Galactic Ripples

Ripples roll across the Milky Way Galaxy. They’re like the ripples on a pond after stones have been tossed into it. In the case of the Milky Way, the ripples were created by several impacts by a single “stone”: another galaxy.
The Sagittarius Dwarf Galaxy is only about one ten-thousandth as heavy as the Milky Way. But it’s plunged through the Milky Way’s disk three times — about five-and-a-half billion, two billion, and one billion years ago.
Each time it passed through, it created “waves” in the Milky Way. They ripple outward, pushing dust and gas in the Milky Way’s disk ahead of them. That triggers the birth of new stars.
Researchers looked at bouts of star formation in our part of the galaxy. They found three busy periods of starbirth. And all of them took place at about the same time the Sagittarius Dwarf was plunging through the Milky Way. That suggests that many of our stellar neighbors were born as a result of these passages. The researchers even speculated that one passage coul