What is expected to happen to the Milky Way and the Andromeda galaxy in about 4 to 5 billion years?

Answer

Andromeda is approaching us at about 110 km per second. Stars are so spread out that few will actually hit each other.

Context

Andromeda (M31) is the largest galaxy of the Local Group and is approaching the Milky Way at roughly 110 kilometres per second, which NASA describes as about 250,000 miles per hour. Its approach speed is measured with the Doppler shift of its light. Its sideways motion is far harder to measure, and Hubble Space Telescope observations repeated over five to seven years showed that the sideways motion is small, so the galaxies are on a nearly head-on path. NASA's 2012 analysis put the first collision about 4 billion years away, with the merger taking a further couple of billion years.

Later work using Gaia data refined the picture. A 2019 study suggested the encounter could be later, around 4.5 to 5 billion years, and a 2021 analysis found about a 50% chance of a merger within 10 billion years, because the mass and motion of the nearby Large Magellanic Cloud and the galaxy Triangulum affect the outcome. The timing is therefore an estimate with real uncertainty.

Stars in each galaxy are separated by such vast distances that direct star-to-star collisions are expected to be extremely rare. Instead, gravity will fling stars into new orbits, and the two spirals are expected to form a single large elliptical galaxy, sometimes nicknamed Milkomeda. Gas clouds will collide, probably triggering bursts of star formation. The Sun is predicted to end up in a more distant part of the merged galaxy, and the Sun itself will be growing steadily brighter over the same period.