The Milky Way has a history of devouring its neighbors, the smaller satellite galaxies that orbit it. Over time our galaxy’s gravity will tug on the near sides of these satellites more strongly than their far sides, slowly stretching them out until they tear apart and their stars assimilate into the Milky Way. A relatively recent field of astronomy called galactic archaeology aims to identify these stars, which tend to group together in long, thin streams against the sky that trace their destroyed parent galaxy’s orbit. Columbia University astrophysicist Kathryn Johnston describes how galactic archaeology can teach us about the Milky Way’s history of cannibalism in her feature “Fossil Hunting in the Milky Way” in the December 2014 Scientific American.

The videos below, created from computer simulations made by Johnston and her students, show how our galaxy engulfs a satellite. In the first animation the blue form at the center represents the Milky Way’s round central bulge and the flat disk that surrounds it—home to most of our galaxy’s stars. The bright ball that begins at the top right corner is an orbiting dwarf galaxy. Color denotes the density of stars, with white-yellow being densest and blue most diffuse.


Credit: Kathryn V. Johnston

As the animation progresses, the dwarf galaxy stretches more and more until finally its stars have escaped to form two thin tails that completely encircle the parent galaxy. Astronomers have now found stars in such tail-shaped patterns around the Milky Way, verifying that this process occurs as the physics-based computer simulations suggest.

The next video shows a similar scenario. The central blue bars are minimalist representations of the Milky Way’s bulge and disk, and the black balls are the satellite galaxies.


Credit: David Hendel: simulations and visualizations

In the left panel the satellite moves in a not-quite-circular orbit. Its stars ultimately form tails that surround the Milky Way almost three times over. The right panel, in contrast, shows the situation for a dwarf galaxy orbiting in a very oblong, or eccentric, orbit that plunges right through the galactic center. Its stars ultimately spread out in many directions and lose the tail shape traditional to destroyed satellite galaxies. “It’s not yet clear to what extent structure like those in the right-hand panel exist” around the Milky Way, Johnston says. She and her graduate student David Hendel are working to predict what the products from this type of event would look like to astronomers searching for the remnant stars from our position on Earth inside the Milky Way.