What is a star?

A star is a hot sphere of plasma held together by its own gravity and powered during much of its life by nuclear fusion in its core. The Sun is a star. Stellar colors, luminosities and lifetimes differ; two similar points of light can have very different physical properties.

Stellar formation and balance

1. From clouds to stars Stars form when gas and dust clouds condense under gravity in suitable conditions. Compressing matter heats and develops into a protostar. Sustained hydrogen fusion can begin when central conditions become sufficient. Not every small condensation forms an ordinary star. 2. Fusion energy Combining light nuclei can release energy in stars. This differs from ordinary chemical burning with oxygen. Energy travels from interior layers toward the surface and radiates into space. The visible surface is not at the same temperature as the core. 3. Balance and the main sequence Inward gravity is balanced by outward support from the pressure gradient. The long phase of converting core hydrogen into helium is called the main sequence. Initial mass strongly influences evolution. More massive stars may contain more fuel but consume it much faster.

Properties and final stages

4. Color, temperature and brightness Surface temperature affects stellar spectra and color; blue stars are generally hotter than red ones. Apparent brightness also depends on distance, so a faint-looking star may emit enormous power. Intrinsic luminosity must be distinguished from the flux reaching us. 5. Stellar endings Sun-like stars can expand, shed outer layers and leave white dwarfs. Some massive stars undergo supernovae and leave neutron stars or black holes. One ending does not apply to every star; interactions in binary systems also matter. 6. Elements and observation Stars and violent events involving them contribute to cosmic element production and distribution. Spectra, brightness variations and binary orbits reveal temperature, composition and mass. Seeing a star means observing its past state because its light takes time to reach us.