What will happen if the Milky Way and Andromeda galaxies collide?

Key Points:

• Early 2012 Hubble observations suggested Andromeda is on a collision course with the Milky Way in about four billion years, but later studies have revised the odds of a collision to roughly 50%, possibly occurring in around eight billion years.
• The collision, if it happens, would involve the galaxies passing through each other over hundreds of millions of years, creating tidal tails of stars, gas, and dust due to gravitational forces stretching them apart.
• Direct star collisions are extremely unlikely given the vast distances between stars, although gas and dust clouds will frequently collide, potentially triggering intense bursts of star formation.
• The most significant risks involve the supermassive black holes at each galaxy’s center, which could become highly active and emit hazardous radiation during the merger, and possibly merge themselves, releasing powerful gravitational waves.
• The sun and Earth will no longer exist by the time of any potential collision, as the sun is expected to become a red giant and then a white dwarf within about eight billion years, sparing us from witnessing these cosmic events.