How do galaxies form?

Some scientists believe that stars form quickly in elliptical galaxies. In these oval-shaped galaxies, stars swarm like gnats. Star formation in spiral galaxies — like our own Milky Way — may happen more slowly, as the galaxy draws mass in a spiral pattern toward its center like a drain sucks water.

Astronomers have seen galaxies in various stages of formation. But they haven’t seen the very earliest stage. Instead, they use computer models to approximate what happens — using clues from light that’s traveled to Earth from a time before the first stars or galaxies.

Scientists believe a Big Bang caused our universe to swell into existence 14 billion years ago. They’ve made maps of the early universe by looking at the distribution of light leftover from the Big Bang — the “cosmic microwave background.” These blurry maps show that mass — the stuff that both you and stars are made of — was scattered unevenly in the early universe.

Masses exert a gravitational pull on each other — the closer they are, the stronger they pull. In the early universe, ripples and bumps in density and temperature might have meant that masses in colder regions were slightly closer together. These clumps might have spawned the first galaxies. Once a galaxy begins to assemble, its own gravity increases as it pulls in more gas. Stars might flare into being a few hundred million years after the Big Bang.

But it might take billions of years for a large galaxy like our own Milky Way to be fully formed.

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