The Milky Way Is Still Growing, Surprising Scientists
“It’s no big secret that galaxies grow over time. The force of gravity is powerful enough to pull smaller galaxies, gas clouds, and star clusters into larger ones, even over distances of millions of light years. Our own Milky Way has likely devoured hundreds of smaller galaxies over its lifetime, and continues to absorb the dwarf satellites which surround us. But there’s a steadier, more subtle way that galaxies grow: by continuing to form stars from the gas already inside. While most of the stars that form will do so in the plane or central bulge of a spiral galaxy like our own, a new study has shown that galaxies also grow outward over time, meaning that their physical extent increases in space. The implication is that our own galaxy is increasing in size by 500 meters per second: growing by a light year every 600,000 years.”
Imagine a galaxy all by its lonesome out there in the Universe. It’s full of stars, with gas, dust, plasma, and dark matter permeating all throughout it. What’s going to happen to the galaxy over time? You might think that it will continue to form new stars in its spiral arms, while older stars burn out and eventually die. All of that is true, but there’s a subtle but important effect that really adds up over cosmic time: the physical extent of where stars can be found grows as even isolated galaxies age. The Milky Way itself is growing at a rate of 500 m/s, typical of spiral galaxies around this size. It means that by time the Universe is three times as old as it presently is, Milky Way-like galaxies will have grown to be twice as large as they presently are.