The evolution of galaxies is a constant work in progress. For example, the Milky Way and Andromeda are bound together by a common pull of gravity. They’re approaching each other at 110 kilometers (68 miles) per second. In about 5 billion years, they will actually pass through each other. They’ll mingle stars, but more importantly, they’ll draw some of each other’s gas and dust out into long, intergalactic, star-forming streamers. Over several billion years, the two will perform a delicate cosmic dance—passing through each other several times before ending up as a giant elliptical galaxy.
