Over on Universe Today, a link to a paper which puts forth the theory that 25% of all of the stars of the Milky Way are originally from other galaxies.
“It turns out that many of the stars and globular star clusters we see when we look into the night sky are not natives, but aliens from other galaxies,” said Duncan Forbes. “They have made their way into our galaxy over the last few billion years.”
Previously astronomers had suspected that some globular star clusters, which each contain between 10000 and several million stars were foreign to our galaxy, but it was difficult to positively identify which ones.”
I think the lede is a little off. Instead of thinking of them as invaders, one might think of them as subjugated stars, since its more likely that the Milky Way *captured* them than any other mechanism. I recall a paper not long ago that identified a former dwarf galaxy within the bounds of the Milky Way, but this paper suggests there are many more dwarf galaxies and globular clusters engulfed by our galaxy than I thought.