Galaxy collisions are a key factor in the evolution of cosmic structures. Between 4 and 6 billion years from now, it will be the turn of the Milky Way and the Andromeda Galaxy to start merging. But what will the consequences be?

This image from the Gemini North telescope in Hawaii shows a pair of spiral galaxies, NGC 4568 (bottom) and NGC 4567 (top), as they begin to collide and merge. The galaxies will eventually form a single elliptical galaxy in about 500 million years.

NGC 4568 and NGC 4567. Credits: International Gemini Observatory/NOIRLab/NSF/AURA Image processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), J. Miller (Gemini Observatory/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab) & D. de Martin (NSF’s NOIRLab)

Another ‘marriage’ similar to this one is expected between 4 and 6 billion years from now, when the Milky Way and the Andromeda Galaxy will collide. Their collision will destroy the existing spiral structures in both galaxies, producing a galaxy with a more dominant central spheroid. It will raise a large amount of dust and will be accompanied by an increase in the mass of the central black hole.

A possibly dramatic-sounding scenario, but one from which individual stars will emerge largely unscathed. In fact, during a merger, galaxies behave somewhat like ghosts, simply interpenetrating each other. This is all because of the large distances (on the order of several light years) between the individual stars that make up the galaxies, which thanks to this typically do not collide during a galactic merger.


Alister W Graham, MNRAS, Volume 522, Issue 3, July 2023, Pages 3588–3601.

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