Star and Galaxy Formation in the Early Universe

After the Big Bang, the universe cooled, leading to the formation of neutral hydrogen and helium atoms. Gravity, as described by Einstein's theory of relativity, caused these tiny atoms and dark matter to exert gravitational pull.

Between 150 million and a billion years after the Big Bang, hydrogen and helium began to collect into denser regions. These large gas clouds, or nebulae, can remain in hydrostatic equilibrium, but if massive enough (above the Jeans mass), gravity overwhelms gas pressure, leading to collapse.

As a gas cloud collapses, it flattens into a disk and heats up over millions of years, reionizing atoms into plasma. Eventually, the inner region becomes so hot that outward pressure forms a protostar in temporary hydrostatic equilibrium. With further mass accretion, temperatures reach millions of degrees, initiating nuclear fusion and birthing a star.

The formation of stars releases tremendous radiation, which reionizes surrounding nebulae and can trigger more star formation. During this period, stars formed across the universe, illuminating the cosmos and ending the dark ages.

Gravity continued to play a crucial role, causing massive stars to collect into dense regions. This led to the formation of the first galaxies, ranging from dwarf galaxies to much larger ones with hundreds of billions of stars.

Galaxies, embedded within dark matter, exerted even greater gravitational influence, leading them to collect into groups, clusters, and superclusters. By the end of this billion-year period, the universe began to resemble its current structure, even though planets had not yet formed.

To understand how planets and other non-stellar objects formed, further knowledge about stars is required, which will be explored in subsequent parts of the series.