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As astronomers look out into the Universe, they’re also looking back in time. By seeing an object 1 billion light-years away, astronomers are seeing how the object looked 1 billion years ago, since the light took a billion years to reach us. This “time machine” effect allows astronomers to study galaxy evolution; how small galaxies built up over time to become the structures we see in the Universe today.
After the Big Bang, the Universe was the same everywhere. But tiny fluctuations in density made some regions attact others with more gravity, and over time, regions began to clump together into larger and larger structure. These clumps became primordial galaxies, and clumps of hydrogen gas within the proto-galaxies became the first stars.
Some of these early objects were small, and became tiny dwarf galaxies, while others were much larger and became the familiar spiral shapes, like our own Milky Way.
Once formed, these galaxies evolved together in larger galactic structures called groups, clusters and superclusters. Galaxies were attracted to one another by gravity, and collided together in a series of mergers. The outcome of these mergers depends on the mass of the galaxies in the collision.
Small galaxies are torn apart by larger galaxies and added to the mass of larger galaxies. Our own Milky Way recently devoured a few dwarf galaxies, turning them into streams of stars that orbit the galactic core. But when large galaxies of similar size come together, they become giant elliptical galaxies. The delicate spiral structure is lost, and the merged galaxies become large and egg-shaped. This will be the outcome for our own Milky Way galaxy when it merges with Andromeda in a few billion years.
And so, galaxy evolution has been occurring over billions of years, and it will continue to happen for the foreseeable future. Galaxies will continue to evolve and change as they interact with other galaxies.
We have written many articles about galaxies for Universe Today. Here’s an article about how a supercomputer will study galaxy evolution.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.