NGC 3201 is an oddball among the 150-or-so globular star clusters in the Milky Way. It is moving very rapidly through the galaxy, and its motion is retrograde, that is, it’s orbiting around the galactic core in the opposite direction of most of the stars in the galaxy. That’s led to speculation that it may have come from outside and have been captured.
Also, it contains a black hole which was revealed by the strange movements of a star being quickly flung around the massive, invisible singularity.
Image Credit: ESA / NASA