The break-up has scattered stars
These "alien" stars could at times be close to our Sun, they say.
The interlopers were spotted on a new survey of the entire sky.
This allowed them to remove obscuring foreground objects and get a good look at what lies around our Milky Way.
Torn apart and devoured
Using data from the Two-Micron All Sky Survey (2Mass) - a study of the sky in infrared light - astronomers are showing that our Milky Way is devouring one of its neighbours.
The analysis is the first to map the full extent of the nearby Sagittarius galaxy, showing how this galaxy, which is 10,000 times smaller in mass than our own, is being stretched out - torn apart and devoured by our Milky Way.
"It's clear who's the bully in the interaction," says Steven Majewski, professor of astronomy at the University of Virginia, US.
"If people had infrared-sensitive eyes, the entrails of Sagittarius would be a prominent fixture sweeping across our sky."
Sagittarius has been smeared across space
The cosmic violence cannot be seen easily because of the stars, gas and dust that are in the way.
To get a better view, astronomers used the 2Mass infrared maps and digitally removed millions of foreground stars to leave a type of star called an M giant.
These large, infrared-bright stars act as tracers because they are populous in the Sagittarius galaxy but uncommon in the outer Milky Way.
"We sifted several thousand interesting stars from a catalogue of half a billion," says professor Michael Skrutskie, also of the University of Virginia.
"By tuning our maps of the sky to the right kind of star, the Sagittarius system jumped into view."
Majewski adds: "This first full-sky map of Sagittarius shows its extensive interaction with the Milky Way."
The new image shows that stars and star clusters now in the outer parts of our Milky Way have been ripped from Sagittarius by our Milky Way's gravity.
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WOW!!! Amazing stuff!
