Dwarf star coming in from the south, below the ecliptic. Supposedly will cross the ecliptic between Jupiter and Mars, agitating astroids out of their orbiting positions, the sign of which will be a marked increase in astroid impacts on Jupiter ... and eventually some into the Earth.
If another star crossed the asteroid belt, I think we would see it coming from a long distance away. I haven’t heard of a comet big enough to look like a star.