Galaxies—where do they come from? How long do they exist? These are questions that astronomers are only just now beginning to answer, using observations of the most distant “baby” galaxies that were born right after the Big Bang. They are also studying our own Milky Way and its continuing evolution. It turns out that galaxies didn’t start out as the gorgeous and fascinating objects we see today. They were built over billions of years by systematic mergers and collisions.
Galaxies are some of the oldest structures in the universe. Most of them began like our own Milky Way—getting their start as cosmic seeds in the earliest epochs of the universe, not long after the universe began in an event called the Big Bang. This occurred 13.8 billion years ago and was the simultaneous creation of our universe and the beginning of an expansion of space and time that continues to this day. For the first few hundred million years, things were hot and opaque and the universe consisted of a soup of primordial atomic particles. Eventually, as this nascent cosmos expanded, it cooled. Tiny fluctuations in the density of this basic soup became the seeds of galaxies, helped along by the gravitational influence of dark matter.
About 400 million years after the Big Bang, the first stars began to shine in those infant galaxies, which were little more than shreds of starlit matter. Eventually these primitive galactic shreds began to combine. They collided with each other to form larger collections of stars. As they did, successive generations of stars were born, lived, and died. It was this way with the infant Milky Way, which first began forming at least 11 to 12 billion years ago. Its earliest stars eventually died and seeded the galactic environment with materials that got recycled into new generations of stars. Some of the remnants of our galaxy’s earliest stellar population still exist as slowly cooling white dwarfs in the galaxy’s gassy halo. However, there are other stars that are as old as our galaxy—they most likely formed in globular clusters that we see hovering and orbiting in the galaxy’s halo.
Today the Milky Way is still cannibalizing neighbors that stray too close. There’s a small spheroid-shaped elliptical galaxy called the Sagittarius Dwarf that is slowly spiraling into our galaxy as it orbits. As it goes past, it leaves behind streams of stars that are metal-poor. This indicates they formed early in the history of the universe, and the exact metal content identifies them as being part of the Sagittarius Dwarf. The discovery of this ancient galaxy and its stars in close proximity to the Milky Way was one of the key pieces of evidence that astronomers needed to show how collisions shaped the Milky Way.
The inexorable pull of gravity might also bring the Large and Small Magellanic Clouds into our galaxy in a few billion years. It appears that these two neighboring galaxies have already been interacting with each other. There’s a river of high-speed gas connecting them called the Magellanic Stream, and it may be left over from a close encounter between the pair more than 2 billion years ago that triggered huge bursts of star formation. Such starbursts are important outcomes of galaxy collisions. For example, the Milky Way and Andromeda (our nearest spiral neighbor) grew through ever-more-complex galaxy mergers and near-misses that took millions and millions of years to complete. When the participating galaxies hit head on, in a full merger, their stars mingled but didn’t collide. As with the recent encounter of the Magellanic Clouds, the close encounters also sent shock waves through the clouds of gas and dust, and that spurred huge bursts of star birth activity. So, as galaxies evolve and merge, their populations of stars are enriched by ongoing bouts of star formation.
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.
Ellipticals are galaxies in the most advanced state of evolution. They are the largest, most massive collections of stars that are moving in random orbits. These galaxies form directly from violent collisions of smaller galaxies, and there’s no chance for spiral arms to exist in them. What’s more, ellipticals all have black holes at their hearts, and some of them are shooting jets of material out into intergalactic space. The elliptical galaxy M87, for example, has a high-speed jet of superheated matter streaming away from the region around its central 6-billion solar-mass black hole.