A theory of galaxy formation — called the “bottom-up model” — says that large galaxies like the Milky Way attained their great size by swallowing up neighboring dwarf galaxies over billions of years. One in one hundred stars in the Milky Way belong to the stellar halo, which is much larger than the Galaxy’s familiar spiral disk. These stars are almost as old as the Universe. Many of the Milky Way’s ancient stars ancient stars, found in a stellar halo of debris surrounding the Milky Way, had been ripped from smaller galaxies by the gravitational forces generated by colliding galaxies.
The early Universe was full of small galaxies which led short and violent lives. These galaxies collided with each other leaving behind debris which eventually settled into more familiar looking galaxies like the Milky Way.
Milky Way has had a steady diet of smaller galaxies during its lifetime. Early in the life of the Milky Way galaxy mergers such as this occurred on a much more frequent basis, contributing substantially to the mass of the Milky Way.
In about three billion years, the Milky Way and other large galaxy of the Local Group – Andromeda – may also collide. The two galactic cores would orbit each other for another three billion years before merging. During that time, the stars making up the two spiral galaxies would slowly coalesce into a more elliptical combo galaxy, “Milkomeda”. It is possiblethat our Solar System may be ejected from the new galaxy some time during the collision. In other case, if the Solar System moves towards the centre of the collision, life on Earth could also end catastrophically. Though majority of astronomers suggests a more peaceful merger of the two galaxies.