The Milky Way and Andromeda are currently hurtling at one another at 110 km/s. In roughly 4 billion years, the galaxies will collide and subsequently form a single, larger galaxy. Though this collision may seem like it will be the end of times, in reality, there is a very small chance that the Solar System will even be effected.
(Rendition of the Milky Way and Andromeda Collision)
This is due to the fact that galaxies, generally speaking, are not very dense. Andromeda is roughly 260,000 ly in diameter and contains about 1 trillion stars. The Milky Way is roughly 100,000 ly in diameter and contains about 300 billion stars. Though the number of stars is much denser near the center of galaxies, the average distance between these stars is still about 160 billion km. Thus, the chances of any star colliding is extremely small.
On the other hand, the super-massive black holes at the centers of Andromeda and the Milky Way will definitely merge. This is because the black holes are located at the center of mass in both of the galaxies. Since the two galaxies are attracted to the other’s center of mass, these two black holes are moving directly toward one another and thus will merge. A new, even more massive black hole will be created and gravitational waves will be released as these black holes get within one light year of one another.
