How gravity pulls a galaxy apart
A small galaxy passing near Andromeda does not need to collide with it in the everyday sense to be changed by it. Andromeda’s gravity acts more strongly on the near side of the smaller galaxy than on the far side. That difference, called a tidal force, stretches the smaller system and can pull stars and gas out of it.
As the two galaxies move onward, the stripped material trails behind in long streams. The stars in those streams still follow their own paths around Andromeda, gradually becoming mixed into its much larger system. The smaller galaxy may lose its shape before it has been completely absorbed.
Why the encounter is not a stellar pileup
A galaxy looks crowded in photographs, but its stars are separated by enormous distances. When galaxies pass through one another, their stars are therefore unlikely to hit directly. Their gravity, however, extends across those distances, so the galaxies can strongly disturb each other without a star-on-star crash.
Gas behaves differently from stars: it can interact and compress as the galaxies overlap, changing where new stars may form. But the broad rearrangement is driven by gravity, not by a swarm of stellar collisions.
A merger built from streams
The result is less like two solid objects smashing together than like one system being gradually unpicked. Stripped stars and gas can remain in visible streams for a long time before dispersing into Andromeda. Those streams are traces of the merger in progress—and a reminder that a galaxy can grow by gathering material its gravity has drawn away from smaller neighbors.
