I have read this question:
When galaxies collide it is not that their stars crash into each other, because their individual cross-sections are extremely small when compared to the space between them. This is dealt with in qualitative terms on the Wikipedia page on the likely collision.
What will happen to the Earth when Milky Way and Andromeda merge?
And this one:
All that being said, the effect would be order of magnitude smaller than the type of seismic events that happen on a daily basis, and would not pose any sort of threat to anything on Earth.
Andromeda & Milky Way Merger: Gravitational Waves
The first one says that the stars (solar systems) will hardly collide with each other.
Now the second one says that the gravitational waves from the merger of the central black holes would be hardly detectable.
So basically, when this merger happens, assuming humanity is still here, and the Earth is still intact, would we even notice the merger? As far as I understand, the black holes would merge in seconds, but I do not know how long the full merger of the galaxies would last. Would we just simply see more stars moving fast in the night sky?
Would we even notice the merger with the Andromeda Galaxy?
Best Answer
The merger would be indirectly noticeable due to a dramatic burst of star formation and supernovas. The gas of the two galaxies will meet at high velocity, clump, and produce new stars. Some will be very heavy and bright, resulting in supernovas and gamma ray bursts: the merged galaxy may become a bit too risky for planet-bound civilizations dependent on an ozone layer. This is amplified by the possibility of gas accretion on the central black holes producing a luminous active galactic core. In the end these processes will tend to blow away much of the gas and end star formation, leaving us with a big elliptical galaxy.
Obviously the process itself stretches over hundreds of million years from start to finish of the final merge, so there would never be anything moving dramatically. The night sky would just keep on getting more complex and rich until the merger winds down.
A civilization observing the stars would likely figure out what is going on, especially by comparing to other galactic mergers.