Formation and evolution of stars and galaxies

The study of how stars and galaxies originate, change over time, and reach their present forms through gravity, gas dynamics, radiation, and cosmic expansion.

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Gravitational collapse: The contraction of matter under its own gravity, often increasing its density and temperature. Dense regions collapse to form protostars and contribute to the growth of galaxies.
Big Bang nucleosynthesis: The formation of light atomic nuclei during the first minutes of the universe. It established the primordial hydrogen and helium available to the first stars.
Population III stars: Hypothetical first-generation stars formed from nearly pristine hydrogen and helium in the early universe. Their short-lived existence connects primordial chemistry to the first enrichment of galaxies.
Initial mass function: A statistical description of the distribution of stellar masses at birth in a population of stars. Its form determines how many massive, short-lived stars drive feedback and enrich galaxies.
Reionization: The era when ultraviolet radiation ionized most neutral hydrogen in the intergalactic medium. The sources and timing of this transition depend on how the first stars and galaxies formed.
Molecular cloud: A cold, dense interstellar cloud composed mainly of molecular hydrogen and dust. Stars form when parts of these clouds become gravitationally unstable.
Cosmic microwave background: The relic radiation released when the early universe became transparent, now observed at microwave wavelengths. Its tiny temperature variations reveal initial density fluctuations that later grew into structure.
Globular cluster: A dense, roughly spherical group of old stars bound by gravity. Its stars preserve evidence about early episodes of star formation in galaxies.
Star formation efficiency: The fraction of available gas converted into stars over a specified region or time interval. Differences in efficiency help distinguish rapidly quenched systems from actively star-forming galaxies.
Early galaxy formation: The emergence and growth of galaxies during the universe's first billion years. Unexpectedly bright early galaxies challenge models of star formation and assembly.