of California at Irvine.

His colleague Chris Purcell, now at the University of Pittsburgh, added, "When all that dark matter first smacked into the Milky Way, 80 percent to 90 percent of it was stripped off.

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

The hypothesis deals a blow to the notion that the Milky Way has somehow inhabited a "quiet, rural galactic neighbourhood", said Curtis Struck of Iowa State University in a commentary carried by Nature. "Beware of wildlife, even in apparently quiet galaxies."

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