The Giant Galaxies We See Today, Even Our Own, Were Built Up From Many Smaller Galaxies, But Construction Isn't Done Yet, It Continues Even Today. Old-grown galaxies approach and interact with each other. They may collide and eventually merge. As the galaxies approach, the tug of gravity creates tides that distort their shapes. Stars and gas stream into new orbits. Sometimes they're completely ejected, trailing into the depths of intergalactic space. Gas clouds compressed in the chaos light up with intense rounds of star formation. Because stars create most of the chemical elements, such episodes have a profound effect on a galaxy's chemical makeup. This infrared image of the entire sky shows half a billion stars, most are in our galaxy. Some are not. These are companion galaxies that orbit our Milky Way. And some are in between. In 1994, astronomers discovered that some of these stars actually belong to a different galaxy. It's called the Sagittarius Dwarf Eliptical, and the Milky Way is tearing it apart. Because the dwarf galaxy passes through the Milky Way's disk, gravitational tides stretch the dwarf's stars into long streams that wrap around the galaxy's orbit. For the dwarf, it's a fatal attraction. For the Milky Way, it's but one of many similar events that shaped our home galaxy. But there's something much bigger headed our way. M31, the Andromeda Galaxy. This is no dwarf. It's the Milky Way's biggest neighbor of roughly the same size, mass and type. Astronomers say the crash will be the end about two billion years from now. This supercomputer simulation shows how the event may unfold over billions of years. The first pass distorts the two great spirals. Stars are tossed into the intergalactic night, like sparks thrown from a campfire. Our Sun, complete with planets in tow, could be similarly ejected. Each pass blurs the identities of each galaxy. Eventually, Andromeda and the Milky Way will merge into a single entity, mass, mass, astronomers call, Milkamaga. How did the shape, structure and chemical content of galaxies change over the sweep of cosmic history? Deep surveys by the James Webb Space Telescope will capture the full panorama. And the earliest dwarfs that formed to the familiar galaxies we see around us today.