Astronomy
Galaxy Formation
Quick fact
The oldest known galaxies formed just 300-500 million years after the Big Bang, and they are tiny compared to the Milky Way – some contain only a million stars.
Why this is interesting
Look up at the night sky and you see a universe of galaxies. But how did the first galaxies form from the near-uniform soup after the Big Bang?
Read the full explanation
Understanding Galaxy Formation
After the Big Bang, the universe was nearly uniform but had tiny density fluctuations. Gravity amplified these fluctuations, making dense regions denser. Dark matter, which doesn't interact with light, clumped first into halos. Normal gas fell into these halos, heated up, then cooled and fragmented into clouds. These clouds collapsed to form the first stars and small galaxies. Over billions of years, such protogalaxies merged and grew, eventually forming the large, majestic structures we see today. This hierarchical process also explains why galaxies have different shapes and sizes.
A deeper explanation
Galaxy formation is driven by the hierarchical assembly of dark matter halos in a universe dominated by cold dark matter. Initially, tiny quantum fluctuations from inflation are imprinted in the cosmic microwave background and expanded by cosmic expansion. Areas of slightly higher density attract matter gravitationally, causing dark matter to collapse into roughly spherical halos. Gas follows, but its collisional nature leads to shock heating and subsequent cooling via atomic transitions (e.g., hydrogen line emission). Cooled gas becomes molecular and forms stars, creating the first galactic building blocks. Feedback from massive stars (supernovae) and active galactic nuclei (from black hole accretion) can expel gas, regulating star formation and shaping the galaxy's final morphology. Mergers between halos redistribute matter and trigger starbursts, while the cosmic web feeds gas along filaments. This interplay explains observed correlations between galaxy mass, morphology, and environment.