Astronomy
Galaxy Formation
Quick fact
The first galaxies formed about 400 million years after the Big Bang, and they were much smaller than today's galaxies, merging over time to create giants.
Why this is interesting
Have you ever looked up at the night sky and wondered how the majestic spiral of the Milky Way came to be? The story of galaxy formation is a cosmic drama spanning billions of years.
Read the full explanation
Understanding Galaxy Formation
Imagine the early universe as a nearly uniform soup of hydrogen, helium, and mysterious dark matter, with only tiny ripples in density. Gravity acts on these ripples: slightly denser regions pull in surrounding material, growing denser and hotter. As these clouds (protogalaxies) collapse, gas cools and fragments into clumps that become stars. The first galaxies were small and irregular, but they constantly merged with neighbors, gradually building larger, more organized structures like spirals and ellipticals. Dark matter provides the gravitational scaffolding, forming a halo that holds everything together.
A deeper explanation
Galaxy formation is driven by two key mechanisms: hierarchical merging and baryonic cooling. In the standard model of cosmology (ΛCDM), dark matter halos form first through gravitational instability, creating a web of filaments. Ordinary gas (baryons) falls into these halos, shocks and heats up, then cools via radiation. Cooling allows the gas to contract further, triggering star formation. Feedback from supernovae and active galactic nuclei regulates this process by heating or expelling gas, preventing runaway growth. Over billions of years, halos merge hierarchically, bringing their galaxies together in violent collisions that transform shapes and ignite starbursts. This process explains the diverse galaxy types we see – from small dwarf galaxies to giant ellipticals – and why galaxies evolve over cosmic time. Understanding galaxy formation is crucial for interpreting observations of the distant universe, testing dark matter models, and revealing how our own home came to be.