galaxy-evolution

The Andromeda Galaxy is approaching the Milky Way at ~110 km/s and will collide in approximately 4.5 billion years. The two galaxies will merge over several billion years of passes, ultimately forming a giant elliptical galaxy — sometimes called 'Milkomeda.' Despite the collision of two galaxies each containing hundreds of billions of stars, individual stellar collisions are extremely rare. Earth will likely survive, though in a dramatically different night sky.

Galaxies are not static objects. They grow, merge, transform, and eventually fall quiet. Over 13.8 billion years the universe has gone from small, chaotic protogalaxies to the spirals and giant ellipticals we see today, and tracing that transformation is one of astronomy's deepest challenges.

Stars do not sit quietly inside galaxies. Their radiation, winds, and supernovae heat, push, and enrich the gas around them. This stellar feedback is why star formation is slow, why dwarf galaxies are faint, and where much of a galaxy's heavy elements end up.