The expansion of the universe carries galaxies apart; gravity within the Local Group is the exception. Andromeda, 2.5 million light-years away, is moving towards us. Hubble measurements published in 2012 showed that its sideways velocity is too small to let it miss: the two galaxies will meet in roughly 4 to 4.5 billion years and, after a series of passes lasting a further few billion, settle into a single elliptical galaxy.

The word collision misleads. The space between stars in a galaxy is millions of times their own diameters, and the chance of two stars striking each other is negligible. What changes is structure: gas clouds will collide and compress, triggering a brief intense burst of star formation, the spiral arms will come apart, and the system will become an elliptical galaxy whose stars follow randomly oriented orbits. The Sun is likely to be thrown much further from the galactic centre in the process.

This is an event of calculation rather than observation — which is exactly why it sits in a band of its own on the timeline. Its scale is remote too: 4.5 billion years is slightly more than the entire age of the Earth so far, and falls in much the same interval as the estimate for the Sun becoming a red giant and rendering the Earth uninhabitable. Its human significance lies less in the event than in the measure it offers: a species that can read the past back 13.8 billion years can compute a comparable distance forward.