Relativistic star systems

All these year of sci-fi/astronomy and I had never encountered this idea — Stars That Race through Space at Nearly the Speed of Light, “hypervelocity” stars. Rogue systems, sure, even ones deep in intergalactic space (a haunting thought). But moving at relativistic speeds?! 🤯 Compared to their frame of reference, which would be the stars around them, or the nearest galaxy. Yet another gift from the weirdness of supermassive black holes.