This new model for the birth of the mysterious bubbles also explains discrepancies in the ages of stars at the galactic middle.

In 2010, sky maps made by NASA's Fermi Gamma-ray Space Telescope revealed two lobes of particles billowing out from the heart of the Milky Way, each one stretching 25,000 light years beyond the galactic plane.

Astronomers suspected the bubbles were inflated by a period of violence in the galactic centre about 10 million years ago, but no one could say what had triggered the outburst.

Earlier this year, Kelly Holley-Bockelmann from Vanderbilt University in Nashville, Tennessee, was discussing the problem with Tamara Bogdanović from the Georgia Institute of Technology in Atlanta.