For decades, astronomers have claimed the Milky Way will merge with Andromeda in ~4 billion years. Here’s why, in 2025, that seems unlikely.
If you had asked any informed astronomer about the fate of our galaxy here in the 21st century, you would have gotten a near-universal story from practically all of them: in about four billion years, the Milky Way would merge with the only galaxy larger than itself in the entire Local Group, Andromeda. Andromeda holds the distinction of being the very first object correctly identified as being outside the Milky Way itself: when Edwin Hubble identified and measured a special class of star (Cepheid variable stars) in Andromeda back in 1923. These stars allowed Hubble, building on the earlier work of Henrietta Leavitt, to measure the distance to Andromeda, determining that it was far, far outside the full extent of the Milky Way.
While nearly all subsequently-discovered galaxies in the Universe are measured to be receding from us — caught up in the expansion of the Universe — Andromeda is different. Instead of moving away, Andromeda is headed towards us at an impressive 109 km/s: about twice as fast as Halley’s comet moves at its maximum speed. Because of this speed, its distance of about 2.5…
