Mosaic image of Earth's nighttime sky as the collision of M31 and the Milky Way unfolds:
1. (Upper left): Present Day - the bright belt of our Milky Way stretches across the sky, while the Andromeda galaxy looks like a faint spindle, several times the diameter of the full Moon.
2. (Upper right): 2 Billion Years - M31's approaching disk is noticeably larger.
3. (2nd row left): 3.75 Billion Years - M31 fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda.
4. (2nd row right)-5(3rd row left): 3.85-3.9 Billion Years - during the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters.
6. (3rd row right): 4 Billion Years - After its first close pass, Andromeda is tidally stretched out. The Milky Way, too, becomes warped.
7. (Final row left): 5.1 Billion Years - During the second close passage, the two galactic cores maintain their separate identity. The level of star formation is much smaller because interstellar gas and dust has been reduced during earlier stages of the collision.
8. (Final row right): 7 Billion Years - The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.