Stars

Stars are luminous spheres of hot plasma held together by gravity. Their energy is produced by nuclear fusion in their cores, where hydrogen is fused into helium and, in later stages, into heavier elements. This energy travels outward, balancing gravity and radiating as light and heat that can traverse vast distances.

Most stars form in giant molecular clouds. Gravitational collapse creates a protostar that heats up until hydrogen fusion begins, at which point the star enters the main sequence phase, the longest part of a star’s life. Stellar lifetimes depend on mass: low- to intermediate-mass stars burn hydrogen slowly and can last billions of years, while massive stars exhaust their fuel quickly and end their lives in spectacular events.

Stars are classified by spectral type, from the hot, blue O and B types through A, F, G, K, to the cool, red M type. These spectra reflect surface temperature and composition. Luminosity classes distinguish size categories, such as dwarfs (main sequence stars like the Sun), giants, and supergiants. The Sun is a G-type main-sequence star, often described as a G dwarf.

The fate of a star depends on its initial mass. Low- to intermediate-mass stars (up to about 8 solar masses) shed outer layers and leave behind white dwarfs. More massive stars experience core collapse, exploding as supernovae and leaving neutron stars or black holes. In all cases, stellar nucleosynthesis enriches the interstellar medium with heavier elements, contributing to the chemical evolution of galaxies.

Stars are fundamental components of galaxies, forming clusters, shaping surrounding gas through winds and radiation, and serving as primary sources of light and energy in the universe. Their life cycles span vast timescales and contribute to the cosmic abundance of elements.